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The Entire Story of a Star_ Explained Slowly · Night Science Stories

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  Speaker 1: Good evening and welcome back to the Sleepy Scientist. It
• 0:02
  Speaker 1: is lovely to have you here. By now I hope
• 0:05
  Speaker 1: the busiest part of your day has started to fade
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• 0:12
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• 0:17
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  Speaker 1: you are listening from tonight. It is always rather nice
• 0:27
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  Speaker 1: is now on Spotify too. There is a link waiting
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• 0:48
  Speaker 1: on without requiring quite so much attention from either of you.
• 0:52
  Speaker 1: If these videos have become part of the way you
• 0:54
  Speaker 1: wind down in the evening, a like and a subscribe
• 0:57
  Speaker 1: are always very much appreciated, and to those of you
• 1:01
  Speaker 1: who keep returning night after night, thank you. I know
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  Speaker 1: there are plenty of things you could be doing with
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  Speaker 1: this part of your evening, and I am very glad
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  Speaker 1: that for a little while you've chosen to spend it here.
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  Speaker 1: Tonight we're going to follow one of the longest stories
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  Speaker 1: nature knows how to tell. It begins in a place
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  Speaker 1: with no star at all. Just a vast, cold cloud
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  Speaker 1: of gas and dust drifting through the darkness. Then gravity
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  Speaker 1: begins to gather that material together. A protostar forms. Nuclear
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  Speaker 1: fusion starts. And for millions, billions, or even trillions of years,
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  Speaker 1: depending on the star, an enormous sphere of plasma shines
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  Speaker 1: steadily into space. But stars do not stay the same forever.
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  Speaker 1: Eventually the fuel changes. The balance shifts. The star expands, contracts,
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  Speaker 1: sheds material, or collapses entirely. Some end as white dwarfs.
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  Speaker 1: Some explode as supernovae. Some leave behind neutron stars so
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  Speaker 1: dense that a tiny amount of their matter would outweigh mountains.
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  Speaker 1: And some collapse so completely that they become black holes.
• 2:14
  Speaker 1: which means that the gentle points of light scattered across
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  Speaker 1: the night sky are not simply objects. They are lives
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  Speaker 1: being lived at different stages. So tonight, we're going to
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  Speaker 1: begin before a star exists at all, inside a cold
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  Speaker 1: molecular cloud, and follow the entire journey slowly from birth
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  Speaker 1: to death. Get comfortable. Let your thoughts loosen their grip
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  Speaker 1: on the day and close your eyes whenever they feel ready.
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  Speaker 1: A star can spend billions of years shining. But its
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  Speaker 1: story begins long before the first light appears. Before there
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  Speaker 1: is a star, there is no glowing sphere, no bright surface,
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  Speaker 1: no nuclear furnace, and no neat object waiting to be
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  Speaker 1: switched on. There is only material spread through space. Much
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  Speaker 1: of it exists inside enormous molecular clouds. vast regions of
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  Speaker 1: cold gas and dust drifting through the galaxy. These clouds
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  Speaker 1: are made mostly of hydrogen, with helium making up most
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  Speaker 1: of the remainder. Mixed through them are much smaller amounts
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  Speaker 1: of heavier elements and microscopic dust grains, the solid particles
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  Speaker 1: built from elements such as carbon, oxygen, silicon and iron.
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  Speaker 1: Some molecular clouds stretch across tens or even hundreds of light-years.
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  Speaker 1: They can contain enough material to form thousands of stars,
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  Speaker 1: yet most of that material is spread incredibly thinly by
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  Speaker 1: everyday standards. Even inside a cloud described as dense by astronomers,
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  Speaker 1: you would hardly feel surrounded by anything at all. Space
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  Speaker 1: has a slightly different definition of crowded. The coldest star-forming
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  Speaker 1: regions can reach temperatures of around 10 Kelvin. That is
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  Speaker 1: only about 10 degrees above absolute zero. making them some
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  Speaker 1: of the coldest naturally occurring environments in the universe. This
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  Speaker 1: extreme cold matters. Warm gas has energetic particles moving rapidly
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  Speaker 1: in many directions, creating pressure that can help resist gravitational contraction.
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  Speaker 1: Colder gas moves more slowly, reducing that internal support. Under
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  Speaker 1: the right conditions, gravity can then begin to gather material
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  Speaker 1: together more effectively. and the material it gathers is often
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  Speaker 1: far older than the star that will eventually emerge from it.
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  Speaker 1: A modern molecular cloud contains hydrogen and helium left over
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  Speaker 1: from the early universe, but it also contains material processed
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  Speaker 1: by previous generations of stars. Long-dead stars have expelled gas
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  Speaker 1: through stellar winds. Sun-like stars have shed their outer layers
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  Speaker 1: near the ends of their lives. Massive stars of scattered
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  Speaker 1: material across space in supernova. Explosions. So the birth of
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  Speaker 1: a star is often partly an act of recycling. The
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  Speaker 1: carbon in the dust, the oxygen mixed into the gas,
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  Speaker 1: and many of the heavier elements present in a star-forming
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  Speaker 1: cloud may have spent part of their earlier history inside
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  Speaker 1: stars that disappeared billions of years ago. Our own galaxy
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  Speaker 1: contains many enormous stellar nurseries where this process is taking
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  Speaker 1: place today. The Orion Molecular Cloud is one of the
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  Speaker 1: best-known examples. It is part of a much larger complex
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  Speaker 1: of gas and dust containing several active regions of star formation.
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  Speaker 1: The famous Orion Nebula glows within this broader environment, illuminated
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  Speaker 1: by young stars that formed from the surrounding material. Another
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  Speaker 1: familiar star-forming region lies inside the Eagle Nebula, where enormous
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  Speaker 1: columns of gas and dust known as the Pillars of
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  Speaker 1: Creation have become some of the most recognisable structures in astronomy.
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  Speaker 1: They look almost solid in photographs, like towering cliffs suspended
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  Speaker 1: in space, but they are vast clouds sculpted by radiation
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  Speaker 1: and stellar winds from nearby young stars. Places like these
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  Speaker 1: also reveal an important difficulty in studying stellar birth. Dust
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  Speaker 1: blocks visible light very effectively. A young star can therefore
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  Speaker 1: be forming deep inside a cloud, while remaining completely hidden
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  Speaker 1: from an ordinary optical telescope. From the outside, the region
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  Speaker 1: may appear almost black. Astronomers overcome this by observing the
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  Speaker 1: universe at other wavelengths. Infrared radiation can pass through dust
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  Speaker 1: more effectively than visible light, allowing telescopes to look into
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  Speaker 1: regions that would otherwise remain obscured. The James Webb Space
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  Speaker 1: Telescope is especially powerful for this kind of work. revealing
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  Speaker 1: young stars, dusty structures and warm material embedded within stellar nurseries.
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  Speaker 1: Radio and millimetre observations provide another way of studying the
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  Speaker 1: colder parts of these clouds. The Atacama Large Millimetre slash
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  Speaker 1: Submillimetre Array, located high in the Atacama Desert of northern Chile,
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  Speaker 1: observes radiation associated with cold gas and dust. It allows
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  Speaker 1: astronomers to investigate structures that are far too cold and
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  Speaker 1: faint to shine brightly in visible light. Together, observations like
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  Speaker 1: these reveal that star-forming clouds are anything but featureless. They
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  Speaker 1: contain filaments, knots, clumps and regions of different density, all
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  Speaker 1: moving and interacting under the influence of turbulence, magnetic fields,
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  Speaker 1: radiation and gravity. Yet before a particular star begins to form,
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  Speaker 1: There is still no single centre that deserves to be
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  Speaker 1: called the star. There are simply enormous numbers of particles.
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  Speaker 1: Atoms and molecules move through the cloud. Dust grains drift
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  Speaker 1: among them. Every piece of matter attracts every other piece
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  Speaker 1: through gravity. But at first those attractions may be too
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  Speaker 1: weak and too disorganised to produce a dramatic collapse. Given
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  Speaker 1: the right conditions, however, one region can become slightly denser
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  Speaker 1: than its surroundings. That small difference matters because gravity becomes
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  Speaker 1: stronger where more mass is concentrated. More material is pulled inward,
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  Speaker 1: increasing the density further. The growing concentration then attracts still
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  Speaker 1: more material. A slow imbalance can begin to reinforce itself.
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  Speaker 1: This is the beginning of a star, although nothing is
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  Speaker 1: shining yet. The process is not the creation of matter
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  Speaker 1: from nothing. Gravity does not manufacture the substance of a star.
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  Speaker 1: It reorganises material that already exists. The atoms that will
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  Speaker 1: one day sit inside a blazing stellar core may begin
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  Speaker 1: widely separated across an almost unimaginably large region of cold
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  Speaker 1: interstellar space. Over time, gravity can bring them together. And
• 9:13
  Speaker 1: once enough matter has gathered into a sufficiently dense region,
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  Speaker 1: the cloud is no longer merely drifting through the galaxy.
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  Speaker 1: It has begun to collapse. A molecular cloud can remain
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  Speaker 1: cold and dark for a very long time without producing
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  Speaker 1: anything that resembles a star. Gravity is present everywhere inside it,
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  Speaker 1: quietly pulling every piece of matter towards every other piece.
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  Speaker 1: But gravity is not acting alone. The gas has pressure.
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  Speaker 1: Its particles are moving, colliding and pushing in different directions,
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  Speaker 1: while magnetic fields and turbulent motions can also help resist collapse.
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  Speaker 1: For a star to begin forming something has to change
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  Speaker 1: in this balance. A region of the cloud must become
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  Speaker 1: dense and massive enough for its own gravity to overcome
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  Speaker 1: the forces supporting it. This condition is known as gravitational instability.
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  Speaker 1: It does not mean that gravity has suddenly appeared. Gravity
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  Speaker 1: was there all along. What changes is that gravity becomes
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  Speaker 1: strong enough relative to the internal support of the cloud
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  Speaker 1: for contraction to feed upon itself. A useful way of
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  Speaker 1: understanding this was developed by the British physicist and astronomer
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  Speaker 1: Sir James Jeans in the early 20th century. Jeans considered
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  Speaker 1: the competition between gravity and gas pressure inside a cloud.
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  Speaker 1: His work led to the idea now known as the
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  Speaker 1: Jeans instability. In simple terms, a cloud has a critical
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  Speaker 1: relationship between its temperature, density and mass. If a region
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  Speaker 1: becomes sufficiently massive and dense, or sufficiently cold, its own
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  Speaker 1: gravity can overwhelm the pressure trying to keep it spread out.
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  Speaker 1: The characteristic threshold is often described using the gene's mass.
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  Speaker 1: A warmer cloud generally requires more mass before gravity can dominate,
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  Speaker 1: because its particles are moving more rapidly and producing greater pressure.
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  Speaker 1: A colder cloud can become unstable with less mass because
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  Speaker 1: its particles provide less resistance to contraction. Density matters as well.
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  Speaker 1: Pack the same amount of material into a smaller region
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  Speaker 1: and gravity becomes increasingly effective at drawing the cloud together.
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  Speaker 1: Once a region crosses the appropriate threshold, a slight contraction
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  Speaker 1: can increase its density, which strengthens gravity, which encourages further contraction.
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  Speaker 1: process begins to reinforce itself. There are several ways a
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  Speaker 1: molecular cloud can be pushed towards this state. One is turbulence.
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  Speaker 1: Molecular clouds are not perfectly still structures. Gas moves through
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  Speaker 1: them at different speeds and in different directions, creating filaments,
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  Speaker 1: knots and compressed regions. Some of these denser patches can
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  Speaker 1: become gravitationally unstable without requiring any dramatic outside event. Clouds
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  Speaker 1: can also interact with one another. Collisions between streams or
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  Speaker 1: larger structures of gas can compress material into a smaller volume,
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  Speaker 1: increasing the density enough to favour collapse. Massive stars can
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  Speaker 1: influence nearby clouds too. Young high-mass stars produce powerful ultraviolet
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  Speaker 1: radiation and strong stellar winds. These can inflate enormous bubbles
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  Speaker 1: in the surrounding gas. As an expanding bubble pushes outward,
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  Speaker 1: it may compress material along its edge. In some environments,
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  Speaker 1: that compression can help create dense regions in which new
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  Speaker 1: stars begin forming. Supernova explosions can also drive shockwaves through
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  Speaker 1: the interstellar medium. A passing shock can squeeze parts of
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  Speaker 1: a molecular cloud, and under suitable conditions, this compression may
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  Speaker 1: contribute to gravitational collapse. It is tempting to imagine every
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  Speaker 1: star beginning with some spectacular cosmic shove, perhaps a supernova
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  Speaker 1: conveniently arriving at exactly the right moment. Nature is not
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  Speaker 1: always quite so theatrical. Many molecular clouds are already turbulent
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  Speaker 1: enough to create dense, unstable regions by themselves. There does
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  Speaker 1: not have to be one identifiable external trigger for every star.
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  Speaker 1: The internal structure of the cloud can provide all the
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  Speaker 1: irregularity needed, and once collapse begins, an enormous cloud does
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  Speaker 1: not usually fall neatly towards one central point. Instead, it
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  Speaker 1: tends to fragment. Different regions become dense enough to collapse independently,
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  Speaker 1: producing many separate concentrations of matter. This is why stars
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  Speaker 1: are so often born in groups, clusters and associations. rather
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  Speaker 1: than appearing one at a time in complete isolation. A
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  Speaker 1: single large star-forming cloud may eventually produce dozens, hundreds or
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  Speaker 1: even thousands of stars. Each collapsing fragment follows its own
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  Speaker 1: local gravitational story. At first, the fragment may be irregular
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  Speaker 1: and poorly organised. Gas falls inward from different directions. Filaments
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  Speaker 1: feed material into denser knots. Turbulence continues to stir the surroundings,
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  Speaker 1: but gravity steadily concentrates more and more mass towards the centre.
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  Speaker 1: As the fragment shrinks, another physical principle becomes increasingly important.
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  Speaker 1: Conservation of angular momentum means that even a cloud that
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  Speaker 1: begins with only a very small amount of rotation can
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  Speaker 1: spin faster as it contracts. The familiar comparison is a
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  Speaker 1: spinning ice skater. When the skater pulls their arms inward,
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  Speaker 1: their rotation speeds up. The same underlying principle applies to
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  Speaker 1: a contracting cloud of gas, although the cloud is vastly larger,
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  Speaker 1: far less tidy and considerably less likely to win points
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  Speaker 1: for presentation. The original molecular material does not need to
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  Speaker 1: be rotating quickly. Any small net rotation can become much
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  Speaker 1: more noticeable once the cloud is contracted by an enormous factor,
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  Speaker 1: This rotation also means that infalling matter cannot all travel
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  Speaker 1: directly towards the centre. Material with angular momentum tends to orbit,
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  Speaker 1: helping create a flattened structure around the growing central concentration.
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  Speaker 1: The centre itself becomes denser with time. More material falls inward,
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  Speaker 1: and the gravitational field of the developing object grows stronger.
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  Speaker 1: The cloud is beginning to lose its earlier diffuse appearance.
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  Speaker 1: and acquire something resembling an organised centre. Gravity is also
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  Speaker 1: doing something less obvious. It is heating the material. When
• 16:14
  Speaker 1: matter falls deeper into a gravitational field, gravitational potential energy
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  Speaker 1: is released. As gas collapses towards the centre, part of
• 16:24
  Speaker 1: that energy is converted into kinetic energy, and then into
• 16:28
  Speaker 1: thermal energy through collisions and compression. The cloud began as
• 16:33
  Speaker 1: an extremely cold environment, perhaps only around 10 Kelvin in
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  Speaker 1: its densest regions. But the contracting centre now becomes progressively warmer.
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  Speaker 1: This heating is not yet the result of sustained hydrogen fusion.
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  Speaker 1: That distinction is important. The object forming at the centre
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  Speaker 1: may begin to look increasingly star-like, and it may eventually
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  Speaker 1: radiate considerable energy, but it has not yet become a
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  Speaker 1: normal main sequence star. Its central temperature is still too
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  Speaker 1: low for sustained hydrogen fusion to provide the dominant long-term
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  Speaker 1: source of energy. For now, gravity is supplying the heat.
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  Speaker 1: The amount of matter collected during this period is already
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  Speaker 1: beginning to shape nearly everything about the object's future. A
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  Speaker 1: star born with relatively little mass will have weaker gravity,
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  Speaker 1: a cooler core, lower luminosity and an extraordinarily long life.
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  Speaker 1: A star born with much more mass will compress its
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  Speaker 1: interior far more strongly. Its core will become hotter, its
• 17:40
  Speaker 1: nuclear reactions will eventually proceed much faster and its lifetime
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  Speaker 1: will be dramatically shorter. Mass will influence its colour, temperature, brightness,
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  Speaker 1: internal structure and eventual fate. So long before the star
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  Speaker 1: has produced its first sustained hydrogen fusion, Some of the
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  Speaker 1: most important features of its entire existence are already being established.
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  Speaker 1: The transformation is still incomplete. There is no stable shining
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  Speaker 1: star yet, only an increasingly dense concentration of matter drawing
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  Speaker 1: material inward, rotating more quickly as it contracts and becoming
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  Speaker 1: warmer as gravitational energy is converted into heat. The once
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  Speaker 1: diffuse cloud now possesses a centre. Gravity has begun to win.
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  Speaker 1: As more and more material falls towards the centre of
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  Speaker 1: the collapsing cloud, the growing object begins to take on
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  Speaker 1: a much more recognisable form. This young object is called
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  Speaker 1: a protostar. A protostar is not yet a true main-sequence star.
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  Speaker 1: It is a contracting stellar object that has not established
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  Speaker 1: sustained hydrogen fusion in its core. Even so, it can
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  Speaker 1: already become remarkably hot and bright. The reason is gravity.
• 19:03
  Speaker 1: As gas continues to fall inward, gravitational potential energy is
• 19:07
  Speaker 1: converted into motion and then into heat through compression and collisions.
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  Speaker 1: The deeper the material falls into the growing gravitational well,
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  Speaker 1: the more energy can be released. For a time, this
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  Speaker 1: gravitational contraction provides much of the protostar's luminosity. Long before
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  Speaker 1: scientists understood nuclear fusion, This basic idea was explored by
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  Speaker 1: Lord Kelvin and Hermann von Helmholtz. They recognised that a
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  Speaker 1: contracting object could convert gravitational energy into heat and light.
• 19:41
  Speaker 1: The process became known as the Kelvin-Helmholtz mechanism. The Kelvin-Helmholtz
• 19:46
  Speaker 1: mechanism could not explain how the Sun had remained bright
• 19:49
  Speaker 1: for billions of years because gravitational contraction alone would run
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  Speaker 1: out of usable energy far too quickly. Nuclear fusion eventually
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  Speaker 1: solved that problem. But for a protostar, gravitational contraction is
• 20:04
  Speaker 1: exactly the sort of energy source that matters. The object
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  Speaker 1: can therefore shine before it has truly joined the main sequence.
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  Speaker 1: Around it, the surrounding material rarely falls directly inward from
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  Speaker 1: every direction. The original cloud almost always contained at least
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  Speaker 1: a small amount of rotation. As the cloud collapses, conservation
• 20:27
  Speaker 1: of angular momentum causes that rotation to become increasingly important.
• 20:33
  Speaker 1: Material moving towards the centre cannot simply lose all of
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  Speaker 1: its sideways motion. Instead, much of it settles into a
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  Speaker 1: flattened rotating structure known as an accretion disk. The result
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  Speaker 1: is a young protostar surrounded by a disk of gas
• 20:49
  Speaker 1: and dust, with additional material from the larger. Envelope continuing
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  Speaker 1: to fall towards it. Accretion is the process through which
• 20:58
  Speaker 1: the protostar continues gaining mass from the surrounding material. Gas
• 21:03
  Speaker 1: moves through the disk, gradually transferring angular momentum outward and
• 21:08
  Speaker 1: allowing some matter to spiral closer to the centre. As
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  Speaker 1: more material joins the protostar, its mass increases, its gravity
• 21:16
  Speaker 1: becomes stronger and the pressure and temperature in its centre
• 21:19
  Speaker 1: continue to rise. This process is not perfectly smooth. Young
• 21:25
  Speaker 1: stellar systems can be highly active environments. Magnetic fields threading
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  Speaker 1: the disk and the rapidly changing material around the protostar
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  Speaker 1: can help launch powerful outflows. In many cases, narrow jets
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  Speaker 1: of material emerge in opposite directions, roughly along the rotational
• 21:43
  Speaker 1: axis of the system. These bipolar jets can travel enormous
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  Speaker 1: distances through the surrounding cloud. They are not decorative extras.
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  Speaker 1: They are physically important. By carrying away angular momentum, jets
• 21:57
  Speaker 1: and outflows help make it possible for additional matter to
• 22:00
  Speaker 1: move inward and continue feeding the protostar. The interaction between gravity, rotation,
• 22:07
  Speaker 1: magnetic fields and accretion is therefore deeply connected to how
• 22:12
  Speaker 1: the young star grows. Astronomers have observed direct evidence of
• 22:16
  Speaker 1: these outflows in objects known as Herbig-Haro objects. They are
• 22:21
  Speaker 1: named after the astronomers George Herbig and Guillermo Haro, who
• 22:25
  Speaker 1: studied bright patches of nebulosity associated with young stars. A
• 22:30
  Speaker 1: Herbig-Hero object forms when a fast jet from a young
• 22:34
  Speaker 1: stellar object crashes into surrounding gas. The collision creates shockwaves
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  Speaker 1: that heat and excite the gas, causing it to glow.
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  Speaker 1: Some Herbig-Hero structures extend across distances of several light-years, which
• 22:49
  Speaker 1: is impressive considering that the star producing them may still
• 22:52
  Speaker 1: be in the very early stages of its existence. As
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  Speaker 1: a low-mass protostar continues evolving, it can eventually become what
• 23:01
  Speaker 1: astronomers call a Tetari star. Tetari stars are young stars
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  Speaker 1: that are no longer deeply buried protostars, but have not
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  Speaker 1: yet settled into stable main-sequence hydrogen fusion. They can be
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  Speaker 1: highly variable, their brightness can change irregularly, their magnetic fields
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  Speaker 1: can be extremely active, and powerful stellar winds can carry
• 23:23
  Speaker 1: material away from their surfaces. They may also show strong
• 23:27
  Speaker 1: flares and intense X-ray emission, compared with more settled stars
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  Speaker 1: such as the present-day Sun. At the same time, they
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  Speaker 1: are still contracting. Gravity continues to draw the star inward,
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  Speaker 1: and that gradual contraction keeps heating the core. Young stellar
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  Speaker 1: systems like these can be difficult to study because they
• 23:48
  Speaker 1: are often surrounded by the same dust that helped create them.
• 23:52
  Speaker 1: Visible light can be blocked or scattered before it ever escapes.
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  Speaker 1: This is why different wavelengths are so useful. The James
• 24:00
  Speaker 1: Webb Space Telescope can observe infrared radiation that passes through
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  Speaker 1: dusty regions more effectively than visible light. It can reveal
• 24:10
  Speaker 1: embedded young stars, warm dust and structures inside disks that
• 24:14
  Speaker 1: would otherwise remain hidden. The Hubble Space Telescope can observe
• 24:18
  Speaker 1: exposed jets. glowing gas and young stars once parts of
• 24:23
  Speaker 1: their surroundings become visible. Radio observatories add another layer of
• 24:28
  Speaker 1: information by tracing cold molecular gas and dust that are
• 24:31
  Speaker 1: still feeding the growing system. Together, these observations show stellar
• 24:36
  Speaker 1: birth not as one simple collapse, but as a complex
• 24:40
  Speaker 1: process involving infall, disks, rotation, magnetic fields, jets and continuing accretion.
• 24:49
  Speaker 1: Not every collapsing object becomes a true star. There is
• 24:53
  Speaker 1: a lower mass limit for sustained hydrogen fusion. An object
• 24:57
  Speaker 1: with less than roughly 7 or 8% of the Sun's
• 25:00
  Speaker 1: mass generally cannot compress its centre, enough to maintain ordinary
• 25:05
  Speaker 1: hydrogen fusion over the long term. Instead, it becomes a
• 25:08
  Speaker 1: brown dwarf. Brown dwarfs occupy the region between the most
• 25:13
  Speaker 1: massive planets and the least massive stars. They can briefly
• 25:17
  Speaker 1: fuse deuterium, and the more massive examples can also fuse
• 25:21
  Speaker 1: lithium for a time, but they never settle into sustained
• 25:25
  Speaker 1: hydrogen fusion in the way that a normal main sequence
• 25:29
  Speaker 1: star does. The exact boundary is not perfectly fixed. It
• 25:35
  Speaker 1: depends somewhat on chemical composition and other physical details, so
• 25:39
  Speaker 1: nature does not place a perfectly neat line between star
• 25:42
  Speaker 1: and brown dwarf. But the difference remains fundamental. A true
• 25:47
  Speaker 1: star must eventually reach conditions in its core where sustained
• 25:51
  Speaker 1: hydrogen fusion becomes possible. For the protostar, everything is moving
• 25:56
  Speaker 1: towards that threshold. Mass continues to arrive. The central pressure rises.
• 26:04
  Speaker 1: The temperature climbs higher. The surrounding disk evolves, while jets
• 26:09
  Speaker 1: and winds gradually reshape the system. Deep inside, the core
• 26:13
  Speaker 1: becomes steadily denser and hotter. as gravity continues its work.
• 26:19
  Speaker 1: Eventually the centre approaches temperatures of several million degrees. At
• 26:23
  Speaker 1: that point, the conditions are beginning to change in a
• 26:27
  Speaker 1: decisive way. Gravity has taken the protostar as far as
• 26:31
  Speaker 1: contraction alone can take it. The core is approaching the
• 26:35
  Speaker 1: point where atomic nuclei can begin to participate in sustained
• 26:40
  Speaker 1: nuclear fusion. The young object is very close to becoming
• 26:44
  Speaker 1: a true star. For millions of years, gravity has been
• 26:48
  Speaker 1: compressing the young object from all directions. The protostar has
• 26:53
  Speaker 1: grown denser, hotter and more compact, while material continues to
• 26:57
  Speaker 1: fall inward from its surroundings. Deep in the centre, the
• 27:02
  Speaker 1: temperature has been rising steadily, eventually reaching several million degrees.
• 27:07
  Speaker 1: At these temperatures, matter behaves very differently from the material
• 27:11
  Speaker 1: we encounter in ordinary life. Atoms can no longer remain intact.
• 27:16
  Speaker 1: Electrons are stripped away from their nuclei, leaving a hot
• 27:20
  Speaker 1: mixture of charged particles known as plasma. Inside this plasma,
• 27:25
  Speaker 1: hydrogen nuclei move rapidly through the stellar core. A hydrogen
• 27:30
  Speaker 1: nucleus is essentially a single proton, and protons carry positive
• 27:35
  Speaker 1: electric charge. Two positively charged particles repel one another, so
• 27:41
  Speaker 1: bringing two protons extremely close together is not easy. This
• 27:46
  Speaker 1: electrical repulsion is known as the Coulomb barrier. For nuclear
• 27:50
  Speaker 1: fusion to occur, nuclei must approach closely enough for the
• 27:55
  Speaker 1: strong nuclear force to become important. The strong nuclear force
• 27:59
  Speaker 1: acts over an incredibly short range. But once nuclei are
• 28:03
  Speaker 1: sufficiently close, it can bind particles together much more strongly
• 28:08
  Speaker 1: than their electrical repulsion can push them apart. Temperature helps
• 28:12
  Speaker 1: by giving the particles enormous kinetic energy. The hotter the
• 28:17
  Speaker 1: core becomes, the faster the nuclei move and the more
• 28:21
  Speaker 1: frequently they collide. But temperature alone does not tell the
• 28:25
  Speaker 1: whole story. Even inside the centre of a star like
• 28:29
  Speaker 1: the Sun, most protons do not have enough energy to
• 28:32
  Speaker 1: cross the Coulomb barrier in the straightforward way predicted by
• 28:36
  Speaker 1: classical physics. Quantum mechanics provides another route. A phenomenon known
• 28:42
  Speaker 1: as quantum tunnelling gives particles a small probability of passing
• 28:46
  Speaker 1: through a barrier that classical physics says they should not
• 28:50
  Speaker 1: have enough energy to cross. This is not a rare
• 28:54
  Speaker 1: curiosity added on top of stellar fusion. Quantum tunnelling is
• 28:59
  Speaker 1: one of the reasons stars can shine at all. Given
• 29:02
  Speaker 1: the staggering number of protons colliding inside a stellar core,
• 29:06
  Speaker 1: Even a very small probability of tunnelling becomes enough for
• 29:10
  Speaker 1: nuclear reactions to occur continuously. In a star with a
• 29:14
  Speaker 1: mass similar to the Sun, the main pathway is called
• 29:18
  Speaker 1: the proton-proton chain. The process begins when two protons interact.
• 29:24
  Speaker 1: Through a series of nuclear reactions, hydrogen is gradually converted
• 29:29
  Speaker 1: into helium. The complete chain involves several intermediate particles and reactions,
• 29:35
  Speaker 1: but the overall result is surprisingly simple. Four hydrogen nuclei
• 29:40
  Speaker 1: ultimately contribute to the production of one helium nucleus, while
• 29:45
  Speaker 1: energy is released along the way. Some of that energy
• 29:48
  Speaker 1: appears as electromagnetic radiation. Some is carried by particles such
• 29:54
  Speaker 1: as neutrinos, which interact so weakly with matter that that
• 29:58
  Speaker 1: many of them escape from the stellar interior almost immediately.
• 30:02
  Speaker 1: The nuclear reactions that power stars were understood in detail
• 30:06
  Speaker 1: only during the 20th century. One of the central figures
• 30:10
  Speaker 1: was the German-American physicist Hans Bethe. Bethe helped explain the
• 30:15
  Speaker 1: reaction pathways through which stars convert hydrogen into helium and
• 30:19
  Speaker 1: release energy. His work on stellar energy production became one
• 30:23
  Speaker 1: of the foundations of modern astrophysics. and he received the
• 30:27
  Speaker 1: Nobel Prize in Physics in 1967. The proton-proton chain is
• 30:32
  Speaker 1: especially important in stars such as the Sun. In hotter
• 30:37
  Speaker 1: and more massive stars, another sequence of reactions becomes increasingly significant.
• 30:43
  Speaker 1: This is the carbon-nitrogen-oxygen cycle. Carbon, nitrogen and oxygen nuclei
• 30:49
  Speaker 1: participate in a series of reactions that ultimately convert hydrogen
• 30:54
  Speaker 1: into helium. The carbon, nitrogen and oxygen act as part
• 30:59
  Speaker 1: of a cycle, rather than simply being consumed as the
• 31:02
  Speaker 1: main fuel. The final broad result is again the transformation
• 31:07
  Speaker 1: of hydrogen into helium, accompanied by the release of energy.
• 31:11
  Speaker 1: The carbon-nitrogen-oxygen cycle is extremely sensitive to temperature, so it
• 31:18
  Speaker 1: becomes far more important in stellar cores that are hotter
• 31:21
  Speaker 1: than the sun's. Despite the familiar phrase that stars are
• 31:25
  Speaker 1: burning hydrogen, none of this is burning in the everyday
• 31:28
  Speaker 1: chemical sense. There is no flame inside a star. There
• 31:32
  Speaker 1: is no oxygen being used as it would be in
• 31:35
  Speaker 1: a fire. Chemical burning rearranges electrons around atoms. Nuclear fusion
• 31:41
  Speaker 1: changes the nuclei themselves. The energy released is also vastly
• 31:46
  Speaker 1: greater than the energy available from ordinary chemical reactions. Part
• 31:50
  Speaker 1: of the reason is that the helium nucleus produced by
• 31:53
  Speaker 1: fusion has slightly less mass than the combined mass of
• 31:56
  Speaker 1: the particles that contributed to making it. That small difference
• 32:01
  Speaker 1: in mass corresponds to released energy through mass-energy equivalents. The
• 32:05
  Speaker 1: missing mass has not vanished without explanation. It has been
• 32:09
  Speaker 1: converted into other forms of energy. Once hydrogen fusion becomes
• 32:14
  Speaker 1: sustained in the central region, the young star experiences a
• 32:17
  Speaker 1: profound change. Until now, gravity has driven the contraction. As
• 32:23
  Speaker 1: the object contracted, it heated up. Fusion now introduces a
• 32:28
  Speaker 1: long-lasting internal source of energy, capable of supporting the star.
• 32:33
  Speaker 1: The interior becomes hot enough that gas pressure and radiation
• 32:36
  Speaker 1: contribute to resisting further gravitational compression. A balance develops. This
• 32:42
  Speaker 1: balance is called hydrostatic equilibrium. Gravity continues pulling every layer
• 32:48
  Speaker 1: of the star inward. At the same time, pressure within
• 32:52
  Speaker 1: the hot stellar material pushes outward. The star does not
• 32:56
  Speaker 1: defeat gravity, and gravity does not disappear. Instead, the two
• 33:02
  Speaker 1: effects reach an approximate balance. If the core contracts slightly,
• 33:07
  Speaker 1: its temperature can rise, increasing the rate of nuclear reactions
• 33:11
  Speaker 1: and strengthening the pressure that opposes further contraction. If the
• 33:16
  Speaker 1: star expands slightly, rate and allowing gravity to regain some influence.
• 33:25
  Speaker 1: This ability to respond to small changes helps make an
• 33:29
  Speaker 1: ordinary star remarkably stable over very long periods. That stability
• 33:34
  Speaker 1: should not be mistaken for stillness. Inside the star, nuclei
• 33:39
  Speaker 1: are reacting continuously. Energy is moving outward. Matter is circulating
• 33:45
  Speaker 1: in regions where convection occurs. Radiation travels through the interior,
• 33:50
  Speaker 1: repeatedly interacting with charged particles. Gravity acts without pause across
• 33:56
  Speaker 1: the entire star. What appears from a distance to be
• 33:59
  Speaker 1: an unchanging point of light is really an enormous system
• 34:03
  Speaker 1: in constant physical activity. Eventually, the newly formed star settles
• 34:08
  Speaker 1: into sustained hydrogen, fusion in its core. Astronomers describe the
• 34:14
  Speaker 1: beginning of this stable phase as the zero-age main sequence.
• 34:18
  Speaker 1: The star has joined the main sequence, the great population
• 34:22
  Speaker 1: of ordinary hydrogen-fusing stars that includes our own Sun. This
• 34:26
  Speaker 1: is the moment when the long period of formation gives
• 34:29
  Speaker 1: way to the longest part of stellar life. A star
• 34:32
  Speaker 1: may remain in this state for millions, billions, or, for
• 34:37
  Speaker 1: the smallest examples, potentially trillions of years. How long it
• 34:42
  Speaker 1: remains there depends strongly on the mass it gathered before
• 34:45
  Speaker 1: fusion ever began. That mass determines how strongly gravity compresses
• 34:50
  Speaker 1: its core. how hot the centre becomes, how rapidly nuclear
• 34:54
  Speaker 1: fuel is consumed, and how brightly the star shines. The
• 34:59
  Speaker 1: star has finally been born, and almost everything about the
• 35:03
  Speaker 1: pace of the life ahead has already been decided. Once
• 35:06
  Speaker 1: a star settles into sustained hydrogen fusion, the pace of
• 35:10
  Speaker 1: its life changes completely. The violent contraction of its birth
• 35:15
  Speaker 1: is over. The surrounding cloud begins to disperse, the accretion
• 35:19
  Speaker 1: disk may gradually thin, and the star enters a period
• 35:23
  Speaker 1: that can last vastly longer than everything that came before it.
• 35:27
  Speaker 1: This is the main sequence. For most stars, the main
• 35:31
  Speaker 1: sequence occupies the overwhelming majority of their active lives. During
• 35:36
  Speaker 1: this time, hydrogen in the central region is steadily converted
• 35:39
  Speaker 1: into helium, releasing the energy that allows the star to
• 35:43
  Speaker 1: shine while helping its interior remain in hydrostatic equilibrium. Our
• 35:48
  Speaker 1: own Sun provides the most familiar example. The Sun is
• 35:52
  Speaker 1: roughly 4.5 billion years old, and it is still comfortably
• 35:57
  Speaker 1: within its main sequence lifetime. It has already spent billions
• 36:01
  Speaker 1: of years converting hydrogen into helium in its core, and
• 36:04
  Speaker 1: is expected to continue doing so for roughly another 5
• 36:07
  Speaker 1: billion years. On human timescales, that is so long that
• 36:12
  Speaker 1: the Sun appears almost perfectly constant. But inside it, an
• 36:16
  Speaker 1: extraordinary amount of activity is taking place every second. Roughly
• 36:21
  Speaker 1: 600 million tonnes of hydrogen are involved in fusion each
• 36:27
  Speaker 1: second in the Sun's core. The resulting helium has slightly
• 36:31
  Speaker 1: less mass than the hydrogen that went into the reactions,
• 36:35
  Speaker 1: and several million tonnes of mass are effectively converted into
• 36:39
  Speaker 1: energy every second. That sounds like an alarming rate of consumption.
• 36:44
  Speaker 1: until the scale of the Sun is remembered. The Sun
• 36:48
  Speaker 1: contains about 2,000 million million million million tonnes of material.
• 36:54
  Speaker 1: Even though only the central region reaches the temperatures and
• 36:58
  Speaker 1: pressures required for sustained fusion, the available hydrogen reservoir is enormous.
• 37:04
  Speaker 1: A star is therefore able to maintain what looks like
• 37:07
  Speaker 1: astonishingly wasteful energy production for billions of years. The energy
• 37:12
  Speaker 1: created in the core does not immediately leap into space.
• 37:16
  Speaker 1: It has a long journey outward. In the Sun, energy
• 37:21
  Speaker 1: from the central regions first moves through what is known
• 37:24
  Speaker 1: as the radiative zone. Here, energy is transported mainly by radiation,
• 37:30
  Speaker 1: interacting repeatedly with the surrounding plasma. A photon does not
• 37:35
  Speaker 1: simply travel in a straight line from the centre of
• 37:38
  Speaker 1: the Sun to the surface. It may be absorbed re-emitted,
• 37:42
  Speaker 1: scattered and replaced many times during the outward transfer of energy.
• 37:48
  Speaker 1: Describing the process as a single photon patiently trying to
• 37:52
  Speaker 1: escape would be slightly misleading, but it gives some sense
• 37:56
  Speaker 1: of how indirect the journey can be. Farther outward, the
• 38:00
  Speaker 1: sun enters its convective zone. Here the material itself moves.
• 38:06
  Speaker 1: Hot plasma rises towards the surface, cools and sinks again.
• 38:11
  Speaker 1: producing enormous circulating motions that transport energy through the outer
• 38:16
  Speaker 1: part of the star. Radiation and convection are therefore two
• 38:20
  Speaker 1: major ways that stars move energy from their interiors towards
• 38:24
  Speaker 1: their surfaces. The balance between them varies from star to star.
• 38:29
  Speaker 1: A star's mass changes its internal temperature, density and opacity,
• 38:35
  Speaker 1: so different stars develop different internal structures. Some low-mass stars
• 38:41
  Speaker 1: can be convective through almost their entire interiors. Stars similar
• 38:45
  Speaker 1: to the Sun possess both radiative and convective regions. More
• 38:50
  Speaker 1: massive stars can develop convective cores surrounded by radiative envelopes. Eventually,
• 38:57
  Speaker 1: energy reaches the visible surface of the star, known as
• 39:00
  Speaker 1: the photosphere. From there, it escapes into space as electromagnetic radiation.
• 39:07
  Speaker 1: Visible light is only part of this output. Stars also
• 39:11
  Speaker 1: emit infrared radiation, ultraviolet radiation, and other wavelengths depending on
• 39:17
  Speaker 1: their temperature and activity. The main sequence itself became clear
• 39:22
  Speaker 1: through one of the most important diagrams in astronomy. In
• 39:26
  Speaker 1: the early 20th century, Danish astronomer Einar Hertzsprung and American
• 39:31
  Speaker 1: astronomer Henry Norris Russell independently explored relationships between stellar brightness
• 39:37
  Speaker 1: and temperature. The Hertzsprung-Russell diagram plots stars according to quantities
• 39:43
  Speaker 1: such as luminosity and surface temperature. When large numbers of
• 39:47
  Speaker 1: stars are placed on this diagram, They do not appear
• 39:50
  Speaker 1: randomly scattered. A broad diagonal band emerges. That band is
• 39:55
  Speaker 1: the main sequence. It stretches from cool, faint, low-mass stars
• 40:02
  Speaker 1: at one end to extremely hot, luminous, high-mass stars at
• 40:06
  Speaker 1: the other. The Sun sits somewhere around the middle of
• 40:10
  Speaker 1: this enormous family. That is an important reminder that the
• 40:14
  Speaker 1: Sun is not a typical template that every other star
• 40:17
  Speaker 1: closely follows. At the low-mass end are red dwarfs. These
• 40:22
  Speaker 1: stars are smaller, cooler and much less luminous than the Sun.
• 40:27
  Speaker 1: Their cores consume hydrogen very slowly and in the least
• 40:31
  Speaker 1: massive red dwarfs, convection can mix material through much of
• 40:35
  Speaker 1: the star. This allows them to make unusually efficient use
• 40:39
  Speaker 1: of their hydrogen supply. At the other end are stars
• 40:43
  Speaker 1: many times more massive than the Sun. They contain more
• 40:46
  Speaker 1: hydrogen but they also possess much stronger gravity. That stronger
• 40:51
  Speaker 1: gravity compresses their cores to greater pressures and temperatures, causing
• 40:56
  Speaker 1: nuclear reactions to proceed far more rapidly. The result is
• 41:01
  Speaker 1: one of the more counterintuitive rules of stellar evolution. A
• 41:05
  Speaker 1: massive star begins life with more fuel, yet lives for
• 41:09
  Speaker 1: less time. Its enormous luminosity comes at a price. The
• 41:14
  Speaker 1: most massive stars can shine tens or hundreds of thousands
• 41:18
  Speaker 1: of times more brightly than the Sun, and may exhaust
• 41:21
  Speaker 1: their core hydrogen after only a few million years. A
• 41:25
  Speaker 1: small red dwarf, by contrast, may survive for hundreds of
• 41:28
  Speaker 1: billions or even trillions of years. The universe itself is
• 41:33
  Speaker 1: only about 13.8 billion years old. This means that the
• 41:38
  Speaker 1: lowest mass red dwarfs born early in cosmic history have
• 41:41
  Speaker 1: not yet had enough time to finish their main sequence lives.
• 41:46
  Speaker 1: No astronomer has ever observed one naturally reaching the final
• 41:50
  Speaker 1: stages predicted for it, because the universe has simply not
• 41:54
  Speaker 1: been around long enough. Stellar ageing therefore runs on very
• 41:58
  Speaker 1: different clocks. Imagine two stars forming at roughly the same
• 42:02
  Speaker 1: time from the same enormous molecular cloud. One might be
• 42:06
  Speaker 1: a small red dwarf. The other might contain 20 times
• 42:11
  Speaker 1: the mass of the Sun. The massive star could pass
• 42:14
  Speaker 1: through its main sequence life, expand, undergo advanced nuclear fusion
• 42:21
  Speaker 1: and die in a supernova while the red dwarf beside
• 42:25
  Speaker 1: it has barely begun using its hydrogen. They share a birthday,
• 42:30
  Speaker 1: but almost nothing resembling the same lifespan. Even during the
• 42:34
  Speaker 1: apparently stable main sequence, a star is not truly unchanging.
• 42:40
  Speaker 1: Hydrogen is gradually disappearing from the central region, while helium accumulates.
• 42:46
  Speaker 1: Helium nuclei contain more mass packed into each particle than
• 42:51
  Speaker 1: hydrogen nuclei, changing the physical conditions in the core. As
• 42:56
  Speaker 1: the composition shifts, the core slowly adjusts, its density increases
• 43:02
  Speaker 1: and its temperature rises. These gradual changes can increase the
• 43:07
  Speaker 1: rate at which nuclear reactions occur. meaning that a main
• 43:11
  Speaker 1: sequence star can slowly become more luminous as it ages.
• 43:16
  Speaker 1: The Sun is a good example. When it first arrived
• 43:19
  Speaker 1: on the main sequence, it was significantly fainter than it
• 43:22
  Speaker 1: is today. Over billions of years, the gradual accumulation of
• 43:27
  Speaker 1: helium in its core has altered its internal structure and
• 43:31
  Speaker 1: increased its luminosity. It will continue to brighten slowly during
• 43:36
  Speaker 1: the remainder of its main sequence life. Nothing dramatic has
• 43:41
  Speaker 1: to happen from one day to the next. There is
• 43:44
  Speaker 1: no moment when the sun suddenly looks older. Stellar ageing
• 43:48
  Speaker 1: is written into changes that unfold over hundreds of millions
• 43:52
  Speaker 1: and billions of years. For now, hydrostatic equilibrium continues to hold.
• 43:59
  Speaker 1: Gravity pulls inward. Pressure supports the star from within. Hydrogen
• 44:04
  Speaker 1: continues becoming helium. energy moves slowly outward and escapes into space.
• 44:10
  Speaker 1: This long balance makes the main sequence appear peaceful, but
• 44:14
  Speaker 1: it cannot last forever. Every fusion reaction changes the composition
• 44:20
  Speaker 1: of the core by a tiny amount, and after enough
• 44:23
  Speaker 1: time has passed, the central supply of usable hydrogen will
• 44:28
  Speaker 1: become too depleted for the star to continue living in
• 44:31
  Speaker 1: quite the same way. Among all the properties a star possesses,
• 44:36
  Speaker 1: One matters more than almost any other. It's mass. The
• 44:40
  Speaker 1: amount of material a star gathers while forming has an
• 44:44
  Speaker 1: enormous influence over what happens during almost every stage of
• 44:48
  Speaker 1: its life. Mass helps determine how hot the centre becomes,
• 44:53
  Speaker 1: how brightly the star shines, how quickly it consumes its fuel,
• 44:58
  Speaker 1: how its interior is arranged, how long it survives, and
• 45:02
  Speaker 1: what sort of remnant it eventually leaves behind. The reason
• 45:06
  Speaker 1: begins with gravity. A more massive star possesses stronger self-gravity.
• 45:12
  Speaker 1: The greater gravitational pull compresses the stellar interior more intensely,
• 45:18
  Speaker 1: raising the pressure and temperature in the core. Higher temperature
• 45:22
  Speaker 1: means the particles inside the core move more rapidly and
• 45:25
  Speaker 1: collide more frequently. Nuclear reactions then occur at a faster rate.
• 45:31
  Speaker 1: So a star with more mass does not merely contain
• 45:34
  Speaker 1: more fuel. It also burns through that fuel much more aggressively.
• 45:40
  Speaker 1: This is why a massive star can be extraordinarily luminous
• 45:44
  Speaker 1: while living for a surprisingly short time. A small star
• 45:48
  Speaker 1: takes a very different path. At the low-mass end of
• 45:52
  Speaker 1: the main sequence are red dwarfs. These stars can contain
• 45:57
  Speaker 1: only a fraction of the Sun's mass. They're cooler, dimmer
• 46:01
  Speaker 1: and far less luminous than the Sun. but in one
• 46:05
  Speaker 1: respect they are remarkably efficient. Many low-mass red dwarfs are
• 46:10
  Speaker 1: convective through most or all of their interiors. Convection allows
• 46:15
  Speaker 1: material to circulate. Hydrogen from farther out can be mixed inward,
• 46:20
  Speaker 1: while helium produced near the centre can be carried elsewhere.
• 46:24
  Speaker 1: This means that a red dwarf can make use of
• 46:27
  Speaker 1: a much larger fraction of its total hydrogen than a
• 46:30
  Speaker 1: star like the Sun. where only the central region reaches
• 46:34
  Speaker 1: the conditions required for sustained hydrogen fusion. At the same time,
• 46:40
  Speaker 1: a red dwarf consumes that hydrogen extremely slowly. The result
• 46:45
  Speaker 1: is a lifetime that can be almost absurdly long. The
• 46:48
  Speaker 1: smallest ordinary hydrogen-fusing stars are expected to remain on the
• 46:52
  Speaker 1: main sequence for trillions of years. That is far longer
• 46:56
  Speaker 1: than the universe has existed so far. The universe is
• 47:00
  Speaker 1: only about 13.8 billion years old, so the least massive
• 47:05
  Speaker 1: red dwarfs have not had enough time to complete their
• 47:09
  Speaker 1: predicted main-sequence evolution. Even the oldest examples are still alive.
• 47:15
  Speaker 1: Astronomers therefore cannot simply observe a population of ancient red
• 47:19
  Speaker 1: dwarfs that have naturally reached the end of their lives.
• 47:23
  Speaker 1: For that part of stellar evolution, they must rely heavily
• 47:26
  Speaker 1: on theoretical models. These models predict that the smallest red
• 47:31
  Speaker 1: dwarf should gradually become hotter and brighter as they use
• 47:35
  Speaker 1: up their hydrogen over immense spans of time, eventually moving
• 47:39
  Speaker 1: towards final states that no telescope can yet observe directly
• 47:44
  Speaker 1: because the universe is not aged enough to produce them.
• 47:48
  Speaker 1: A star like the Sun occupies a middle ground. It
• 47:52
  Speaker 1: is massive enough to have a relatively hot central core.
• 47:56
  Speaker 1: and a main sequence lifetime measured in billions rather than
• 48:00
  Speaker 1: trillions of years. Its internal structure includes both radiative and
• 48:04
  Speaker 1: convective regions. It will spend roughly 10 billion years fusing
• 48:09
  Speaker 1: hydrogen before its central supply becomes depleted enough to force
• 48:14
  Speaker 1: a major change in structure. And unlike a sufficiently massive star,
• 48:19
  Speaker 1: The Sun will never develop the conditions required to fuse
• 48:23
  Speaker 1: a long sequence of progressively heavier elements all the way
• 48:27
  Speaker 1: towards iron. Its mass is simply too low. At the
• 48:32
  Speaker 1: high mass end, the story becomes dramatically faster. A star
• 48:37
  Speaker 1: several times the mass of the Sun experiences far greater
• 48:41
  Speaker 1: gravitational compression. Its central temperature rises high enough for nuclear
• 48:46
  Speaker 1: reactions to proceed at a tremendous rate. These stars can
• 48:51
  Speaker 1: be tens of thousands or even hundreds of thousands of
• 48:54
  Speaker 1: times more luminous than the Sun. Their brightness is spectacular,
• 48:59
  Speaker 1: but it is purchased with time. Some of the most
• 49:02
  Speaker 1: massive stars may complete their entire lives in only a
• 49:05
  Speaker 1: few million years. A small red dwarf born at the
• 49:09
  Speaker 1: same time could still be near the beginning of its existence,
• 49:13
  Speaker 1: after the massive star has already died. This relationship between
• 49:18
  Speaker 1: mass and lifetime is one of the defining ideas of
• 49:21
  Speaker 1: stellar evolution. More mass usually means more fuel, but it
• 49:26
  Speaker 1: also means vastly faster fuel consumption. The second effect wins.
• 49:33
  Speaker 1: There is also an important transition between stars that end
• 49:37
  Speaker 1: relatively gently, and stars capable of undergoing core collapse. A
• 49:43
  Speaker 1: useful rough dividing region lies around eight times the mass
• 49:46
  Speaker 1: of the Sun at birth. Stars below this general range
• 49:50
  Speaker 1: usually do not develop cores massive and hot enough to
• 49:53
  Speaker 1: proceed through the full sequence of advanced fusion stages that
• 49:58
  Speaker 1: lead towards iron and catastrophic collapse. Instead, after giant faces
• 50:04
  Speaker 1: and extensive mass loss, they generally leave behind white dwarfs.
• 50:09
  Speaker 1: Stars born above roughly this region may eventually develop cores
• 50:13
  Speaker 1: capable of proceeding towards core collapse and, in many cases,
• 50:18
  Speaker 1: supernova explosions. But this boundary is not a perfectly sharp
• 50:23
  Speaker 1: line drawn through nature. A star born with just under
• 50:27
  Speaker 1: or just over 8 solar masses cannot have its fate
• 50:31
  Speaker 1: predicted from the number alone. Other properties matter. One of
• 50:36
  Speaker 1: them is metallicity. In astronomy, metallicity refers to the abundance
• 50:41
  Speaker 1: of elements heavier than helium. This does not mean that
• 50:45
  Speaker 1: the star is full of ordinary metal in the everyday sense.
• 50:49
  Speaker 1: It is simply the astronomical convention for describing the proportion
• 50:53
  Speaker 1: of heavier elements, such as carbon, oxygen, silicon and iron.
• 50:59
  Speaker 1: Metallicity can affect a star's opacity, meaning how easily radiation
• 51:04
  Speaker 1: travels through its interior. It can also influence stellar winds.
• 51:09
  Speaker 1: Stars with different metallicities can therefore lose different amounts of
• 51:14
  Speaker 1: mass over the course of their lives, altering what remains
• 51:17
  Speaker 1: when the star reaches its final stages. Rotation matters too.
• 51:23
  Speaker 1: A rapidly rotating star can experience additional internal mixing. Material
• 51:29
  Speaker 1: can be transported between different regions, changing how much fuel
• 51:34
  Speaker 1: reaches the core and how the chemical composition develops over time,
• 51:39
  Speaker 1: rotation can also affect the star's shape, mass loss, and
• 51:43
  Speaker 1: later evolution. For some massive stars, stellar winds become so
• 51:49
  Speaker 1: powerful that they remove a substantial fraction of the star's
• 51:53
  Speaker 1: original mass before the end of its life. A star
• 51:56
  Speaker 1: may therefore be born extremely massive but arrive at its
• 52:00
  Speaker 1: final collapse with far less material than it once possessed.
• 52:05
  Speaker 1: And then there are binary stars. A large fraction of stars,
• 52:09
  Speaker 1: particularly massive ones, belong to systems containing two or more stars.
• 52:15
  Speaker 1: Once a companion is introduced, the simple picture becomes much
• 52:19
  Speaker 1: less tidy. Two stars can exchange material. One star can
• 52:25
  Speaker 1: strip away the outer layers of another. A star can
• 52:28
  Speaker 1: gain mass and evolve differently from the way it would have,
• 52:33
  Speaker 1: in isolation. Two stars can even merge. These interactions can
• 52:38
  Speaker 1: completely change the final outcome. A star that might otherwise
• 52:42
  Speaker 1: have become a red giant with a massive envelope can
• 52:46
  Speaker 1: lose that envelope to a companion. Another star can gain
• 52:50
  Speaker 1: enough material to alter its later evolution. Compact remnants such
• 52:56
  Speaker 1: as white dwarfs can also accrete matter from companions that
• 53:00
  Speaker 1: creating entirely new possibilities that single stars do not experience.
• 53:06
  Speaker 1: So mass is not destiny in an absolute sense. It
• 53:10
  Speaker 1: is better thought of as the most important starting condition.
• 53:15
  Speaker 1: The initial mass of a star provides the clearest first
• 53:18
  Speaker 1: indication of how bright it will be, how quickly it
• 53:21
  Speaker 1: will evolve, and what kinds of nuclear reactions its core
• 53:25
  Speaker 1: may eventually support. But the final fate depends on the
• 53:30
  Speaker 1: entire history of the star. Its chemical composition matters. Its
• 53:35
  Speaker 1: rotation matters. Its stellar winds matter. Its final core mass matters.
• 53:42
  Speaker 1: And if it has a companion, that companion may interfere
• 53:46
  Speaker 1: rather enthusiastically with the original plan. Still, if only one
• 53:51
  Speaker 1: fact about a newborn star were available, mass would be
• 53:55
  Speaker 1: the most useful fact to know. From that one property,
• 54:00
  Speaker 1: astronomers can already make remarkably strong predictions. A small star
• 54:05
  Speaker 1: will probably live slowly. A sun-like star will spend billions
• 54:10
  Speaker 1: of years in relative stability, before ending as a white dwarf.
• 54:16
  Speaker 1: A sufficiently massive star may race through its nuclear fuel,
• 54:20
  Speaker 1: build heavier elements in its core, and eventually reach conditions
• 54:25
  Speaker 1: where gravity becomes overwhelming once again. The star's life began
• 54:30
  Speaker 1: with gravity gathering material together. And the amount of material
• 54:36
  Speaker 1: gravity managed to gather remains one of the strongest influences
• 54:40
  Speaker 1: over everything that follows. For billions of years, a star
• 54:45
  Speaker 1: like the Sun can remain remarkably stable. Hydrogen fusion continues
• 54:51
  Speaker 1: in the core. Helium accumulates slowly. Gravity and pressure remain
• 54:56
  Speaker 1: in balance. and from the outside the star seems almost unchanged.
• 55:02
  Speaker 1: But eventually the central supply of usable hydrogen becomes depleted.
• 55:07
  Speaker 1: This does not mean that every hydrogen atom in the
• 55:10
  Speaker 1: star has disappeared. Far from it. Large amounts of hydrogen
• 55:15
  Speaker 1: still remain in the outer regions. The important change is
• 55:19
  Speaker 1: that the central core has become rich in helium. While
• 55:23
  Speaker 1: the hydrogen concentration there has fallen too low for the
• 55:26
  Speaker 1: old pattern of sustained core fusion to continue in the
• 55:29
  Speaker 1: same way, the balance that maintained the star throughout its
• 55:33
  Speaker 1: main sequence life begins to shift. The helium core is
• 55:38
  Speaker 1: not yet hot enough to fuse helium efficiently, so for
• 55:41
  Speaker 1: a time it becomes inert. Gravity then gains ground. Without
• 55:47
  Speaker 1: the same level of energy production in the centre, the
• 55:50
  Speaker 1: helium core begins to contract under its own weight. As
• 55:54
  Speaker 1: it contracts, it heats. The temperature rises not because helium
• 56:00
  Speaker 1: fusion has begun, but because gravitational potential energy is once
• 56:05
  Speaker 1: again being converted into thermal energy. This is the same
• 56:09
  Speaker 1: basic principle that helped heat the star before hydrogen fusion
• 56:14
  Speaker 1: first started, now returning much later in its life. The
• 56:19
  Speaker 1: region just outside the helium core also becomes hotter. There,
• 56:25
  Speaker 1: hydrogen is still available. Eventually the temperature in this surrounding
• 56:30
  Speaker 1: layer becomes high enough for hydrogen fusion to continue vigorously
• 56:35
  Speaker 1: in a shell around the inert helium core. This is
• 56:39
  Speaker 1: called hydrogen shell burning. The star is no longer producing
• 56:43
  Speaker 1: most of its energy through hydrogen fusion at the very centre. Instead,
• 56:49
  Speaker 1: Hydrogen is being converted into helium in a surrounding shell.
• 56:54
  Speaker 1: As that shell burns, fresh helium is deposited onto the
• 56:58
  Speaker 1: core beneath it. The helium core therefore grows in mass.
• 57:03
  Speaker 1: At the same time, the energy output from the shell
• 57:06
  Speaker 1: becomes substantial. The outer layers of the star respond. They expand.
• 57:13
  Speaker 1: A star that once resembled the present-day Sun begins to
• 57:17
  Speaker 1: swell to many times its former size. At first, it
• 57:22
  Speaker 1: passes through a subjuant phase. This marks the transition away
• 57:26
  Speaker 1: from the main sequence as the core contracts and the
• 57:29
  Speaker 1: outer envelope begins to expand. On the Hertzsprung-Russell diagram, the
• 57:35
  Speaker 1: star moves away from the main sequence band where it
• 57:38
  Speaker 1: spent most of its life. Its luminosity changes. Its surface
• 57:43
  Speaker 1: temperature changes. Its radius begins growing dramatically. Eventually, the star
• 57:50
  Speaker 1: becomes a red giant. The name can be slightly misleading
• 57:54
  Speaker 1: if taken too literally. A red giant is not red
• 57:58
  Speaker 1: because it has somehow become cold overall. Its interior is
• 58:02
  Speaker 1: hotter in important regions than before. The redness comes from
• 58:06
  Speaker 1: the surface. As the outer envelope expands, the surface becomes cooler.
• 58:12
  Speaker 1: than it was during the main sequence. A cooler stellar
• 58:15
  Speaker 1: surface emits a greater proportion of its visible light at
• 58:19
  Speaker 1: longer wavelengths, giving the star a more orange or reddish appearance.
• 58:24
  Speaker 1: Yet the total luminosity can rise enormously. This happens because
• 58:29
  Speaker 1: the star's surface area has become vastly larger. A cooler
• 58:33
  Speaker 1: surface spread across a gigantic sphere can still produce far
• 58:38
  Speaker 1: more total energy than a hotter but much smaller star.
• 58:44
  Speaker 1: The future Sun provides the most familiar example. It is
• 58:48
  Speaker 1: expected to begin leaving the main sequence in roughly 5
• 58:51
  Speaker 1: billion years. From there, its structure will change over an
• 58:56
  Speaker 1: extended sequence of phases before it eventually becomes a red giant.
• 59:01
  Speaker 1: At its largest, the Sun is expected to expand to
• 59:05
  Speaker 1: well over 100 times its current radius. An object that
• 59:10
  Speaker 1: now has a radius of about 700,000 kilometres will grow
• 59:15
  Speaker 1: until its outer layers reach tens of millions of kilometres
• 59:19
  Speaker 1: into space. The inner solar system will be transformed. Mercury
• 59:24
  Speaker 1: is expected to be engulfed Venus is also expected to
• 59:28
  Speaker 1: be swallowed by the expanding Sun. Earth's fate is more complicated.
• 59:34
  Speaker 1: As the Sun loses mass through increasingly strong stellar winds,
• 59:39
  Speaker 1: its gravitational pull will weaken slightly and planetary orbits can
• 59:43
  Speaker 1: move outward. That might sound encouraging for Earth. Unfortunately, the
• 59:49
  Speaker 1: expanding solar atmosphere creates another effect. Tidal interactions and drag
• 59:55
  Speaker 1: can act in the opposite direction. Detailed models do not
• 59:59
  Speaker 1: reduce Earth's future to one perfectly simple outcome, although many
• 1:00:04
  Speaker 1: studies suggest that the planet is likely to be engulfed
• 1:00:07
  Speaker 1: during the Sun's giant evolution. Long before that question becomes
• 1:00:12
  Speaker 1: physically relevant, conditions on Earth will already have become completely
• 1:00:17
  Speaker 1: unsuitable for present-day life, because the Sun will gradually brighten,
• 1:00:23
  Speaker 1: even before it reaches the red giant stage. So there
• 1:00:27
  Speaker 1: is no need to adjust next week's plans. The sun's
• 1:00:30
  Speaker 1: red giant future lies billions of years away. It is
• 1:00:34
  Speaker 1: also important not to confuse a red giant like the
• 1:00:37
  Speaker 1: future sun with a red supergiant. The names sound similar,
• 1:00:42
  Speaker 1: but the stars involved can be very different. A sun-like
• 1:00:46
  Speaker 1: star becomes a red giant as it leaves the main
• 1:00:49
  Speaker 1: sequence and undergoes major structural changes around an inert helium core.
• 1:00:55
  Speaker 1: A genuinely massive star can eventually become a red supergiant,
• 1:01:00
  Speaker 1: reaching far greater sizes and following a different sequence of
• 1:01:04
  Speaker 1: advanced nuclear burning. Betelgeuse, for example, is a red supergiant.
• 1:01:10
  Speaker 1: The future sun will never become a star of that kind.
• 1:01:14
  Speaker 1: Its mass is far too low. Back inside our sun-like
• 1:01:18
  Speaker 1: red giant, the growing helium core continues to contract. Hydrogen
• 1:01:24
  Speaker 1: shell burning keeps adding more helium to it. The core
• 1:01:27
  Speaker 1: becomes denser and hotter. An unusual quantum mechanical effect also
• 1:01:33
  Speaker 1: becomes increasingly important. As the core becomes extremely dense, the
• 1:01:39
  Speaker 1: electrons within it begin providing what is known as electron
• 1:01:42
  Speaker 1: degeneracy pressure. This pressure behaves differently from ordinary gas pressure.
• 1:01:49
  Speaker 1: It does not depend on temperature in the usual way
• 1:01:51
  Speaker 1: and helps support the core even as it continues to heat.
• 1:01:55
  Speaker 1: Outside the core, the giant star can be a sawmiss. Inside, however,
• 1:02:01
  Speaker 1: the most important changes are occurring in a compact region
• 1:02:04
  Speaker 1: that is becoming steadily hotter. The temperature climbs into the
• 1:02:08
  Speaker 1: tens of millions of degrees. Eventually, it approaches around 100
• 1:02:13
  Speaker 1: million degrees. At those temperatures, helium nuclei begin to have
• 1:02:19
  Speaker 1: access to an entirely new set of nuclear reactions. Hydrogen
• 1:02:23
  Speaker 1: was the first long-lasting fuel of the star. Its depletion
• 1:02:27
  Speaker 1: did not switch the star off. Instead, it forced gravity
• 1:02:32
  Speaker 1: to compress the core until the conditions became extreme enough
• 1:02:36
  Speaker 1: for another element to take its place. The star has
• 1:02:40
  Speaker 1: grown older, larger and brighter. And deep inside the red giant,
• 1:02:46
  Speaker 1: helium is finally approaching the point where it too can
• 1:02:49
  Speaker 1: begin to fuse. Deep inside the red giant, the helium
• 1:02:53
  Speaker 1: core has been growing denser and hotter. For a long time,
• 1:02:57
  Speaker 1: helium has simply accumulated as the ash left behind by
• 1:03:01
  Speaker 1: hydrogen fusion. Now the conditions are changing. As the contracting
• 1:03:06
  Speaker 1: core approaches temperatures of around 100 million degrees, helium nuclei
• 1:03:12
  Speaker 1: begin to participate in nuclear reactions of their own. A
• 1:03:16
  Speaker 1: second major fuel has become available. The most important process
• 1:03:21
  Speaker 1: is called the triple alpha process. A helium nucleus contains
• 1:03:26
  Speaker 1: two protons and two neutrons and is also known as
• 1:03:29
  Speaker 1: an alpha particle. In the first step, two helium nuclei
• 1:03:34
  Speaker 1: collide and briefly form an unstable nucleus of beryllium. The
• 1:03:39
  Speaker 1: beryllium nucleus normally falls apart almost immediately. But if a
• 1:03:44
  Speaker 1: third helium nucleus arrives at just the right moment, the
• 1:03:47
  Speaker 1: combined system can form carbon. Three helium nuclei have in
• 1:03:52
  Speaker 1: effect been transformed into one carbon nucleus. This sounds simple
• 1:03:56
  Speaker 1: when reduced to a sentence. In reality, it depends on
• 1:04:00
  Speaker 1: an extraordinary piece of nuclear physics. The British astronomer Fred
• 1:04:05
  Speaker 1: Hoyle realised that the observed abundance of carbon in the
• 1:04:08
  Speaker 1: universe could not easily be explained unless carbon possessed a
• 1:04:12
  Speaker 1: particular excited nuclear energy state that made the triple alpha
• 1:04:16
  Speaker 1: process much more likely. In the early 1950s, Hoyle predicted
• 1:04:22
  Speaker 1: that such a state should exist. Experiments later found it.
• 1:04:27
  Speaker 1: It became known as the Hoyle state. Without this nuclear resonance,
• 1:04:32
  Speaker 1: stars would produce carbon far less efficiently. The prediction became
• 1:04:37
  Speaker 1: one of the most famous examples of astrophysical reasoning pointing
• 1:04:40
  Speaker 1: towards a previously unknown property of an atomic nucleus. The
• 1:04:45
  Speaker 1: American nuclear physicist William Fowler was central to the broader
• 1:04:49
  Speaker 1: effort to understand how nuclear reactions inside stars manufacture the
• 1:04:53
  Speaker 1: chemical elements. Through laboratory measurements, theoretical work and collaboration with
• 1:04:59
  Speaker 1: other researchers, Fowler helped establish the detailed reaction networks now
• 1:05:04
  Speaker 1: known as stellar nucleosynthesis. The simple fact that stars can
• 1:05:09
  Speaker 1: make carbon and oxygen is therefore tied to decades of
• 1:05:13
  Speaker 1: work connecting nuclear physics on Earth with events occurring inside
• 1:05:17
  Speaker 1: stellar cores. Once carbon has been produced, another important reaction
• 1:05:23
  Speaker 1: can take place. A carbon nucleus can capture another helium nucleus.
• 1:05:28
  Speaker 1: The result is oxygen. Helium burning therefore produces substantial quantities
• 1:05:33
  Speaker 1: of both carbon and oxygen, although the precise balance between
• 1:05:37
  Speaker 1: them depends on temperature, density and the rates of the
• 1:05:40
  Speaker 1: competing nuclear reactions. For a star similar in mass to
• 1:05:45
  Speaker 1: the Sun, the beginning of helium fusion is made especially
• 1:05:48
  Speaker 1: unusual by the state of the core. Before ignition, the
• 1:05:52
  Speaker 1: helium core is supported partly by electron degeneracy pressure. Electron
• 1:05:57
  Speaker 1: degeneracy pressure is a quantum mechanical effect that becomes important
• 1:06:01
  Speaker 1: when matter is squeezed to extremely high densities. Electrons belong
• 1:06:06
  Speaker 1: to a family of particles known as fermions. The Pauli
• 1:06:10
  Speaker 1: exclusion principle states that identical fermions cannot all occupy the
• 1:06:15
  Speaker 1: same quantum state. As matter is compressed, electrons are forced
• 1:06:20
  Speaker 1: into a wider range of available momentum states. This creates
• 1:06:24
  Speaker 1: a pressure that resists further compressions The important feature is
• 1:06:28
  Speaker 1: that degeneracy pressure does not depend on temperature in the
• 1:06:31
  Speaker 1: same way as ordinary gas pressure. That changes the behaviour
• 1:06:35
  Speaker 1: of the core dramatically. In normal stellar gas, an increase
• 1:06:39
  Speaker 1: in temperature usually creates an increase in pressure. The material expands.
• 1:06:45
  Speaker 1: Expansion cools the gas and helps regulate the nuclear reaction rate.
• 1:06:49
  Speaker 1: A degenerate helium core does not initially respond so conveniently.
• 1:06:54
  Speaker 1: When helium fusion begins, the temperature rises. The fusion rate
• 1:06:58
  Speaker 1: increases very rapidly with temperature. But because the pressure supporting
• 1:07:03
  Speaker 1: the core is dominated by electron degeneracy, rather than ordinary
• 1:07:08
  Speaker 1: thermal pressure, the core does not immediately expand enough to
• 1:07:12
  Speaker 1: cool itself. The reaction rate can therefore rise extremely quickly.
• 1:07:17
  Speaker 1: This event is called the helium flash. The name makes
• 1:07:21
  Speaker 1: it sound as though the entire red giant suddenly explodes.
• 1:07:25
  Speaker 1: It does not. From outside the star, there is no
• 1:07:29
  Speaker 1: spectacular blast racing across the sky. The helium flash occurs
• 1:07:34
  Speaker 1: deep inside the core. Its significance is internal. A huge
• 1:07:38
  Speaker 1: amount of energy is released over a relatively short period,
• 1:07:42
  Speaker 1: but much of that energy goes into changing the physical
• 1:07:45
  Speaker 1: state of the core rather than blowing the star apart.
• 1:07:48
  Speaker 1: As the core heats … The electrons eventually become less degenerate.
• 1:07:53
  Speaker 1: Ordinary thermal pressure becomes important again. The core can then expand.
• 1:07:58
  Speaker 1: Expansion causes cooling. The runaway behaviour ends. Helium fusion settles
• 1:08:05
  Speaker 1: into a much more stable phase. For a time, the
• 1:08:08
  Speaker 1: star regains a new kind of balance. Helium is now
• 1:08:11
  Speaker 1: being fused steadily in the core, while hydrogen can continue
• 1:08:15
  Speaker 1: burning in a shell farther out. The star is still
• 1:08:19
  Speaker 1: old compared with its main sequence youth, but it has
• 1:08:22
  Speaker 1: reached another relatively stable period. Astronomers can see populations of
• 1:08:27
  Speaker 1: stars in this stage. In many star clusters, helium-burning stars
• 1:08:32
  Speaker 1: occupy a region of the Hertzsprung-Russell diagram known as the
• 1:08:35
  Speaker 1: horizontal branch. In populations richer in heavier elements, including many
• 1:08:42
  Speaker 1: stars in the Milky Way's disk, Similar core helium burning
• 1:08:46
  Speaker 1: stars often gather into a feature called the red clump.
• 1:08:49
  Speaker 1: These names describe observational groupings rather than completely different physical principles.
• 1:08:55
  Speaker 1: In both cases, the underlying story is that helium is
• 1:08:58
  Speaker 1: being fused in the core Not every star experiences a
• 1:09:02
  Speaker 1: helium flash. More massive stars develop helium cores that do
• 1:09:07
  Speaker 1: not become strongly electron degenerate before reaching the temperature required
• 1:09:11
  Speaker 1: for fusion. In these stars, helium ignition occurs more gradually.
• 1:09:16
  Speaker 1: As the temperature rises, ordinary pressure can respond immediately. The
• 1:09:21
  Speaker 1: core expands and regulates itself without the same sudden runaway.
• 1:09:26
  Speaker 1: So the helium flash is not a universal feature of
• 1:09:29
  Speaker 1: every star. It is associated with stars in the appropriate
• 1:09:33
  Speaker 1: lower mass range, where a degenerate helium core forms before
• 1:09:37
  Speaker 1: helium fusion begins. Whatever the exact route to ignition, the
• 1:09:43
  Speaker 1: broader consequence is profound. The stars begun manufacturing carbon and
• 1:09:48
  Speaker 1: oxygen in large quantities. These elements will later become part
• 1:09:53
  Speaker 1: of the material returned to interstellar space. Carbon will become
• 1:09:57
  Speaker 1: central to the chemistry of planets and living systems. Oxygen
• 1:10:02
  Speaker 1: will become one of the most abundant heavy elements in
• 1:10:05
  Speaker 1: rocky worlds and in the wider galaxy. Neither element was
• 1:10:09
  Speaker 1: present in large quantities during the earliest moments of the universe.
• 1:10:13
  Speaker 1: Stars had to make them. A red giant quietly fusing
• 1:10:18
  Speaker 1: helium in its core is therefore participating in the gradual
• 1:10:21
  Speaker 1: chemical enrichment of the cosmos. But helium cannot support the
• 1:10:25
  Speaker 1: star for as long as hydrogen once did. The conditions
• 1:10:29
  Speaker 1: required for helium fusion are more extreme, and the available
• 1:10:33
  Speaker 1: phase is much shorter. A sun-like star can spend roughly
• 1:10:37
  Speaker 1: 10 billion years on the main sequence. Its core helium-burning
• 1:10:42
  Speaker 1: stage lasts only a small fraction of that time. on
• 1:10:45
  Speaker 1: the order of around 100 million years. Still an unimaginably
• 1:10:51
  Speaker 1: long span by human standards, but brief compared with what
• 1:10:54
  Speaker 1: came before. The rhythm of stellar evolution is beginning to accelerate.
• 1:11:00
  Speaker 1: Hydrogen sustained the star through most of its life. Helium
• 1:11:04
  Speaker 1: provides a second major source of nuclear energy, but not
• 1:11:08
  Speaker 1: for nearly as long. The core is changing composition again.
• 1:11:13
  Speaker 1: Helium is gradually disappearing. Carbon and oxygen are taking its place,
• 1:11:19
  Speaker 1: and as this second fuel is consumed, the star is
• 1:11:23
  Speaker 1: moving closer to a point where its mass will no
• 1:11:26
  Speaker 1: longer be enough to unlock another long-lasting source of nuclear energy. Eventually,
• 1:11:32
  Speaker 1: the supply of helium in the core begins to run
• 1:11:34
  Speaker 1: low as well. The star has already spent billions of
• 1:11:38
  Speaker 1: years converting hydrogen into helium, then a much shorter period
• 1:11:43
  Speaker 1: converting helium into heavier nuclei. Now the centre is composed
• 1:11:47
  Speaker 1: mainly of carbon and oxygen. For a star with roughly
• 1:11:51
  Speaker 1: the mass of the Sun, this creates another important limit.
• 1:11:55
  Speaker 1: The core can contract and heat, but not enough to
• 1:11:59
  Speaker 1: ignite carbon fusion in a sustained way. Carbon requires much
• 1:12:04
  Speaker 1: more extreme conditions than helium. A more massive star can
• 1:12:08
  Speaker 1: eventually reach those temperatures. A Sun-like star cannot. So the
• 1:12:13
  Speaker 1: chain of nuclear burning effectively stops here. The carbon and
• 1:12:17
  Speaker 1: oxygen remain as an inert core. Around that core, however,
• 1:12:22
  Speaker 1: fusion is not completely finished. Helium can continue burning in
• 1:12:26
  Speaker 1: a shell surrounding the centre. Farther out, hydrogen can still
• 1:12:30
  Speaker 1: fuse in another shell. The star enters a late stage
• 1:12:34
  Speaker 1: known as the asymptotic giant branch. During this phase, the
• 1:12:39
  Speaker 1: outer envelope expands enormously once again. The star becomes cool
• 1:12:44
  Speaker 1: at the surface, extremely luminous overall and increasingly unstable. Its
• 1:12:49
  Speaker 1: interior now has a layered structure. At the centre sits
• 1:12:54
  Speaker 1: the compact carbon-oxygen core. Around that lies a helium-burning shell.
• 1:13:00
  Speaker 1: Outside that is a hydrogen-burning shell. Beyond both of them
• 1:13:04
  Speaker 1: extends the vast outer envelope. These shells do not always
• 1:13:09
  Speaker 1: burn steadily. The helium-burning shell can become unstable in episodes
• 1:13:14
  Speaker 1: known as thermal pulses. During a thermal pulse, helium fusion
• 1:13:18
  Speaker 1: increases dramatically for a relatively short period. The energy released
• 1:13:23
  Speaker 1: can cause large changes inside the star. Layers expand. Convection strengthens.
• 1:13:30
  Speaker 1: The luminosity shifts. Then the star settles again for a
• 1:13:34
  Speaker 1: time before another pulse occurs. These repeated episodes can alter
• 1:13:39
  Speaker 1: the star's structure again and again. They also help bring
• 1:13:42
  Speaker 1: material from the interior towards the surface. This process is
• 1:13:47
  Speaker 1: known as dredge-up. Deep convection can reach into regions that
• 1:13:51
  Speaker 1: contain material altered by nuclear reactions. That material is then
• 1:13:57
  Speaker 1: carried upward and mixed into the outer envelope. As a result,
• 1:14:01
  Speaker 1: the chemical composition visible at the surface can change. Carbon,
• 1:14:06
  Speaker 1: for example, can become much more abundant in the atmosphere
• 1:14:09
  Speaker 1: of some evolved stars. In certain cases, enough carbon reaches
• 1:14:13
  Speaker 1: the surface for the star to become what astronomers call
• 1:14:16
  Speaker 1: a carbon star. The atmosphere can then contain more carbon
• 1:14:20
  Speaker 1: than oxygen. This changes the molecules that form there and
• 1:14:24
  Speaker 1: can give the star a strikingly deep red appearance. The
• 1:14:28
  Speaker 1: asymptotic giant branch also provides important conditions for making some
• 1:14:32
  Speaker 1: elements heavier than iron. One of the main processes involved
• 1:14:37
  Speaker 1: is the slow neutron capture process. It is called slow
• 1:14:41
  Speaker 1: because atomic nuclei capture neutrons at a rate slow enough
• 1:14:44
  Speaker 1: that unstable nuclei often have time to undergo radioactive decay
• 1:14:49
  Speaker 1: before another neutron arrives. Over long periods, this allows nuclei
• 1:14:54
  Speaker 1: to move gradually towards heavier elements. The slow neutron capture
• 1:14:59
  Speaker 1: process helps produce elements such as strontium, barium and lead.
• 1:15:04
  Speaker 1: So even though a sun-like star never reaches the temperatures
• 1:15:07
  Speaker 1: required to fuse carbon in its core, it can still
• 1:15:11
  Speaker 1: contribute to the production of elements heavier than iron through
• 1:15:15
  Speaker 1: a completely different mechanism. At the same time, the star
• 1:15:19
  Speaker 1: is becoming increasingly good at losing material. Its outer layers
• 1:15:24
  Speaker 1: are only weakly bound by gravity because the star has
• 1:15:28
  Speaker 1: expanded to such an enormous size. Pulsations move the atmosphere
• 1:15:33
  Speaker 1: in and out, Radiation pushes on dust grains that form
• 1:15:37
  Speaker 1: in the cool outer regions. Those dust grains collide with
• 1:15:41
  Speaker 1: surrounding gas and help drive material away from the star.
• 1:15:45
  Speaker 1: The result is a powerful stellar wind. Over time, this
• 1:15:49
  Speaker 1: wind can remove a substantial fraction of the star's mass.
• 1:15:54
  Speaker 1: Gas and dust flow outward into interstellar space. Some of
• 1:15:58
  Speaker 1: that material contains carbon and other elements that were manufactured
• 1:16:02
  Speaker 1: or redistributed during the star's life. In this way, the
• 1:16:06
  Speaker 1: star begins returning processed matter to the galaxy long before
• 1:16:10
  Speaker 1: the central remnant itself has finished evolving. The loss of
• 1:16:14
  Speaker 1: the outer envelope eventually becomes so extensive that the hot
• 1:16:18
  Speaker 1: inner core starts to emerge. Once the surrounding material becomes
• 1:16:23
  Speaker 1: thin enough, intense ultraviolet radiation from the exposed core begins
• 1:16:28
  Speaker 1: ionising the gas that was expelled earlier. That gas starts
• 1:16:32
  Speaker 1: to glow. The result is what astronomers call a planetary nebula.
• 1:16:37
  Speaker 1: The name is historical and slightly unfortunate. A planetary nebula
• 1:16:42
  Speaker 1: has nothing to do with planets. Early astronomers looking through
• 1:16:46
  Speaker 1: small telescopes sometimes saw these objects as round, softly glowing disks.
• 1:16:52
  Speaker 1: Their appearance reminded them vaguely of planets. The name remained,
• 1:16:57
  Speaker 1: even after their true nature became clear. Some of the
• 1:17:01
  Speaker 1: most beautiful examples are now familiar images in astronomy. The
• 1:17:06
  Speaker 1: Helix Nebula surrounds a dying star with wide rings and
• 1:17:09
  Speaker 1: intricate structures. The Ring Nebula appears as a bright shell
• 1:17:14
  Speaker 1: of glowing gas with a hot remnant near the centre.
• 1:17:17
  Speaker 1: The Cat's Eye Nebula contains a more complex arrangement of shells,
• 1:17:22
  Speaker 1: knots and arcs shaped by episodes of mass loss. These
• 1:17:26
  Speaker 1: structures reveal that the shedding of a star's outer layers
• 1:17:29
  Speaker 1: is not always smooth or perfectly spherical. Pulsations, magnetic fields,
• 1:17:35
  Speaker 1: stellar rotation and binary companions can all influence the shape
• 1:17:40
  Speaker 1: of the escaping material. Some planetary nebulae form rings. Others
• 1:17:45
  Speaker 1: develop lobes. Some contain multiple shells produced by separate episodes
• 1:17:50
  Speaker 1: of mass loss. The glowing gas can appear spectacular in photographs.
• 1:17:56
  Speaker 1: but the phase does not last for very long by
• 1:17:58
  Speaker 1: stellar standards. Over tens of thousands of years, the expelled
• 1:18:03
  Speaker 1: material continues expanding. Its density falls. The gas becomes too
• 1:18:09
  Speaker 1: thin to glow brightly. Eventually the nebula disperses into the
• 1:18:13
  Speaker 1: surrounding interstellar medium. The material mixes with the galaxy again.
• 1:18:19
  Speaker 1: Some of it may later become part of another molecular cloud.
• 1:18:23
  Speaker 1: Some may one day enter newly forming stars and planets.
• 1:18:28
  Speaker 1: The original giant, meanwhile, is nearly gone. Its envelope has
• 1:18:32
  Speaker 1: been stripped away. The nuclear burning shells fade. What remains
• 1:18:37
  Speaker 1: is the compact carbon-oxygen core that the star was never
• 1:18:41
  Speaker 1: massive enough to heat further. The brilliant giant has effectively
• 1:18:45
  Speaker 1: dismantled itself. Its outer layers have been returned to space.
• 1:18:49
  Speaker 1: its core has been exposed, and the object left behind
• 1:18:53
  Speaker 1: is no longer a normal star sustained by nuclear fusion.
• 1:18:57
  Speaker 1: Once the planetary nebula fades and the last of the
• 1:19:00
  Speaker 1: outer layers drift away, the exposed core remains behind. This
• 1:19:05
  Speaker 1: compact remnant is a white dwarf. For The Sun. This
• 1:19:10
  Speaker 1: is the expected final state. The Sun will never become
• 1:19:14
  Speaker 1: a neutron star and it will never collapse into a
• 1:19:17
  Speaker 1: black hole. It is simply not massive enough. After its
• 1:19:21
  Speaker 1: red giant faces and the loss of its outer envelope,
• 1:19:25
  Speaker 1: it will leave behind a white dwarf composed mainly of
• 1:19:28
  Speaker 1: carbon and oxygen. Many other stars follow the same broad fate.
• 1:19:33
  Speaker 1: A typical white dwarf can contain a large fraction of
• 1:19:36
  Speaker 1: the Sun's mass while being only about the size of Earth.
• 1:19:40
  Speaker 1: That immediately produces an extraordinary density. Matter that once occupied
• 1:19:46
  Speaker 1: a star more than a million kilometres across becomes compressed
• 1:19:50
  Speaker 1: into an object only thousands of kilometres in radius. A
• 1:19:54
  Speaker 1: spoonful of white dwarf material would weigh enormously more than
• 1:19:58
  Speaker 1: any ordinary material encountered on Earth. The object is still hot.
• 1:20:03
  Speaker 1: In fact, A newly formed white dwarf can have a
• 1:20:06
  Speaker 1: surface temperature of tens of thousands of degrees. But something
• 1:20:11
  Speaker 1: crucial has changed. There is no longer sustained nuclear fusion
• 1:20:15
  Speaker 1: providing a fresh supply of energy. The white dwarf shines
• 1:20:20
  Speaker 1: because it is still hot from its earlier life. It
• 1:20:23
  Speaker 1: is radiating stored thermal energy into space. In that sense,
• 1:20:27
  Speaker 1: the object is more like a glowing ember than an
• 1:20:30
  Speaker 1: active star. It is no longer producing large quantities of
• 1:20:34
  Speaker 1: new energy through fusion. It is simply cooling. That raises
• 1:20:39
  Speaker 1: an obvious question. If gravity once squeezed the original star
• 1:20:43
  Speaker 1: into this tiny object, what prevents the white dwarf from
• 1:20:47
  Speaker 1: collapsing even further? The answer comes from quantum mechanics. The
• 1:20:52
  Speaker 1: Austrian physicist Wolfgang Pauli introduced a principle in 1925 that
• 1:20:58
  Speaker 1: became known as the Pauli Exclusion Principle. It applies to
• 1:21:02
  Speaker 1: particles known as fermions, including electrons. The basic rule is
• 1:21:07
  Speaker 1: that identical fermions cannot all occupy exactly the same quantum state.
• 1:21:13
  Speaker 1: Inside a white dwarf, matter is compressed so strongly that
• 1:21:16
  Speaker 1: the electrons are forced into an extremely crowded quantum environment.
• 1:21:21
  Speaker 1: They cannot simply all settle into the lowest possible state.
• 1:21:25
  Speaker 1: As the density increases, electrons must occupy higher and higher
• 1:21:30
  Speaker 1: momentum states. This creates what is known as electron degeneracy pressure.
• 1:21:36
  Speaker 1: Electron degeneracy pressure resists further compression. It does not behave
• 1:21:41
  Speaker 1: like ordinary gas pressure. A normal hot gas can lose
• 1:21:45
  Speaker 1: pressure as it cools because the particles move more slowly.
• 1:21:49
  Speaker 1: Electron degeneracy pressure is different. It arises from the quantum
• 1:21:54
  Speaker 1: mechanical restrictions on how electrons are allowed to arrange themselves.
• 1:21:59
  Speaker 1: That means a white dwarf does not need to remain
• 1:22:02
  Speaker 1: extremely hot in order to support itself against gravity. It
• 1:22:07
  Speaker 1: can cool while degeneracy pressure continues to resist collapse. This
• 1:22:12
  Speaker 1: is why an isolated white dwarf can remain stable for
• 1:22:15
  Speaker 1: immensely long periods. There is, however, a limit. The Indian-American
• 1:22:21
  Speaker 1: astrophysicist Subramanian Chandrasekhar worked out that electron degeneracy pressure cannot
• 1:22:28
  Speaker 1: support a white dwarf above a certain mass. That maximum
• 1:22:32
  Speaker 1: is known as the Chandrasekhar limit. It is roughly 1.4
• 1:22:36
  Speaker 1: times the mass of the Sun, although the exact value
• 1:22:40
  Speaker 1: depends somewhat on composition and other details. Below that broad limit,
• 1:22:45
  Speaker 1: electron degeneracy pressure can support the remnant. Above it, a
• 1:22:50
  Speaker 1: white dwarf cannot remain stable in the same way. For
• 1:22:53
  Speaker 1: an isolated star like the Sun, this is not a problem.
• 1:22:57
  Speaker 1: The Sun will lose a large fraction of its mass
• 1:23:00
  Speaker 1: before the white dwarf stage, leaving a remnant comfortably below
• 1:23:03
  Speaker 1: the Chandrasekhar limit. The white dwarf can then continue cooling.
• 1:23:09
  Speaker 1: And cooling is what it will do for an almost
• 1:23:11
  Speaker 1: unimaginable length of time. Over billions of years, the surface
• 1:23:16
  Speaker 1: temperature falls, the remnant becomes dimmer, its glow gradually shifts
• 1:23:21
  Speaker 1: towards lower energies. but the process is extremely slow because
• 1:23:27
  Speaker 1: white dwarfs begin very hot and contain a great deal
• 1:23:30
  Speaker 1: of stored thermal energy. Their interiors can also undergo another
• 1:23:35
  Speaker 1: remarkable transition. As the white dwarf cools, the positively charged
• 1:23:41
  Speaker 1: atomic nuclei, or ions, in the dense interior can gradually
• 1:23:46
  Speaker 1: settle into a more ordered arrangement. They can form a
• 1:23:49
  Speaker 1: solid-like lattice, This process is called crystallisation. The word can
• 1:23:55
  Speaker 1: sound slightly surprising when applied to something that began as
• 1:23:59
  Speaker 1: a star, but under the immense pressures inside a white dwarf,
• 1:24:03
  Speaker 1: the interior can indeed develop a crystalline structure. Astronomers have
• 1:24:08
  Speaker 1: found observational evidence that white dwarf crystallisation occurs in real
• 1:24:14
  Speaker 1: stellar remnants. The European Space Agency's Gaia mission has been
• 1:24:19
  Speaker 1: especially valuable for studying large populations of nearby white dwarfs,
• 1:24:25
  Speaker 1: allowing researchers to compare their temperatures, luminosities and cooling behaviour
• 1:24:30
  Speaker 1: with theoretical models. The cooling process continues far beyond the
• 1:24:35
  Speaker 1: present age of the universe. In theory, a white dwarf
• 1:24:39
  Speaker 1: would eventually radiate away almost all of its remaining thermal energy.
• 1:24:44
  Speaker 1: The final state is sometimes called a black dwarf. A
• 1:24:48
  Speaker 1: black dwarf would be a cold, dark, stellar remnant that
• 1:24:53
  Speaker 1: no longer emits significant visible light. None are expected to
• 1:24:57
  Speaker 1: exist today. The universe is only about 13.8 billion years old.
• 1:25:03
  Speaker 1: That is nowhere near long enough for even the oldest
• 1:25:06
  Speaker 1: white dwarfs to have cooled all the way to the
• 1:25:09
  Speaker 1: black dwarf state. So black dwarfs remain theoretical objects belonging
• 1:25:15
  Speaker 1: to the extremely distant future. An isolated white dwarf therefore
• 1:25:20
  Speaker 1: has a remarkably quiet ending. It does not explode. It
• 1:25:24
  Speaker 1: does not collapse. It simply becomes cooler and dimmer. A
• 1:25:29
  Speaker 1: white dwarf with a companion star, however, can experience a
• 1:25:33
  Speaker 1: much less peaceful future. In a binary system, the white
• 1:25:37
  Speaker 1: dwarf may pull material away from its companion. This is
• 1:25:41
  Speaker 1: known as accretion. Hydrogen or helium accumulating on the white
• 1:25:45
  Speaker 1: dwarf's surface can sometimes undergo sudden nuclear burning, producing a nova.
• 1:25:52
  Speaker 1: In more extreme circumstances, the white dwarf may gain enough
• 1:25:55
  Speaker 1: mass or merge with another white dwarf. These systems can
• 1:25:59
  Speaker 1: produce thermonuclear supernova. The detailed pathways are still an active
• 1:26:04
  Speaker 1: area of research. Some explosions may involve white dwarfs that
• 1:26:08
  Speaker 1: approach the Chandrasekhar limit. Others may result from mergers between
• 1:26:13
  Speaker 1: two white dwarfs. Some models allow an explosion to begin
• 1:26:17
  Speaker 1: while the total mass is still below the Chandrasekhar limit,
• 1:26:21
  Speaker 1: so there is no single identical route that explains every
• 1:26:25
  Speaker 1: thermonuclear supernova. What matters is that these explosions are fundamentally
• 1:26:31
  Speaker 1: different from the core-collapse supernovae produced by genuinely massive stars.
• 1:26:37
  Speaker 1: A core collapse supernova occurs when the central core of
• 1:26:40
  Speaker 1: a massive star can no longer support itself and collapses
• 1:26:44
  Speaker 1: under gravity. A thermonuclear supernova involving a white dwarf is
• 1:26:50
  Speaker 1: driven by runaway nuclear burning inside degenerate matter. Both can
• 1:26:55
  Speaker 1: produce enormous explosions, but the physics leading to them is different.
• 1:27:00
  Speaker 1: For an isolated star like the Sun, none of that
• 1:27:03
  Speaker 1: is expected. Its future is much calmer. The outer layers
• 1:27:08
  Speaker 1: will drift into space, the glowing planetary nebula will fade,
• 1:27:13
  Speaker 1: and at the centre, a white dwarf will remain, small, dense,
• 1:27:19
  Speaker 1: hot at first, then gradually cooling. A star that once
• 1:27:23
  Speaker 1: illuminated a planetary system will have become a compact quantum-supported remnant,
• 1:27:29
  Speaker 1: no longer powered by fusion, but still carrying the heat
• 1:27:32
  Speaker 1: and history of everything it once was. A star born
• 1:27:36
  Speaker 1: with far more mass than the Sun follows a very
• 1:27:40
  Speaker 1: different life. The basic ingredients are the same. Gravity compresses
• 1:27:45
  Speaker 1: the star. Nuclear fusion releases energy. Pressure pushes outward, while
• 1:27:51
  Speaker 1: gravity pulls inward. But the scale changes almost everything. A
• 1:27:57
  Speaker 1: useful dividing region lies around eight times the mass of
• 1:28:01
  Speaker 1: the Sun at birth. Stars above roughly this range can...
• 1:28:05
  Speaker 1: under the right conditions, eventually develop cores capable of proceeding
• 1:28:10
  Speaker 1: towards gravitational collapse. That boundary is not perfectly sharp. The
• 1:28:15
  Speaker 1: exact outcome depends on composition, rotation, mass loss, binary interaction,
• 1:28:22
  Speaker 1: and the detailed structure of the stellar core. But as
• 1:28:26
  Speaker 1: a broad guide, a star born with many times the
• 1:28:30
  Speaker 1: mass of the Sun enters a much faster and more
• 1:28:32
  Speaker 1: extreme form of stellar evolution. The reason begins with stronger gravity.
• 1:28:39
  Speaker 1: A massive star compresses its interior more intensely than a
• 1:28:43
  Speaker 1: Sun-like star. Its central pressure rises. Its core temperature becomes
• 1:28:48
  Speaker 1: far higher. Nuclei move more rapidly, collisions become more frequent,
• 1:28:54
  Speaker 1: and nuclear reactions proceed at a much greater rate. For
• 1:28:58
  Speaker 1: a star like the Sun, Hydrogen fusion is dominated mainly
• 1:29:02
  Speaker 1: by the proton-proton chain. In a hotter massive star, the
• 1:29:07
  Speaker 1: carbon-nitrogen-oxygen cycle becomes far more important. Carbon, nitrogen and oxygen
• 1:29:13
  Speaker 1: nuclei participate in a sequence of reactions that ultimately converts
• 1:29:18
  Speaker 1: hydrogen into helium. Because the reaction rate is extremely sensitive
• 1:29:23
  Speaker 1: to temperature, the hotter cores of massive stars allow the
• 1:29:27
  Speaker 1: carbon-nitrogen-oxygen cycle to operate far more vigorously. The result is
• 1:29:33
  Speaker 1: an enormous energy output. A sufficiently massive star can shine
• 1:29:38
  Speaker 1: tens of thousands, or even hundreds of thousands, of times
• 1:29:42
  Speaker 1: more brightly than the sun. Some extreme stars can exceed
• 1:29:46
  Speaker 1: even that. Such luminosity might suggest that a star has
• 1:29:52
  Speaker 1: an enormous supply of fuel and therefore a very long future.
• 1:29:56
  Speaker 1: The opposite is usually true. Massive stars do contain more hydrogen,
• 1:30:02
  Speaker 1: but their cores consume it at a vastly faster rate.
• 1:30:06
  Speaker 1: The Sun is expected to remain on the main sequence
• 1:30:10
  Speaker 1: for roughly 10 billion years in total. A very massive
• 1:30:14
  Speaker 1: star can exhaust its core hydrogen after only a few
• 1:30:18
  Speaker 1: million years. So two stars can form from the same
• 1:30:23
  Speaker 1: large molecular cloud at approximately the same time, and have
• 1:30:27
  Speaker 1: completely different experiences. The smaller star may continue quietly fusing
• 1:30:33
  Speaker 1: hydrogen long after the massive star has already passed through
• 1:30:36
  Speaker 1: its later stages and died. More fuel does not guarantee
• 1:30:41
  Speaker 1: more time. If the rate of consumption rises even faster
• 1:30:46
  Speaker 1: than the fuel supply, the lifetime becomes shorter. Massive stars
• 1:30:52
  Speaker 1: also face conditions that are less important in lower-mass stars.
• 1:30:56
  Speaker 1: One of these is radiation pressure. Radiation carries momentum. Inside
• 1:31:03
  Speaker 1: an extremely luminous star, the enormous outward flow of radiation
• 1:31:08
  Speaker 1: can contribute significantly to the pressure supporting the star and
• 1:31:13
  Speaker 1: can help drive material away from its surface. This leads
• 1:31:18
  Speaker 1: to powerful stellar winds. These winds can remove tremendous amounts
• 1:31:23
  Speaker 1: of mass over the star's lifetime. In some cases, a
• 1:31:28
  Speaker 1: massive star can lose several times the mass of the
• 1:31:31
  Speaker 1: Sun before it ever reaches its final collapse. That lost
• 1:31:35
  Speaker 1: material is carried into the surrounding interstellar environment, enriching it
• 1:31:40
  Speaker 1: with material processed inside the star. Mass loss also changes
• 1:31:46
  Speaker 1: the star itself. A star may begin life with one
• 1:31:50
  Speaker 1: mass and reach its final stages with considerably less. This
• 1:31:56
  Speaker 1: is one reason the fate of a massive star cannot
• 1:31:59
  Speaker 1: be predicted from birth mass alone. The path is also
• 1:32:03
  Speaker 1: not as simple as every massive star becoming a red
• 1:32:06
  Speaker 1: supergiant and then exploding. Some do become red supergiants. These
• 1:32:12
  Speaker 1: stars possess enormous radii and relatively cool visible surfaces while
• 1:32:18
  Speaker 1: remaining extremely luminous. Betelgeuse is one of the best known
• 1:32:23
  Speaker 1: examples It is a red supergiant in the constellation Orion
• 1:32:28
  Speaker 1: and is expected eventually to undergo core collapse, but the
• 1:32:32
  Speaker 1: exact mass with which Betelgeuse was born is uncertain and
• 1:32:36
  Speaker 1: nobody can give a precise date for its eventual explosion.
• 1:32:40
  Speaker 1: It could happen on an astronomically short timescale, while still
• 1:32:45
  Speaker 1: being far beyond anything useful for a diary. Other massive
• 1:32:49
  Speaker 1: stars can remain hotter and appear as blue supergiants. Some
• 1:32:55
  Speaker 1: lose so much of their outer material that they become compact, hot,
• 1:33:00
  Speaker 1: stripped stars with very different surface compositions. Stellar winds can
• 1:33:06
  Speaker 1: expose layers that were once buried deep inside the star.
• 1:33:10
  Speaker 1: Binary companions can make this even more complicated. Massive stars
• 1:33:16
  Speaker 1: are very often found in binary or multiple systems. If
• 1:33:20
  Speaker 1: two stars orbit closely enough, One can begin transferring material
• 1:33:25
  Speaker 1: to the other. A companion can strip away hydrogen-rich outer layers.
• 1:33:31
  Speaker 1: It can remove much of the stellar envelope before the
• 1:33:34
  Speaker 1: core reaches collapse. The receiving star can gain mass and
• 1:33:39
  Speaker 1: have its own evolution altered. Two stars can even merge.
• 1:33:44
  Speaker 1: So the appearance of a massive star near the end
• 1:33:47
  Speaker 1: of its life may depend not only on what happened
• 1:33:50
  Speaker 1: inside it, but also on whether another star has spent
• 1:33:54
  Speaker 1: millions of years pulling pieces from it. This has important
• 1:33:58
  Speaker 1: consequences for the explosion that may eventually be seen. A
• 1:34:03
  Speaker 1: massive star that retains a large hydrogen envelope can produce
• 1:34:09
  Speaker 1: a different kind of supernova spectrum from a star whose
• 1:34:12
  Speaker 1: hydrogen has already been stripped away. If helium is also removed,
• 1:34:18
  Speaker 1: the resulting explosion can look different again. The observable supernova
• 1:34:23
  Speaker 1: therefore carries information about the star's final outer layers, not
• 1:34:28
  Speaker 1: simply the mass it possessed at birth. Beneath all of
• 1:34:32
  Speaker 1: these variations, however, one central difference remains. A massive star
• 1:34:38
  Speaker 1: can reach conditions that a sun-like star never will. When
• 1:34:43
  Speaker 1: the sun exhausts its helium, it will be left with
• 1:34:46
  Speaker 1: a carbon and oxygen core that never becomes hot enough
• 1:34:49
  Speaker 1: for sustained carbon fusion. A sufficiently massive star is not
• 1:34:54
  Speaker 1: stopped there. Its stronger gravity can compress the core to
• 1:34:58
  Speaker 1: much higher temperatures. Carbon can begin to fuse. Later, still
• 1:35:03
  Speaker 1: heavier fuels can become available. The star that lived for
• 1:35:07
  Speaker 1: much less time than the Sun is able to travel
• 1:35:11
  Speaker 1: much farther through the sequence of nuclear reactions. Its life
• 1:35:15
  Speaker 1: is shorter, but its interior becomes more extreme. Gravity keeps
• 1:35:21
  Speaker 1: forcing the core towards higher temperatures. Fusion keeps finding new
• 1:35:26
  Speaker 1: fuels for as long as the star is massive enough
• 1:35:29
  Speaker 1: to make those reactions possible. And each new stage pushes
• 1:35:34
  Speaker 1: the star closer to a point where there will eventually
• 1:35:37
  Speaker 1: be no energy-producing fuel left capable of holding gravity back.
• 1:35:42
  Speaker 1: Once a massive star has exhausted the helium in its core,
• 1:35:46
  Speaker 1: it reaches a point where a sun-like star would be
• 1:35:49
  Speaker 1: forced to stop. The centre is now rich in carbon
• 1:35:53
  Speaker 1: and oxygen. But the gravity inside a massive star is
• 1:35:57
  Speaker 1: strong enough to continue the process. With helium no longer
• 1:36:01
  Speaker 1: providing substantial energy in the core, gravity compresses the central
• 1:36:06
  Speaker 1: region again. Compression raises the temperature and density until entirely
• 1:36:11
  Speaker 1: new nuclear reactions become possible. Carbon begins to burn. This
• 1:36:16
  Speaker 1: does not resemble a flame. Carbon nuclei collide and undergo
• 1:36:21
  Speaker 1: nuclear reactions in an environment hundreds of millions of degrees hot,
• 1:36:26
  Speaker 1: producing a mixture of heavier nuclei and releasing energy. For
• 1:36:30
  Speaker 1: a time, this provides another source of pressure against gravity.
• 1:36:35
  Speaker 1: But carbon is not the last fuel available. When carbon
• 1:36:39
  Speaker 1: in the central region becomes depleted, the core contracts again.
• 1:36:43
  Speaker 1: The temperature rises further. In sufficiently massive stars, conditions become
• 1:36:49
  Speaker 1: extreme enough for reactions involving neon. After that comes oxygen burning. Eventually,
• 1:36:56
  Speaker 1: silicon burning begins under temperatures of several billion degrees. Each
• 1:37:00
  Speaker 1: stage requires more extreme conditions than the one before it.
• 1:37:04
  Speaker 1: The star has entered a remarkable rhythm. If fuel is consumed,
• 1:37:09
  Speaker 1: energy production weakens, gravity compresses the core, the temperature rises,
• 1:37:16
  Speaker 1: A heavier fuel begins reacting. For much of its life,
• 1:37:20
  Speaker 1: the star was dominated by one central process– hydrogen fusion.
• 1:37:25
  Speaker 1: Near the end, its interior becomes much more complicated. Different
• 1:37:30
  Speaker 1: nuclear reactions can occur simultaneously at different depths. The result
• 1:37:35
  Speaker 1: is often described as an onion-like structure. At the centre
• 1:37:39
  Speaker 1: lies the heaviest material– Around it are shells associated with
• 1:37:44
  Speaker 1: progressively lighter fuels. An evolved massive star may contain an
• 1:37:49
  Speaker 1: innermost region dominated by iron group nuclei, surrounded by zones
• 1:37:54
  Speaker 1: where silicon, oxygen, neon and carbon reactions have occurred or
• 1:37:59
  Speaker 1: are still occurring. Farther out lie helium-rich material and, beyond that,
• 1:38:05
  Speaker 1: hydrogen-rich layers if the star has not already lost them
• 1:38:09
  Speaker 1: through winds or interaction with a companion. The comparison with
• 1:38:15
  Speaker 1: an onion is useful because the star develops distinct layers,
• 1:38:19
  Speaker 1: but unlike an onion sitting peacefully on a kitchen worktop,
• 1:38:23
  Speaker 1: each layer can contain nuclear reactions occurring at temperatures of
• 1:38:27
  Speaker 1: millions or billions of degrees. Understanding how stars create elements
• 1:38:33
  Speaker 1: was one of the great achievements of 20th century astrophysics.
• 1:38:38
  Speaker 1: Margaret Burbage, Geoffrey Burbage, William Fowler and Fred Hoyle produced
• 1:38:44
  Speaker 1: a landmark account of stellar nucleosynthesis in the 1950s. Their
• 1:38:49
  Speaker 1: work helped establish how nuclear reactions inside stars can build
• 1:38:53
  Speaker 1: many of the elements found throughout the universe. The broad
• 1:38:57
  Speaker 1: principle is now central to astronomy. Stars do not merely shine.
• 1:39:02
  Speaker 1: They change the chemical composition of matter. Hydrogen fusion produces helium.
• 1:39:08
  Speaker 1: Helium fusion produces large quantities of carbon and oxygen. More
• 1:39:14
  Speaker 1: massive stars can continue through additional stages. building nuclei farther
• 1:39:19
  Speaker 1: along the periodic table. But something peculiar happens to the timescale.
• 1:39:24
  Speaker 1: Each new burning stage is shorter than the stages before it.
• 1:39:28
  Speaker 1: Hydrogen can sustain a massive star for millions of years.
• 1:39:33
  Speaker 1: Helium burning is much shorter. Carbon burning in a massive
• 1:39:37
  Speaker 1: star may last only centuries, although the exact duration depends
• 1:39:41
  Speaker 1: strongly on the star's mass and internal structure. The later
• 1:39:45
  Speaker 1: stages become faster still. Neon burning may proceed over something
• 1:39:50
  Speaker 1: closer to years. Oxygen burning can be shorter. Silicon burning
• 1:39:55
  Speaker 1: near the end of the process may last only days
• 1:39:58
  Speaker 1: in some models. There is no single timetable that applies
• 1:40:02
  Speaker 1: to every massive star, but the trend is clear. The
• 1:40:06
  Speaker 1: closer the star gets to the end, the faster its
• 1:40:09
  Speaker 1: internal evolution becomes. One reason is that progressively heavier fuels
• 1:40:14
  Speaker 1: release less usable energy per unit mass as the reactions
• 1:40:19
  Speaker 1: move towards the most tightly bound atomic nuclei. Another is
• 1:40:23
  Speaker 1: that the core becomes so hot that it loses enormous
• 1:40:27
  Speaker 1: amounts of energy through neutrinos. Neutrinos are extraordinarily light, electrically
• 1:40:34
  Speaker 1: neutral particles that interact only weakly with ordinary matter. Earlier
• 1:40:39
  Speaker 1: in stellar life, Photons play a major role in carrying
• 1:40:43
  Speaker 1: energy outward through the star. In these advanced stages, neutrinos
• 1:40:48
  Speaker 1: become an extremely effective way for energy to escape directly.
• 1:40:53
  Speaker 1: From the core, because they interact so weakly, they can
• 1:40:57
  Speaker 1: pass through the surrounding stellar material with remarkable ease. The
• 1:41:02
  Speaker 1: hotter the core becomes, the more important neutrino cooling can become.
• 1:41:07
  Speaker 1: Energy disappears from the central region rapidly. Gravity responds by
• 1:41:12
  Speaker 1: driving further contraction. That contraction raises the temperature and accelerates
• 1:41:17
  Speaker 1: the star's evolution even more. Eventually, silicon burning transforms material
• 1:41:23
  Speaker 1: into nuclei near the iron group. This is where the
• 1:41:26
  Speaker 1: long sequence of energy-producing fusion reactions reaches a fundamental boundary.
• 1:41:32
  Speaker 1: Iron group nuclei are among the most tightly bound nuclei
• 1:41:36
  Speaker 1: in nature. Fusing light nuclei up towards this region can
• 1:41:40
  Speaker 1: release energy, because the resulting nuclei are more tightly bound.
• 1:41:45
  Speaker 1: Once the core is dominated by iron group material, continuing
• 1:41:50
  Speaker 1: to fuse nuclei into substantially heavier elements no longer provides
• 1:41:54
  Speaker 1: the same source of energy. Instead of releasing the energy
• 1:41:58
  Speaker 1: needed to help support the star, such reactions require energy.
• 1:42:03
  Speaker 1: The star has reached a nuclear dead end. it is
• 1:42:06
  Speaker 1: worth being careful with the familiar phrase iron core. The
• 1:42:10
  Speaker 1: centre of the star is not a cold, solid ball
• 1:42:13
  Speaker 1: of metallic iron like an enormous cannonball hidden inside the star.
• 1:42:19
  Speaker 1: The temperatures are far too high for anything resembling ordinary
• 1:42:22
  Speaker 1: solid metal. The core contains an extremely hot and dense
• 1:42:27
  Speaker 1: mixture of iron group nuclei, electrons and other particles under
• 1:42:32
  Speaker 1: conditions that have no everyday equivalent. Calling it an iron
• 1:42:36
  Speaker 1: core is a useful shorthand for its nuclear composition. It
• 1:42:41
  Speaker 1: is not a description of its familiar material state. The
• 1:42:45
  Speaker 1: story of where the heavier elements come from also becomes
• 1:42:48
  Speaker 1: more complicated here. Not every element heavier than iron is
• 1:42:52
  Speaker 1: simply manufactured by ordinary fusion inside a massive star. Some
• 1:42:57
  Speaker 1: heavy nuclei are produced through the slow neutron capture process
• 1:43:01
  Speaker 1: inside evolved stars. Other elements require much more neutron-rich or
• 1:43:06
  Speaker 1: explosive conditions. Rapid neutron capture can occur in extreme environments,
• 1:43:12
  Speaker 1: with neutron star mergers now known to be an important
• 1:43:15
  Speaker 1: site for producing heavy elements such as gold and platinum.
• 1:43:19
  Speaker 1: Explosive nucleosynthesis during supernovae can also create and alter a
• 1:43:23
  Speaker 1: wide range of nuclei. So the periodic table is the
• 1:43:27
  Speaker 1: result of several astrophysical processes rather than one simple production
• 1:43:31
  Speaker 1: line hidden inside a single kind of star. Inside the
• 1:43:35
  Speaker 1: massive star, however, the immediate problem has become very simple.
• 1:43:40
  Speaker 1: Gravity has not stopped. It continues pressing inward on the core.
• 1:43:45
  Speaker 1: For millions of years, every major contraction could eventually be
• 1:43:49
  Speaker 1: answered by another nuclear fuel. Hydrogen gave way to helium.
• 1:43:54
  Speaker 1: Helium gave way to carbon. Then came increasingly advanced reactions
• 1:44:00
  Speaker 1: involving neon, oxygen and silicon. Each time, the star found
• 1:44:05
  Speaker 1: another temporary source of energy. Now that sequence has effectively ended,
• 1:44:10
  Speaker 1: the centre has become dominated by iron group nuclei. There
• 1:44:14
  Speaker 1: is no new energy-producing fusion stage waiting beyond them that
• 1:44:18
  Speaker 1: can maintain the same balance. The outer star may still
• 1:44:22
  Speaker 1: look enormous. It may still shine brilliantly. Different shells may
• 1:44:27
  Speaker 1: still be carrying out nuclear reactions. But deep inside, the
• 1:44:31
  Speaker 1: central problem has changed completely. Gravity is still pulling inward,
• 1:44:36
  Speaker 1: and for the first time in the massive star's life,
• 1:44:39
  Speaker 1: there is no heavier nuclear fuel capable of providing the
• 1:44:42
  Speaker 1: energy needed to hold it back. At the centre of
• 1:44:46
  Speaker 1: the massive star, the iron-rich core continues to grow. Around it,
• 1:44:51
  Speaker 1: shells of silicon and other elements are still undergoing nuclear reactions,
• 1:44:55
  Speaker 1: steadily adding more iron group material to the centre. For
• 1:44:59
  Speaker 1: a while, the core remained supported, but there is no
• 1:45:02
  Speaker 1: new fuel waiting inside the iron-rich centre that can release
• 1:45:06
  Speaker 1: energy through ordinary fusion. The long sequence that began with
• 1:45:11
  Speaker 1: hydrogen has reached its limit. Gravity has not. As more
• 1:45:16
  Speaker 1: material is added, the core becomes increasingly massive, dense and
• 1:45:21
  Speaker 1: difficult to support. Thermal pressure still contributes, and quantum mechanical
• 1:45:27
  Speaker 1: effects involving electrons also resist compression. but the balance is
• 1:45:32
  Speaker 1: becoming fragile. Eventually the core approaches conditions where several processes
• 1:45:38
  Speaker 1: begin working against its remaining support at almost the same time.
• 1:45:42
  Speaker 1: One of the most important is electron capture. At the
• 1:45:46
  Speaker 1: extraordinary densities inside the core, electrons are packed closely together
• 1:45:50
  Speaker 1: with atomic nuclei. Under these conditions, an electron can combine
• 1:45:55
  Speaker 1: with a proton. The result is a neutron and a neutrino.
• 1:45:59
  Speaker 1: This process changes the composition of the core. More importantly
• 1:46:04
  Speaker 1: for the immediate stability of the star it reduces the
• 1:46:07
  Speaker 1: number of free electrons available to provide electron degeneracy pressure.
• 1:46:12
  Speaker 1: That pressure had been helping resist gravity. As electron capture
• 1:46:17
  Speaker 1: continues some of that support disappears. The core becomes even
• 1:46:21
  Speaker 1: more vulnerable to collapse. At the same time the temperature
• 1:46:25
  Speaker 1: has become so extreme that energetic photons begin interacting destructively
• 1:46:30
  Speaker 1: with heavy nuclei. This process is called photodisintegration. Instead of
• 1:46:36
  Speaker 1: helping build heavier nuclei and releasing energy, energetic photons can
• 1:46:41
  Speaker 1: break ion group nuclei apart into lighter components. Breaking those
• 1:46:46
  Speaker 1: nuclei apart requires energy, so energy that might otherwise have
• 1:46:50
  Speaker 1: contributed to pressure within the core is consumed. The star
• 1:46:54
  Speaker 1: is now losing support through more than one route. Electron
• 1:46:58
  Speaker 1: capture reduces electron pressure. Photodisintegration absorbs thermal energy. Gravity continues
• 1:47:06
  Speaker 1: pulling inward. The situation begins to run away. A small
• 1:47:11
  Speaker 1: amount of contraction increases the density and temperature. That encourages
• 1:47:16
  Speaker 1: more electron capture and further changes in the nuclear material.
• 1:47:21
  Speaker 1: The support weakens even more. then the core can no
• 1:47:25
  Speaker 1: longer remain in anything resembling hydrostatic equilibrium. It collapses. The
• 1:47:32
  Speaker 1: timescale is astonishing. A region that had taken millions of
• 1:47:36
  Speaker 1: years to develop can change fundamentally in a fraction of
• 1:47:40
  Speaker 1: a second. Material begins falling inward at enormous speeds. The
• 1:47:45
  Speaker 1: central core, initially extending across thousands of kilometres, contracts towards
• 1:47:51
  Speaker 1: a region only tens of kilometres across. The collapse becomes
• 1:47:55
  Speaker 1: one of the fastest large-scale transformations that occurs in stellar evolution.
• 1:48:00
  Speaker 1: Density rises by extraordinary amounts. Matter is squeezed beyond anything
• 1:48:05
  Speaker 1: found in ordinary stars, planets or laboratories on Earth. Eventually
• 1:48:11
  Speaker 1: the innermost part of the collapsing core reaches densities comparable
• 1:48:14
  Speaker 1: to those inside atomic nuclei. At this point, the behaviour
• 1:48:19
  Speaker 1: of matter changes dramatically. The nuclear material becomes extremely difficult
• 1:48:25
  Speaker 1: to compress further. The innermost core stiffens. Its rapid collapse
• 1:48:30
  Speaker 1: is suddenly halted. The surrounding layers, however, are still falling inward.
• 1:48:35
  Speaker 1: They crash into the newly stiffened inner region. This sudden
• 1:48:39
  Speaker 1: deceleration launches a shockwave moving outward through the collapsing material.
• 1:48:45
  Speaker 1: It would be tempting to imagine that the problem is
• 1:48:47
  Speaker 1: now solved. The shock forms. It races outward. The star explodes.
• 1:48:54
  Speaker 1: For many years, simplified descriptions of supernovae often made the
• 1:48:58
  Speaker 1: process sound almost this straightforward. Modern calculations show that the
• 1:49:03
  Speaker 1: reality is more complicated. The initial shock loses energy. As
• 1:49:08
  Speaker 1: it moves through the infalling material, it must break apart
• 1:49:11
  Speaker 1: heavy nuclei. That costs energy. It also has to fight
• 1:49:16
  Speaker 1: against enormous amounts of matter that are still falling inward.
• 1:49:20
  Speaker 1: As a result, the shock can stall. Instead of immediately
• 1:49:23
  Speaker 1: blasting through the entire star, it can become trapped at
• 1:49:27
  Speaker 1: a distance of only a few hundred kilometres from the centre.
• 1:49:31
  Speaker 1: For a successful core collapse supernova, something must help revive it.
• 1:49:36
  Speaker 1: A major part of the answer appears to involve neutrinos.
• 1:49:40
  Speaker 1: The formation of the dense central remnant releases an almost
• 1:49:44
  Speaker 1: unimaginable amount of gravitational energy. Most of that energy does
• 1:49:49
  Speaker 1: not emerge as visible light. It leaves as neutrinos. The
• 1:49:53
  Speaker 1: newly forming compact core becomes an object known as a
• 1:49:57
  Speaker 1: protoneutron star. Temperatures inside it reach tens of billions of degrees.
• 1:50:03
  Speaker 1: Enormous numbers of neutrinos are produced. Although neutrinos normally pass
• 1:50:08
  Speaker 1: through matter with extraordinary ease, the densities inside the collapsing
• 1:50:12
  Speaker 1: core are so extreme that even they interact frequently enough
• 1:50:15
  Speaker 1: to become temporarily trapped. Over the following seconds, they diffuse
• 1:50:20
  Speaker 1: outward and escape. The total energy carried away by neutrinos
• 1:50:24
  Speaker 1: is vastly greater than the energy that eventually appears as
• 1:50:28
  Speaker 1: visible light from the supernova. only a small fraction of
• 1:50:32
  Speaker 1: that neutrino energy needs to be transferred back into the
• 1:50:35
  Speaker 1: material behind the stalled shock to have a dramatic effect.
• 1:50:40
  Speaker 1: Hans Bett, whose earlier work helped explain the nuclear reactions
• 1:50:44
  Speaker 1: that power ordinary stars, also contributed significantly to 20th century
• 1:50:49
  Speaker 1: theories of supernova explosions. One important idea developed through work
• 1:50:55
  Speaker 1: by Bethe and others is the neutrino heating mechanism. Some
• 1:51:00
  Speaker 1: of the immense neutrino flow from the protoneutron star deposits
• 1:51:04
  Speaker 1: energy and material behind the stalled shock. This heating can
• 1:51:08
  Speaker 1: help the shock begin moving outward again. But neutrinos are
• 1:51:12
  Speaker 1: only part of the story. The collapsing star is not
• 1:51:15
  Speaker 1: a perfectly spherical collection of matter behaving in one dimension.
• 1:51:20
  Speaker 1: Real stellar material flows. It overturns. It develops convection. Turbulence
• 1:51:26
  Speaker 1: creates irregular structures. Material can move upward in some regions
• 1:51:31
  Speaker 1: and downward in others. Modern computer simulations show that these
• 1:51:36
  Speaker 1: multi-dimensional motions can help keep matter in the region, where
• 1:51:40
  Speaker 1: neutrino heating is effective for longer periods. They can also
• 1:51:44
  Speaker 1: distort the stalled shock and contribute to conditions that allow
• 1:51:48
  Speaker 1: the explosion to develop. There are additional instabilities and fluid
• 1:51:53
  Speaker 1: motions that researchers continue to study in detail. So although
• 1:51:57
  Speaker 1: the broad physical picture of core collapse is well established,
• 1:52:01
  Speaker 1: the precise mechanism by which every successful explosion develops remains
• 1:52:07
  Speaker 1: an active field of computational astrophysics. Researchers simulate collapsing stars
• 1:52:13
  Speaker 1: in three dimensions, include increasingly detailed neutrino transport and nuclear physics,
• 1:52:20
  Speaker 1: and compare their results with real supernova observations. One of
• 1:52:25
  Speaker 1: the most important real events occurred in 1987. A massive
• 1:52:29
  Speaker 1: star in the Large Magellanic Cloud exploded as Supernova 1987A.
• 1:52:36
  Speaker 1: Before the visible supernova reached its peak brightness, detectors on
• 1:52:40
  Speaker 1: Earth recorded a brief burst of neutrinos. Kamiokande 2 in
• 1:52:45
  Speaker 1: Japan detected neutrinos. The Irvine, Michigan Brookhaven detector in the
• 1:52:49
  Speaker 1: United States recorded them as well. The Baxar Neutrino Observatory
• 1:52:54
  Speaker 1: in the Soviet Union also detected events associated with the burst.
• 1:52:58
  Speaker 1: Only a few dozen neutrinos were recorded in total. That
• 1:53:02
  Speaker 1: may not sound like many, but the fact that they
• 1:53:06
  Speaker 1: were detected at all was extraordinary. The explosion occurred roughly 168,000
• 1:53:11
  Speaker 1: light-years away. Neutrinos interact so weakly with matter that detecting
• 1:53:18
  Speaker 1: even this small number from another galaxy provided powerful evidence
• 1:53:22
  Speaker 1: that an enormous neutrino release accompanies stellar core collapse. Those
• 1:53:28
  Speaker 1: detections gave astronomers a direct glimpse into the otherwise hidden
• 1:53:32
  Speaker 1: centre of a dying massive star. The surface of the
• 1:53:37
  Speaker 1: star had not yet revealed the full explosion, but the
• 1:53:40
  Speaker 1: core had already collapsed. Deep inside, the central region had
• 1:53:44
  Speaker 1: been crushed to nuclear densities. A proton-neutron star had formed.
• 1:53:50
  Speaker 1: A vast pulse of neutrinos had escaped, and the shock,
• 1:53:55
  Speaker 1: after struggling against the infalling material, was beginning the process
• 1:53:59
  Speaker 1: that could tear the outer star apart. The shock that
• 1:54:03
  Speaker 1: formed deep inside the collapsing star now begins moving outward again.
• 1:54:08
  Speaker 1: For a brief time, the star contains two very different
• 1:54:12
  Speaker 1: motions at once. Material in the outer regions is still
• 1:54:16
  Speaker 1: falling inward, pulled towards the collapsed core by gravity. At
• 1:54:21
  Speaker 1: the same time, the revived shock is pushing outward through
• 1:54:24
  Speaker 1: those layers with enormous force. As the shock gains ground,
• 1:54:29
  Speaker 1: the balance changes. The inward fall is reversed. Layer after
• 1:54:34
  Speaker 1: layer of the star is heated, compressed and accelerated away
• 1:54:38
  Speaker 1: from the centre. The massive star begins to tear itself apart.
• 1:54:43
  Speaker 1: This is the beginning of a core collapse supernova. The
• 1:54:47
  Speaker 1: energy involved is difficult to picture. A typical supernova can
• 1:54:51
  Speaker 1: place roughly 10 to the power of 44 joules of
• 1:54:55
  Speaker 1: kinetic energy into the expanding stellar debris. That is the
• 1:55:00
  Speaker 1: energy carried by the material rushing outward. Even this staggering
• 1:55:04
  Speaker 1: amount represents only a fraction of the total energy released
• 1:55:08
  Speaker 1: when the core collapses. Most of the gravitational energy escapes
• 1:55:12
  Speaker 1: in the form of neutrinos. The visible explosion is therefore
• 1:55:16
  Speaker 1: only one part of a much larger event. By the
• 1:55:19
  Speaker 1: time the shock reaches the outer surface of the star,
• 1:55:22
  Speaker 1: the star can brighten enormously. For a period of days
• 1:55:27
  Speaker 1: or weeks, a single exploding star can rival the light
• 1:55:31
  Speaker 1: output of an entire galaxy containing billions of ordinary stars.
• 1:55:37
  Speaker 1: That does not mean the supernova contains the long-term energy
• 1:55:41
  Speaker 1: output of a galaxy. It means an enormous amount of
• 1:55:44
  Speaker 1: energy is being released and redistributed over a very short period.
• 1:55:50
  Speaker 1: Astronomers did not always recognise supernovae as a separate category
• 1:55:54
  Speaker 1: of event. In the early 20th century, stars that suddenly
• 1:55:58
  Speaker 1: became brighter were often grouped under the broad idea of novae.
• 1:56:03
  Speaker 1: The astronomers Walter Baerde and Fritz Zwicky helped establish the
• 1:56:07
  Speaker 1: distinction between ordinary novae and the far more powerful explosions
• 1:56:12
  Speaker 1: now called supernovae. During the 1930s, they also proposed an
• 1:56:17
  Speaker 1: extraordinary idea– A supernova, they suggested, could mark the transformation
• 1:56:23
  Speaker 1: of an ordinary stellar core into a neutron star. That
• 1:56:27
  Speaker 1: proposal was remarkably close to the physical picture developed by
• 1:56:31
  Speaker 1: modern astrophysics. The light from a core collapse supernova does
• 1:56:36
  Speaker 1: not come from one simple source. The initial shock heats
• 1:56:40
  Speaker 1: the stellar material dramatically. As the expanding debris cools, other
• 1:56:45
  Speaker 1: energy sources become important. One of the most important involves
• 1:56:50
  Speaker 1: radioactive nuclei created during the explosion. Nickel is produced in
• 1:56:55
  Speaker 1: large quantities under the extreme temperatures and densities associated with
• 1:56:59
  Speaker 1: the blast. A particularly important isotope of nickel is radioactive.
• 1:57:04
  Speaker 1: It decays into cobalt. The cobalt then decays towards iron.
• 1:57:09
  Speaker 1: These radioactive decays release energy into the expanding debris and
• 1:57:14
  Speaker 1: help power the changing brightness of many supernovae long after
• 1:57:19
  Speaker 1: the initial shock has passed through the star. By carefully
• 1:57:23
  Speaker 1: measuring how the light from a supernova rises and fades,
• 1:57:26
  Speaker 1: astronomers can learn about the amount of radioactive material produced
• 1:57:30
  Speaker 1: and the structure of the explosion. Not every core collapse
• 1:57:34
  Speaker 1: supernova looks the same. A great deal depends on what
• 1:57:38
  Speaker 1: remained around the core at the moment of collapse. Some
• 1:57:41
  Speaker 1: massive stars still possess large hydrogen-rich envelopes. When these stars explode,
• 1:57:47
  Speaker 1: hydrogen produces strong signatures in the light that astronomers analyse.
• 1:57:52
  Speaker 1: Other massive stars have lost much or all of their
• 1:57:55
  Speaker 1: hydrogen before the core ever collapses. Strong stellar winds can
• 1:58:00
  Speaker 1: strip material away over millions of years. A nearby companion
• 1:58:04
  Speaker 1: star can do the same thing much more efficiently by
• 1:58:08
  Speaker 1: pulling material away through gravity. interaction. If most of the
• 1:58:12
  Speaker 1: hydrogen envelope has gone, the resulting supernova looks different. If
• 1:58:17
  Speaker 1: helium has also been stripped away, it can look different again.
• 1:58:21
  Speaker 1: The explosion itself is therefore shaped by the entire history
• 1:58:25
  Speaker 1: of the star. The spectrum observed after the blast does
• 1:58:29
  Speaker 1: not simply reveal what happened in the final second. It
• 1:58:33
  Speaker 1: can preserve evidence of processes that had been reshaping the
• 1:58:36
  Speaker 1: star for millions of years. Once the explosion is underway,
• 1:58:41
  Speaker 1: stellar material expands into space at thousands of kilometres per second.
• 1:58:47
  Speaker 1: The star is no longer a coherent object. Its outer
• 1:58:51
  Speaker 1: layers become a rapidly growing cloud of hot gas, radioactive material,
• 1:58:56
  Speaker 1: and newly synthesised nuclei. Over time, this ejector collides with
• 1:59:02
  Speaker 1: the surrounding interstellar medium. The interaction creates shockwaves that heat
• 1:59:07
  Speaker 1: gas to enormous temperatures and cause the expanding debris to
• 1:59:11
  Speaker 1: glow across many parts of the electromagnetic spectrum. After years,
• 1:59:17
  Speaker 1: centuries and even thousands of years, the result is known
• 1:59:21
  Speaker 1: as a supernova remnant, The Crab Nebula is one of
• 1:59:25
  Speaker 1: the most famous examples. It is the remains of a
• 1:59:29
  Speaker 1: supernova observed from Earth in the year 1054. Today, the
• 1:59:35
  Speaker 1: expanding cloud contains filaments of gas surrounding a rapidly rotating
• 1:59:39
  Speaker 1: neutron star. Another important example is Cassiopeia A. It is
• 1:59:45
  Speaker 1: one of the youngest known supernova remnants in the Milky Way.
• 1:59:49
  Speaker 1: and provides astronomers with an unusually detailed view of the
• 1:59:52
  Speaker 1: aftermath of a massive stellar explosion. Different telescopes reveal different
• 1:59:58
  Speaker 1: parts of these remnants. The Chandra X-ray Observatory detects extremely
• 2:00:03
  Speaker 1: hot gas and energetic structures that shine strongly in X-rays.
• 2:00:08
  Speaker 1: The Hubble Space Telescope can resolve fine optical filaments in
• 2:00:12
  Speaker 1: expanding debris. The James Webb Space Telescope studies infrared emission
• 2:00:17
  Speaker 1: from dust, molecules and material heated by the expanding shock. Together,
• 2:00:23
  Speaker 1: these observatories allow astronomers to reconstruct how the explosion unfolded
• 2:00:28
  Speaker 1: and which elements are distributed through the remnant. Supernovae are
• 2:00:33
  Speaker 1: deeply connected to the chemical evolution of galaxies. Some new
• 2:00:37
  Speaker 1: nuclei are produced during the extreme conditions of the explosion itself.
• 2:00:42
  Speaker 1: Equally important, The blast carries outward elements that were manufactured
• 2:00:46
  Speaker 1: much earlier inside the star. Oxygen created during earlier stages
• 2:00:52
  Speaker 1: of stellar fusion can be thrown into interstellar space. Silicon, sulphur,
• 2:00:57
  Speaker 1: calcium and iron group material can be dispersed as well.
• 2:01:02
  Speaker 1: The explosion takes material that had been locked deep inside
• 2:01:05
  Speaker 1: a star and spreads it across light years. Eventually some
• 2:01:09
  Speaker 1: of that material may become incorporated into future molecular clouds.
• 2:01:14
  Speaker 1: Later generations of stars and planets can then form from
• 2:01:18
  Speaker 1: gas already enriched by stars that lived and died before them.
• 2:01:22
  Speaker 1: There is, however, an old simplification that needs a little care.
• 2:01:26
  Speaker 1: It is sometimes said that every element heavier than iron
• 2:01:30
  Speaker 1: was created in a supernova. That is not correct. The
• 2:01:36
  Speaker 1: origins of the heavy elements involve several different processes. The
• 2:01:40
  Speaker 1: slow neutron capture process inside Evolve with giant stars can
• 2:01:45
  Speaker 1: produce a range of nuclei heavier than iron over long periods.
• 2:01:49
  Speaker 1: Rapid neutron capture requires much more extreme neutron-rich conditions. Neutron-star
• 2:01:56
  Speaker 1: mergers are now known to be important sites for this
• 2:01:59
  Speaker 1: rapid neutron capture process and can produce very heavy elements,
• 2:02:05
  Speaker 1: including large quantities of elements such as gold and platinum.
• 2:02:09
  Speaker 1: Supernovae also contribute to heavy element production. but they are
• 2:02:13
  Speaker 1: one part of a broader story. The phrase that we
• 2:02:17
  Speaker 1: are made of star material is therefore true in spirit,
• 2:02:20
  Speaker 1: but nature has used more than one kind of stellar
• 2:02:23
  Speaker 1: event to assemble the periodic table around us. As the
• 2:02:27
  Speaker 1: expanding supernova debris races outward, very little of the original
• 2:02:32
  Speaker 1: star remains where it once stood. Most of the envelope
• 2:02:36
  Speaker 1: has been launched into space. The familiar shape of the
• 2:02:39
  Speaker 1: star is gone, but the core may survive. At the centre,
• 2:02:44
  Speaker 1: hidden behind the expanding debris, matter has been compressed to
• 2:02:48
  Speaker 1: conditions far beyond anything found in an ordinary star. If
• 2:02:53
  Speaker 1: the collapsed core has the right mass, it can stabilise
• 2:02:57
  Speaker 1: as a neutron star. If it is too massive to
• 2:03:01
  Speaker 1: be supported even in that extraordinary state, collapse can continue
• 2:03:05
  Speaker 1: towards a black hole. So the supernova is both an
• 2:03:09
  Speaker 1: ending and a separation. the outer star is scattered back
• 2:03:13
  Speaker 1: into the galaxy, the inner core becomes something entirely different.
• 2:03:18
  Speaker 1: After millions of years of shining, the massive star has
• 2:03:22
  Speaker 1: finally destroyed its own familiar form, but the material it
• 2:03:27
  Speaker 1: created is now moving outward into space, while at the centre,
• 2:03:32
  Speaker 1: an extraordinarily compact remnant remains. If the collapsed core left
• 2:03:37
  Speaker 1: behind by a massive star is not too heavy, Gravity
• 2:03:41
  Speaker 1: can be stopped again. But the object that remains bears
• 2:03:45
  Speaker 1: almost no resemblance to the star that existed before the explosion.
• 2:03:50
  Speaker 1: It becomes a neutron star. A neutron star can contain
• 2:03:55
  Speaker 1: roughly one or two times the mass of the Sun
• 2:03:57
  Speaker 1: packed into a sphere only around 20 to 30 kilometres across.
• 2:04:02
  Speaker 1: That is roughly the size of a city. The result
• 2:04:05
  Speaker 1: is matter compressed to extraordinary densities. A volume of neutron
• 2:04:11
  Speaker 1: star material small enough to fit into an ordinary kitchen
• 2:04:14
  Speaker 1: would contain an almost unimaginable amount of mass. This makes
• 2:04:20
  Speaker 1: neutron stars some of the densest naturally occurring objects known
• 2:04:24
  Speaker 1: in the universe. It is common to describe one as
• 2:04:27
  Speaker 1: something like a gigantic atomic nucleus. That comparison is useful,
• 2:04:33
  Speaker 1: but only up to a point. A neutron star is
• 2:04:37
  Speaker 1: not simply an enormous ball made from ordinary neutrons packed
• 2:04:41
  Speaker 1: together in one uniform state. Its interior is expected to
• 2:04:45
  Speaker 1: contain several distinct layers, and the behaviour of matter changes
• 2:04:50
  Speaker 1: enormously as the pressure increases towards the centre. The outermost
• 2:04:55
  Speaker 1: solid region is known as the crust. Near the surface,
• 2:04:59
  Speaker 1: atomic nuclei are arranged within a lattice, while electrons move
• 2:05:03
  Speaker 1: around them. As the density increases deeper inside, the nuclei
• 2:05:08
  Speaker 1: become increasingly rich in neutrons. Eventually conditions become so extreme
• 2:05:14
  Speaker 1: that some neutrons are no longer bound inside individual nuclei.
• 2:05:18
  Speaker 1: They begin to form a neutron-rich fluid surrounding the remaining
• 2:05:22
  Speaker 1: nuclear structures. This deeper region is known as the inner crust.
• 2:05:27
  Speaker 1: The arrangement of matter there may become extremely unusual. Under
• 2:05:31
  Speaker 1: the competition between the nuclear force and electrical repulsion, nuclei
• 2:05:36
  Speaker 1: may form elongated rods, flat sheets and other complicated structures.
• 2:05:42
  Speaker 1: These predicted arrangements are sometimes collectively called nuclear pasta. The
• 2:05:48
  Speaker 1: name is unusually cheerful for matter sitting inside an object
• 2:05:51
  Speaker 1: with roughly the mass of the sun compressed into the
• 2:05:54
  Speaker 1: size of a city, but the comparison reflects the shapes
• 2:05:58
  Speaker 1: predicted by some models. deeper still lies the core. Here,
• 2:06:04
  Speaker 1: the density rises beyond that found in ordinary atomic nuclei.
• 2:06:09
  Speaker 1: Exactly what happens to matter under these conditions remains uncertain.
• 2:06:14
  Speaker 1: Neutrons and protons are certainly important, but physicists also investigate
• 2:06:19
  Speaker 1: whether other particles or more exotic states of matter appear
• 2:06:24
  Speaker 1: at the highest densities. The relationship between pressure and density
• 2:06:29
  Speaker 1: inside a neutron star is described by what physicists call
• 2:06:33
  Speaker 1: the equation of state. Determining the correct equation of state
• 2:06:37
  Speaker 1: for ultra-dense matter is one of the major problems in
• 2:06:41
  Speaker 1: modern nuclear astrophysics. It matters because the behaviour of that
• 2:06:45
  Speaker 1: matter determines how large a neutron star can be for
• 2:06:50
  Speaker 1: a given mass and how much mass it can support
• 2:06:54
  Speaker 1: before gravity wins again. The theoretical foundations for understanding this
• 2:06:59
  Speaker 1: limit were developed during the early history of relativistic stellar physics.
• 2:07:04
  Speaker 1: Richard Tolman studied the structure of spherical objects within Albert
• 2:07:08
  Speaker 1: Einstein's general theory of relativity. Julius Robert Oppenheimer and George
• 2:07:14
  Speaker 1: Volkoff then applied relativistic physics to an idealised star made
• 2:07:19
  Speaker 1: from neutrons. Their work helped establish that neutron stars cannot
• 2:07:24
  Speaker 1: support unlimited mass. There is a maximum beyond which no
• 2:07:28
  Speaker 1: stable neutron star can exist. The exact value of that
• 2:07:32
  Speaker 1: maximum is still not known perfectly. It depends on the
• 2:07:35
  Speaker 1: uncertain equation of state of matter at extreme nuclear density.
• 2:07:40
  Speaker 1: Observations have nevertheless shown that some neutron stars contain around
• 2:07:45
  Speaker 1: twice the mass of the Sun. Any realistic theory of
• 2:07:48
  Speaker 1: dense matter must therefore be capable of supporting neutron stars
• 2:07:52
  Speaker 1: at least this massive. a newly formed neutron star can
• 2:07:56
  Speaker 1: also rotate astonishingly quickly. The reason is conservation of angular momentum.
• 2:08:03
  Speaker 1: Before the collapse, the stellar core was thousands of kilometres across. Afterwards,
• 2:08:09
  Speaker 1: much of its mass has been squeezed into an object
• 2:08:11
  Speaker 1: only tens of kilometres wide. As the radius shrinks, the
• 2:08:16
  Speaker 1: rotation rate can increase enormously. The same broad principle appears
• 2:08:21
  Speaker 1: when a spinning skater pulls their arms inward, although the
• 2:08:24
  Speaker 1: stellar version is slightly more committed. Some young neutron stars
• 2:08:28
  Speaker 1: can rotate many times every second. Older neutron stars in
• 2:08:32
  Speaker 1: binary systems can be spun up even further by gaining
• 2:08:36
  Speaker 1: matter and angular momentum from a companion. Some rotate hundreds
• 2:08:41
  Speaker 1: of times per second. The collapse also intensifies the magnetic field.
• 2:08:47
  Speaker 1: Magnetic field lines associated with the original stellar core become
• 2:08:52
  Speaker 1: concentrated into a vastly smaller region. Combined with other processes
• 2:08:57
  Speaker 1: inside the forming neutron star, this can produce magnetic fields
• 2:09:02
  Speaker 1: trillions of times stronger than Earth's. A particularly extreme class
• 2:09:06
  Speaker 1: of neutron stars is known as magnetars. Magnetars possess some
• 2:09:11
  Speaker 1: of the strongest magnetic fields known in the universe. Their
• 2:09:15
  Speaker 1: magnetic activity can produce powerful bursts of X-rays and gamma
• 2:09:19
  Speaker 1: rays as stresses in the crust and magnetic field release
• 2:09:23
  Speaker 1: enormous amounts of energy. Another remarkable form of neutron star
• 2:09:28
  Speaker 1: was first recognised in 1967. Jocelyn Bell Burnell, then a
• 2:09:34
  Speaker 1: postgraduate researcher, discovered an unusually regular radio signal while analysing
• 2:09:40
  Speaker 1: data from a radio telescope. the source appeared to pulse
• 2:09:44
  Speaker 1: with extraordinary precision. Her supervisor, Anthony Hewish, and other researchers
• 2:09:50
  Speaker 1: were involved in establishing the astronomical nature of the signal.
• 2:09:54
  Speaker 1: The object became known as a pulsar. A pulsar is
• 2:09:57
  Speaker 1: a rotating neutron star whose magnetic field and radiation beams
• 2:10:02
  Speaker 1: are not necessarily aligned with its axis of rotation. Radiation
• 2:10:07
  Speaker 1: can be emitted in narrow beams from regions associated with
• 2:10:11
  Speaker 1: the magnetic poles. As the neutron star spins, those beams
• 2:10:15
  Speaker 1: sweep through space. If one crosses Earth, astronomers detect a pulse.
• 2:10:22
  Speaker 1: The effect is similar to a lighthouse beam sweeping past
• 2:10:25
  Speaker 1: an observer. The neutron star is not repeatedly switching itself
• 2:10:30
  Speaker 1: on and off. It is rotating. Each time the beam
• 2:10:34
  Speaker 1: points towards us, another pulse arrives. Some pulses are so
• 2:10:39
  Speaker 1: regular that their timing can rival the precision of extremely
• 2:10:43
  Speaker 1: accurate clocks. One of the most famous examples is the
• 2:10:47
  Speaker 1: Crab Pulsar. It lies at the centre of the Crab Nebula,
• 2:10:51
  Speaker 1: the expanding remains of the stellar explosion seen from Earth
• 2:10:54
  Speaker 1: in the year 1054. The pulsar provides a direct connection
• 2:10:59
  Speaker 1: between a historical supernova and the neutron star left behind
• 2:11:03
  Speaker 1: by the collapsing core. Astronomers continue to use neutron stars
• 2:11:08
  Speaker 1: as laboratories for physics, that cannot easily be reproduced anywhere else.
• 2:11:14
  Speaker 1: The National Aeronautics and Space Administration Neutron Star Interior Composition Explorer,
• 2:11:20
  Speaker 1: operating aboard the International Space Station, has measured X-rays from
• 2:11:24
  Speaker 1: neutron stars with extraordinary timing precision. By studying how rapidly
• 2:11:30
  Speaker 1: rotating hot regions move in and out of view, researchers
• 2:11:34
  Speaker 1: can estimate neutron star radii and masses Those measurements help
• 2:11:39
  Speaker 1: constrain the equation of state, and therefore the behaviour of
• 2:11:42
  Speaker 1: matter at extreme density. A neutron star is no longer
• 2:11:47
  Speaker 1: a normal star. There is no sustained hydrogen fusion in
• 2:11:51
  Speaker 1: its centre. There is no long chain of nuclear fuel
• 2:11:55
  Speaker 1: supporting it against gravity. The active life of the massive
• 2:11:58
  Speaker 1: star has ended. What remains is the compact core. Gravity
• 2:12:03
  Speaker 1: has compressed it to nuclear densities. Quantum mechanics and the
• 2:12:08
  Speaker 1: behaviour of ultra-dense matter now provide the resistance that prevents
• 2:12:12
  Speaker 1: further collapse. For the right amount of mass that is enough,
• 2:12:17
  Speaker 1: the neutron star can remain small enough to fit across
• 2:12:20
  Speaker 1: a city, massive enough to outweigh the Sun, and dense
• 2:12:25
  Speaker 1: enough to preserve in one tiny remnant the final collapsed
• 2:12:29
  Speaker 1: heart of a star that once stretched across millions of kilometres.
• 2:12:34
  Speaker 1: A neutron star represents an extraordinary victory over gravity. An
• 2:12:39
  Speaker 1: entire stellar core can be compressed into an object only
• 2:12:43
  Speaker 1: tens of kilometres across, yet the collapse can still stop,
• 2:12:47
  Speaker 1: but only up to a point. If the surviving core
• 2:12:51
  Speaker 1: contains too much mass, the forms of pressure that can
• 2:12:54
  Speaker 1: support a neutron star are no longer enough. Gravity wins again.
• 2:12:59
  Speaker 1: There is no single birth mass at which every star
• 2:13:02
  Speaker 1: automatically becomes a black hole. The real situation is considerably
• 2:13:07
  Speaker 1: more complicated. A star may begin life with an enormous
• 2:13:11
  Speaker 1: mass and then lose a large fraction of it through
• 2:13:14
  Speaker 1: stellar winds. Its metallicity can influence how effectively those winds
• 2:13:19
  Speaker 1: remove material. Rotation can change how matter is mixed through
• 2:13:23
  Speaker 1: the interior. A binary companion can strip away the outer
• 2:13:28
  Speaker 1: layers or transfer additional mass. the strength of the eventual
• 2:13:33
  Speaker 1: supernova explosion also matters. What determines the final outcome is
• 2:13:38
  Speaker 1: not simply how massive the star was when it formed,
• 2:13:42
  Speaker 1: but how massive and compact the central remnant becomes after
• 2:13:45
  Speaker 1: millions of years of evolution. Even after a supernova begins,
• 2:13:50
  Speaker 1: the story may not be finished. Some material thrown outward
• 2:13:54
  Speaker 1: can fail to escape. The explosion may give it considerable
• 2:13:58
  Speaker 1: speed without giving it quite enough energy to overcome the
• 2:14:01
  Speaker 1: gravity of the central remnant. The material then slows, turns
• 2:14:06
  Speaker 1: around and falls back. This process is known as fallback.
• 2:14:12
  Speaker 1: If enough matter returns, it can increase the mass of
• 2:14:15
  Speaker 1: a newly formed neutron star until the object can no
• 2:14:18
  Speaker 1: longer support itself. Collapse then resumes. A neutron star may
• 2:14:25
  Speaker 1: therefore exist only briefly before becoming a black hole. In
• 2:14:29
  Speaker 1: other cases, the collapse of a very massive stellar core
• 2:14:32
  Speaker 1: may produce only a weak conventional explosion. Large amounts of
• 2:14:36
  Speaker 1: the star can fall inward rather than being successfully ejected.
• 2:14:41
  Speaker 1: At the most extreme end, models allow something approaching direct collapse,
• 2:14:47
  Speaker 1: where a black hole forms with comparatively little bright supernova display.
• 2:14:52
  Speaker 1: This means that not every black hole born from a
• 2:14:55
  Speaker 1: star must announce itself with the same spectacular explosion. Some
• 2:15:00
  Speaker 1: stellar deaths may be much less visually dramatic. The basic
• 2:15:05
  Speaker 1: theory describing what happens when gravity becomes this strong comes
• 2:15:09
  Speaker 1: from general relativity. In 1916, the German physicist Karl Schwarzschild
• 2:15:16
  Speaker 1: found the first exact solution to Albert Einstein's newly developed
• 2:15:20
  Speaker 1: theory the spacetime surrounding a spherical, non-rotating mass. The solution
• 2:15:26
  Speaker 1: revealed a characteristic radius. If sufficient mass were compressed within
• 2:15:31
  Speaker 1: that radius, the geometry of spacetime would contain a boundary
• 2:15:35
  Speaker 1: from beyond which escape would no longer be possible. Today,
• 2:15:40
  Speaker 1: that characteristic scale is known as the Schwarzschild radius. For
• 2:15:44
  Speaker 1: an object with roughly the mass of the Sun, the
• 2:15:47
  Speaker 1: Schwarzschild radius would be only a few kilometres, The Sun
• 2:15:51
  Speaker 1: itself is nowhere near this compact, but a collapsing stellar
• 2:15:55
  Speaker 1: core can become dramatically smaller. The idea that a sufficiently
• 2:15:59
  Speaker 1: massive star could continue collapsing was explored much more directly
• 2:16:03
  Speaker 1: in 1939 by Julius Robert Oppenheimer and Hartland Snyder. Their
• 2:16:08
  Speaker 1: calculation showed that, within general relativity, continued gravitational collapse could
• 2:16:14
  Speaker 1: produce an object from which light could no longer escape.
• 2:16:20
  Speaker 1: The modern name for such an object is a black hole.
• 2:16:23
  Speaker 1: The defining feature is the event horizon. The event horizon
• 2:16:27
  Speaker 1: is not a solid surface. There is no shell or
• 2:16:30
  Speaker 1: wall waiting there. It is a boundary in spacetime. Once
• 2:16:34
  Speaker 1: matter or light crosses inward through that boundary, every future-directed
• 2:16:39
  Speaker 1: path available to it remains inside. Even light cannot travel
• 2:16:43
  Speaker 1: back out. That is why the object appears black when
• 2:16:47
  Speaker 1: considered by itself. It is not because the black hole
• 2:16:51
  Speaker 1: is made from some unusually dark material. Light simply cannot
• 2:16:55
  Speaker 1: escape from within the horizon to reach a distant observer.
• 2:16:59
  Speaker 1: Black holes are sometimes described as though they possess a
• 2:17:02
  Speaker 1: special form of gravity that reaches across space and drags
• 2:17:06
  Speaker 1: everything towards them. That is misleading. From sufficiently far away,
• 2:17:11
  Speaker 1: the gravitational influence of a black hole … behaves according
• 2:17:15
  Speaker 1: to its mass just as the gravitational influence of any
• 2:17:19
  Speaker 1: other compact object would. If the Sun could somehow be
• 2:17:23
  Speaker 1: replaced by a black hole with exactly the same mass,
• 2:17:27
  Speaker 1: Earth would continue orbiting at roughly the same distance. It
• 2:17:31
  Speaker 1: would become very cold for more obvious reasons, but it
• 2:17:34
  Speaker 1: would not suddenly be sucked into the centre. The extraordinary
• 2:17:39
  Speaker 1: behaviour appears because a black hole allows mass to be
• 2:17:42
  Speaker 1: concentrated within an extremely compact region where spacetime becomes strongly curved.
• 2:17:50
  Speaker 1: Real black holes formed from stars are also unlikely to
• 2:17:53
  Speaker 1: be perfectly non-rotating. The stars that create them rotate. Their
• 2:17:58
  Speaker 1: cores rotate. Material falling into the final remnant carries angular momentum.
• 2:18:04
  Speaker 1: This makes the rotating black hole solution discovered by the
• 2:18:08
  Speaker 1: New Zealand mathematician Roy Kerr especially important. Kerr showed that
• 2:18:13
  Speaker 1: general relativity allows a rotating black hole, with properties different
• 2:18:18
  Speaker 1: from the simple non-rotating Schwarzschild case. A rotating black hole
• 2:18:23
  Speaker 1: has a more complicated spacetime around it, including a region
• 2:18:27
  Speaker 1: where rotation drags spacetime itself around the object. Black holes
• 2:18:32
  Speaker 1: formed from individual stars are known as stellar-mass black holes.
• 2:18:37
  Speaker 1: They should not be confused with the supermassive black holes
• 2:18:40
  Speaker 1: found at the centres of large galaxies. A stellar-mass black
• 2:18:44
  Speaker 1: hole may contain several or many times the mass of
• 2:18:47
  Speaker 1: the Sun. A supermassive black hole can contain millions or
• 2:18:52
  Speaker 1: billions of solar masses. Those enormous objects cannot be explained
• 2:18:57
  Speaker 1: simply as untouched remnants of individual ordinary stars. Their growth
• 2:19:02
  Speaker 1: involves a much longer history of accretion, mergers and galactic evolution.
• 2:19:08
  Speaker 1: A solitary stellar black hole can be difficult to detect.
• 2:19:12
  Speaker 1: If no matter is falling into it, the black hole
• 2:19:15
  Speaker 1: itself gives off no ordinary light. Astronomers therefore look for
• 2:19:19
  Speaker 1: its influence on surrounding objects. A visible companion star may
• 2:19:24
  Speaker 1: orbit something massive that cannot be seen directly. gas pulled
• 2:19:29
  Speaker 1: from a companion can form an accretion disk around the
• 2:19:31
  Speaker 1: black hole. As that gas spirals inward, friction and other
• 2:19:36
  Speaker 1: processes can heat it to tremendous temperatures, producing intense radiation
• 2:19:42
  Speaker 1: before the material crosses the event horizon. There is also
• 2:19:46
  Speaker 1: another method that does not depend on light at all.
• 2:19:50
  Speaker 1: Two black holes orbiting one another lose energy through gravitational waves.
• 2:19:55
  Speaker 1: Their orbit shrinks. They spiral together. Finally, they merge. The
• 2:20:01
  Speaker 1: changing gravitational field sends ripples through spacetime that travel outward
• 2:20:06
  Speaker 1: at the speed of light. The Laser Interferometer Gravitational Wave
• 2:20:11
  Speaker 1: Observatory made the first direct detection of gravitational waves from
• 2:20:15
  Speaker 1: a black hole merger in 2015. The Virgo collaboration later
• 2:20:20
  Speaker 1: joined these observations, and together these instruments have detected many
• 2:20:25
  Speaker 1: compact object mergers. These signals provide direct evidence that populations
• 2:20:30
  Speaker 1: of stellar mass black holes exist and that some of
• 2:20:33
  Speaker 1: them eventually find companions, spiral together and collide. The journey
• 2:20:39
  Speaker 1: from a cold molecular cloud to a black hole is
• 2:20:43
  Speaker 1: difficult to reconcile with the appearance of the final object.
• 2:20:47
  Speaker 1: At the beginning there was diffuse gas and dust spread
• 2:20:51
  Speaker 1: across an enormous region. Gravity gathered part of it together.
• 2:20:56
  Speaker 1: A protostar formed. Hydrogen fusion began. For millions of years,
• 2:21:01
  Speaker 1: the star shone. Its core manufactured heavier elements. Its fuel changed.
• 2:21:07
  Speaker 1: Its structure changed. Eventually, the central support failed. The core collapsed.
• 2:21:14
  Speaker 1: And if enough mass remained concentrated in that final remnant,
• 2:21:18
  Speaker 1: even neutron star matter could not stop gravity. Almost nothing
• 2:21:23
  Speaker 1: about the resulting black hole resembles the enormous shining star
• 2:21:27
  Speaker 1: that once occupied the same region of space. Yet the
• 2:21:31
  Speaker 1: connection is continuous. The black hole is not something that
• 2:21:35
  Speaker 1: appeared from nowhere. It is the last stage of a
• 2:21:38
  Speaker 1: chain that began with ordinary interstellar material. A cloud became
• 2:21:42
  Speaker 1: a star. The star became a collapsing core. And the
• 2:21:47
  Speaker 1: core became an object so compact … that the geometry
• 2:21:51
  Speaker 1: of space-time itself prevents anything inside its horizon from returning
• 2:21:55
  Speaker 1: to the outside universe. At the beginning, there was only
• 2:22:00
  Speaker 1: a cold cloud. Hydrogen, helium, heavier elements and microscopic dust
• 2:22:06
  Speaker 1: were spread through interstellar space, almost unimaginably thin by everyday standards.
• 2:22:13
  Speaker 1: Nothing in that cloud looked like a star. There was
• 2:22:16
  Speaker 1: no glowing surface, no blazing core and no obvious centre.
• 2:22:20
  Speaker 1: Then gravity began to gather part of the material together.
• 2:22:24
  Speaker 1: A denser region contracted. It became warmer. Rotation increased as
• 2:22:29
  Speaker 1: the cloud shrank, a disk formed around the centre, and
• 2:22:34
  Speaker 1: eventually a protostar emerged from material that had once been
• 2:22:37
  Speaker 1: scattered across a vast region of space. Still, it was
• 2:22:42
  Speaker 1: not yet a true main sequence star. That required something more.
• 2:22:48
  Speaker 1: The centre had to become hot and dense enough for
• 2:22:50
  Speaker 1: hydrogen nuclei to begin fusing. Once sustained nuclear fusion started,
• 2:22:57
  Speaker 1: the star entered the long period that defines most of
• 2:23:00
  Speaker 1: its active life. From that point onward, one balance mattered
• 2:23:05
  Speaker 1: more than almost anything else. Hydrostatic equilibrium. Gravity continually tried
• 2:23:13
  Speaker 1: to compress the star. Pressure from the hot interior pushed back.
• 2:23:18
  Speaker 1: Neither side permanently defeated the other. For millions or billions
• 2:23:22
  Speaker 1: of years, depending on the star, the result could be
• 2:23:25
  Speaker 1: remarkably stable. Inside, however, the star was always changing. Hydrogen
• 2:23:31
  Speaker 1: was being converted into helium. Energy was being released. Helium
• 2:23:36
  Speaker 1: was gradually accumulating in the core. The star was shining
• 2:23:40
  Speaker 1: because nuclear reactions were transforming matter. And those transformations eventually
• 2:23:45
  Speaker 1: determined what happened when the original hydrogen fuel began to
• 2:23:49
  Speaker 1: run low. Mass decided how far the story could go.
• 2:23:53
  Speaker 1: The smallest red dwarfs burn hydrogen so slowly that their
• 2:23:58
  Speaker 1: predicted lifetimes can stretch into trillions of years. The universe
• 2:24:03
  Speaker 1: is only about 13.8 billion years old, so the smallest
• 2:24:08
  Speaker 1: examples have not yet had time to complete their natural
• 2:24:11
  Speaker 1: main sequence evolutions. They are, in a sense, stars whose
• 2:24:16
  Speaker 1: full ending belongs to a future far beyond the present
• 2:24:18
  Speaker 1: age of the cosmos. A star more like the Sun
• 2:24:22
  Speaker 1: follows a different path. The Sun is now roughly four
• 2:24:26
  Speaker 1: and a half billion years old. It remains a main
• 2:24:29
  Speaker 1: sequence star and is expected to continue fusing hydrogen in
• 2:24:33
  Speaker 1: its core for roughly another five billion years. Eventually the
• 2:24:37
  Speaker 1: hydrogen available for central fusion will become too depleted to
• 2:24:41
  Speaker 1: maintain the same structure. The core will contract. Hydrogen will
• 2:24:46
  Speaker 1: continue burning in a shell. The outer layers will expand
• 2:24:50
  Speaker 1: and the Sun will become a red giant. Helium will
• 2:24:53
  Speaker 1: eventually become a fuel of its own. Carbon and oxygen
• 2:24:57
  Speaker 1: will accumulate in the core. But the Sun will never
• 2:25:00
  Speaker 1: become hot enough to continue through all the advanced fusion stages,
• 2:25:05
  Speaker 1: reached by much more massive stars. Instead, it will lose
• 2:25:10
  Speaker 1: its outer layers. A glowing planetary nebula will form for
• 2:25:14
  Speaker 1: a relatively brief period. The exposed core will remain behind
• 2:25:19
  Speaker 1: as a white dwarf. No core-collapsed supernova. No neutron star.
• 2:25:25
  Speaker 1: No black hole. Just a small, dense remnant slowly cooling
• 2:25:29
  Speaker 1: through the distant future. Massive stars live much faster. Their
• 2:25:35
  Speaker 1: stronger gravity creates hotter cores. Their nuclear reactions proceed at
• 2:25:40
  Speaker 1: far greater rates. Hydrogen gives way to helium, then carbon, neon,
• 2:25:47
  Speaker 1: oxygen and silicon in increasingly rapid succession. Eventually, the core
• 2:25:52
  Speaker 1: becomes dominated by iron group nuclei. At that point, the
• 2:25:57
  Speaker 1: long sequence of energy-releasing fusion reactions has effectively reached its limit.
• 2:26:02
  Speaker 1: Gravity is still there. The next useful fuel is not.
• 2:26:07
  Speaker 1: The core loses support and collapses. For some stars, the
• 2:26:11
  Speaker 1: result is a core collapse supernova. The outer layers are
• 2:26:15
  Speaker 1: thrown into space at tremendous speed, while the central remnant
• 2:26:19
  Speaker 1: may become a neutron star. If the core is too
• 2:26:23
  Speaker 1: massive for even neutron star matter to support, collapse can
• 2:26:27
  Speaker 1: continue towards a stellar mass black hole. These endings can
• 2:26:31
  Speaker 1: look very different from one another, but all of them
• 2:26:34
  Speaker 1: are connected to the same basic life story. Gravity gathers matter.
• 2:26:40
  Speaker 1: Fusion changes it. Gravity returns when the available nuclear support
• 2:26:44
  Speaker 1: finally fails. And yet, calling this death can hide an
• 2:26:49
  Speaker 1: important part of what happens. A star does not simply disappear.
• 2:26:54
  Speaker 1: Much of its material is redistributed. Sun-like stars return gas
• 2:26:59
  Speaker 1: and dust to space through stellar winds and the loss
• 2:27:02
  Speaker 1: of their outer envelopes. Massive stars can eject enormous quantities
• 2:27:07
  Speaker 1: of material during supernova explosions. Over time, this material mixes
• 2:27:13
  Speaker 1: into the interstellar medium. It changes the chemical composition of
• 2:27:18
  Speaker 1: the galaxy. Carbon produced and dredged up in evolved stars
• 2:27:23
  Speaker 1: can be released into space. Oxygen and many other elements
• 2:27:27
  Speaker 1: forged through stellar fusion can be dispersed. Additional nuclei are
• 2:27:32
  Speaker 1: created or redistributed through processes such as slow neutron capture,
• 2:27:37
  Speaker 1: explosive nucleosynthesis, and rapid neutron capture in extreme environments. The
• 2:27:45
  Speaker 1: chemical history of the universe is therefore not the work
• 2:27:48
  Speaker 1: of one single type of event. It is the accumulated
• 2:27:52
  Speaker 1: result of many different stars, many different deaths and several.
• 2:27:58
  Speaker 1: different kinds of astrophysical process. There is also an important
• 2:28:02
  Speaker 1: correction to one of the most familiar phrases in astronomy.
• 2:28:06
  Speaker 1: It is often said that we are made from stars.
• 2:28:09
  Speaker 1: That is broadly true, but not every atom in a
• 2:28:12
  Speaker 1: planet or a human body was manufactured inside a star.
• 2:28:17
  Speaker 1: Most hydrogen was produced in the early universe. Helium was
• 2:28:21
  Speaker 1: also created in large quantities shortly after the Big Bang.
• 2:28:25
  Speaker 1: Many heavier elements, however, were formed later through stellar fusion
• 2:28:30
  Speaker 1: and other high-energy astrophysical processes. The carbon in living tissue,
• 2:28:36
  Speaker 1: the oxygen in water, the calcium in bones and the
• 2:28:39
  Speaker 1: iron in blood all belonged to a chemical story that
• 2:28:43
  Speaker 1: was profoundly shaped by earlier generations of stars. When those
• 2:28:48
  Speaker 1: stars died, their material did not vanish. It returned to space.
• 2:28:54
  Speaker 1: Over millions of years, some of that material can drift
• 2:28:57
  Speaker 1: through the galaxy and become part of another molecular cloud.
• 2:29:01
  Speaker 1: Gravity can gather it once again. Another protostar can form.
• 2:29:07
  Speaker 1: Another disk. Another planetary system. Another star. So the life
• 2:29:14
  Speaker 1: of a star is not really an isolated story with
• 2:29:16
  Speaker 1: a perfectly clean beginning and end. It is part of
• 2:29:20
  Speaker 1: a much larger cycle. A star inherits material from the
• 2:29:24
  Speaker 1: universe that came before it. It changes some of that
• 2:29:28
  Speaker 1: material through nuclear reactions. Then, eventually, it returns part of
• 2:29:34
  Speaker 1: itself to the galaxy. The shining phase ends, the consequences
• 2:29:40
  Speaker 1: do not, and that changes the way the original darkness looks.
• 2:29:45
  Speaker 1: A cold molecular cloud is no longer just empty black
• 2:29:50
  Speaker 1: space with a little gas floating through it. It can
• 2:29:53
  Speaker 1: contain hydrogen from the early universe, dust built from elements
• 2:29:58
  Speaker 1: made in older stars, material expelled from red giants, atoms
• 2:30:05
  Speaker 1: scattered by supernovae, the remnants of stellar lives that ended
• 2:30:09
  Speaker 1: long ago. And somewhere inside that darkness may be the
• 2:30:14
  Speaker 1: raw material of stars that have not yet begun to shine.
• 2:30:17
  Speaker 1: A star can live for millions, billions, or even trillions
• 2:30:23
  Speaker 1: of years. But when its light finally fades, its story
• 2:30:28
  Speaker 1: does not vanish with it. Some of the matter continues.
• 2:30:32
  Speaker 1: Some of the chemistry continues. And the galaxy quietly carries
• 2:30:37
  Speaker 1: both forward.