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How Stars Die (Including the One That Keeps Us Alive) · Star Trails: From Backyard Astronomy to Cosmic Wonder

0:07
Howdy Star Gazers and welcome to this episode
0:10
of Star Trails. My name is Drew and I'll be your
0:13
guide to the night sky for the week of February
0:16
the 22nd to the 28th. This week we wrap up our
0:20
month -long series on stars by exploring how
0:24
they end their lives. For some it's a quiet process
0:28
like embers cooling after a campfire. Others
0:32
explode and some turn into black holes. destined
0:36
to live the remainder of their lives like cosmic
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vampires facing eternity. Later in the show,
0:43
we'll take a look at what you can see in the
0:45
night sky this week. Whether you're tuning in
0:48
from the backyard or the balcony, I'm glad you're
0:51
here. So grab a comfortable spot under the night
0:53
sky and let's get started. When I was a kid,
0:59
I distinctly remember the night when I learned
1:01
our son was at some point going to die. And to
1:06
be honest, it wasn't a great feeling. Sitting
1:10
on the floor in the den of my grandmother's house
1:12
as twilight fell, I opened up my favorite astronomy
1:16
tome, The Universe by National Geographic. and
1:20
read a surprisingly long and detailed account
1:23
of how in some five billion years our Sun would
1:27
grow hotter, all life on Earth would be vaporized,
1:31
and the Sun would expand into a red giant, enveloping
1:35
Venus, Mercury, and the Earth. This was terrifying.
1:39
It was the first realization that the Sun and
1:42
Earth aren't permanent. Sure, I knew I wouldn't
1:45
be around in five billion years to see the Sun
1:48
collapse, but even as a seven or eight year old
1:51
child, knowing that the end was approaching like
1:54
a timer counting down, created a sense of existential
1:57
dread and anxiety. So tonight, we're wrapping
2:01
up our month -long theme about stars. We've talked
2:05
about how they form, what they are, and what
2:08
they do during their epic lifespans. We've done
2:11
some statistical analysis related to star formation
2:15
and what they leave behind. But now it's time
2:18
to talk about how stars die, including the one
2:22
that keeps us alive. The first thing to remember
2:26
is most stars don't explode. Our imagination
2:31
is filled with supernova. Stellar death is often
2:35
portrayed as violent and spectacular. But the
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vast majority of stars don't end that way. As
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we've mentioned a few times this month, most
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stars in the universe are small red dwarfs that
2:48
burn their hydrogen so slowly they will outlive
2:51
the current age of the cosmos by orders of magnitude.
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They won't explode, they'll simply fade away.
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Larger stars, like our sun, live for about 10
3:01
billion years in what astronomers call the main
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sequence phase. During this time, hydrogen fuses
3:09
into helium in the core. Fusion releases energy.
3:13
That energy produces outward pressure. Gravity
3:16
pulls inward and the two forces balance one another
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in what is called hydrostatic equilibrium. For
3:24
billions of years, the Sun has existed in that
3:27
quiet stalemate. But hydrogen in the core isn't
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infinite. And this is where the story becomes
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more subtle than most of us expect. Counter to
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what we might think as a star dies, some, like
3:41
our sun, actually grow in size. When hydrogen
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in the sun's core is mostly depleted, fusion
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in the very center begins to slow. The outward
3:51
pressure weakens. Gravity, which never takes
3:54
a day off, begins to win. The core starts to
3:58
contract. And here's the key piece. When gas
4:02
is compressed, it heats up. Gravitational potential
4:06
energy is converted into thermal energy as the
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core shrinks. Think about how a diesel engine
4:13
works. Unlike a normal gas engine which uses
4:17
spark plugs to touch off the combustion, a diesel
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engine compresses the air to ignite the fuel.
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Now imagine that process happening with half
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the mass of a star. It's not just a small contained
4:30
explosion, it's nuclear fusion. As the core contracts,
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it becomes hotter and denser. Fusion in the center
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has largely ceased, but the rising temperature
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ignites hydrogen in a shell surrounding the inert
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helium core. Instead of fusion happening at the
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center, it now happens in a thin, intense layer
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around it. And that shell fusion is more energetic
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than the Sun's original core fusion was. The
4:59
increased energy output pushes outward on the
5:02
star's outer layers. Those layers respond by
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expanding dramatically. Our Sun will swell to
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perhaps a hundred times its current diameter,
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engulfing the inner planets. Paradoxically, gravity
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causes the expansion. Gravity compresses the
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core. Compression increases temperature. Higher
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temperature ignites shell fusion, and shell fusion
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increases energy output. Thus, the outer layers
5:31
expand. As the Sun expands, its surface temperature
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drops. A larger surface area radiates energy
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more efficiently, so the outer layers cool to
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around 3 ,000 to 4 ,000 Kelvin. The Sun will
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appear redder and dimmer per unit area. even
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though its total luminosity increases. And this
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is the red giant phase. You can look up at the
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night sky right now and see some famous red giants.
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Betelgeuse in Orion is the classic example. It's
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already in its death throes, hundreds of times
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larger than the sun. If placed at the center
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of our solar system, it would extend past Mars.
6:16
possibly even toward Jupiter, depending on how
6:18
you measure its outer atmosphere. This star will
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not become a white dwarf. It will explode as
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a supernova within the next hundred thousand
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years or so, astronomically soon, though not
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calendar soon. Aldebaran is closer to the Sun's
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eventual fate. It's a red giant, not massive
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enough to explode, but expanded and cooling.
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This is a preview of what our Sun will look like
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in about 5 billion years. It's steady. It's not
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violent. It's just old. Eventually in our Sun
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the contracting helium core will become hot enough,
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about 100 million Kelvin, to ignite helium fusion
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into carbon and oxygen. This ignition happens
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in what astronomers call the helium flash. a
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rapid internal event that barely disturbs the
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outer star. After a brief period of renewed stability,
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fusion once again shifts outward into shells,
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and the outer layers grow increasingly unstable.
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Finally, the Sun will shed those layers entirely,
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creating a glowing planetary nebula, a delicate
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shell of gas expanding into space, and at the
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center remains the exposed core. And this is
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where the physics turns truly strange. What remains
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after the Sun sheds its outer layers is a white
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dwarf, roughly Earth -sized, containing about
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half the Sun's current mass. It no longer generates
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energy through fusion. It shines because it's
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hot, not because it's burning. The density inside
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a white dwarf is extraordinary. A teaspoon of
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its material would weigh several tons. At this
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stage, the matter is packed so tightly that classical
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physics alone cannot explain what holds it up.
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The answer lies in quantum mechanics. Electrons
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obey a rule known as the Pauli exclusion principle,
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which states no two electrons can occupy the
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same quantum state simultaneously. When gravity
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attempts to compress the white dwarf further,
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electrons are forced into higher energy states
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simply because there are no lower ones available.
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This creates a pressure called electron degeneracy
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pressure that doesn't depend on temperature.
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The white dwarf isn't supported by heat. It's
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supported by the structure of quantum reality
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itself. Afterwards, it will simply cool over
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trillions of years, fading gradually into darkness.
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There are planetary nebulae you can see right
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now that represent what the sun's far future
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could look like. The ring nebula, M57, is the
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exposed outer layers of a sun -like star that
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died thousands of years ago. At its center is
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a white dwarf, the compressed remnant core. The
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Dumbbell Nebula is larger and easier to observe
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in small telescopes than the Ring Nebula. It's
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another Sun -like star shedding its atmosphere.
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And while you aren't likely to be able to spot
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it, we know of another fairly close white dwarf.
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Sirius, the brightest star in our sky, has a
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tiny companion, Sirius B. A white dwarf about
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the size of Earth, but nearly as massive as the
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Sun. It's difficult to see visually because it's
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overwhelmed by the glare of Sirius A, but it's
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been imaged and studied exclusively by Hubble
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and X -ray telescopes. That tiny dot represents
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quantum mechanics holding up half a star. If
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a star begins its life more massive than the
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Sun, the ending changes dramatically. Massive
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stars burn through their fuel quickly. fusing
10:18
heavier and heavier elements in their cores.
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Hydrogen becomes helium, helium becomes carbon,
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carbon becomes oxygen, and so on, building an
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onion -like structure of nested fusion layers.
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This process continues until iron accumulates
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in the core. But iron is a dead end because fusing
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iron consumes energy instead of releasing it.
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When the core becomes iron -rich, fusion can
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no longer support it, and gravity wins completely.
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The core collapses in less than a second. Electrons
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are crushed into protons forming neutrons and
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releasing an enormous burst of neutrinos. The
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collapsing core rebounds, driving a supernova
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explosion that briefly outshines entire galaxies.
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What remains depends on the core's mass. If the
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remnant core is between roughly one and a half
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and about two or three solar masses, collapse
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halts at an even more extreme stage. Electrons
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and protons merge into neutrons. The result is
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a neutron star, typically about 20 kilometers
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across, yet containing more mass than the Sun.
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Its density is so extreme that a sugar cube -sized
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piece would outweigh a mountain. Once again,
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quantum mechanics intervenes. Neutron degeneracy
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pressure, the same exclusion principle applied
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to neutrons, presents further collapse. Inside,
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matter may exist as a superfluid, possibly containing
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exotic particles or even free quarks. We're still
12:00
probing this frontier with gravitational wave
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observations and nuclear physics experiments.
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Some neutron stars spin rapidly and emit beams
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of radiation, appearing to us as pulsars. The
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Crab Nebula, M1, is the remnant of a supernova
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observed in 1054 AD. At its center lies a neutron
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star, the Crab Pulsar, spinning about 30 times
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per second. Other neutron stars possess magnetic
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fields so intense they defy comprehension. These
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are magnetars, neutron stars with extraordinarily
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amplified magnetic fields, trillions of times
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stronger than Earth's. The leading explanation
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is that rapid rotation during collapse amplifies
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magnetic fields like a dynamo. The result is
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an object capable of releasing immense bursts
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of X -rays and gamma rays. They are rare and
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violent, but still the product of stellar collapse.
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And that finally brings us to black holes. If
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the collapsing core exceeds the mass that neutron
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degeneracy pressure can support, no known force
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can halt the implosion. Gravity overwhelms every
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resistance. The core continues collapsing until
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an event horizon forms. That's a boundary beyond
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which even light cannot escape. A black hole
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isn't a cosmic vacuum cleaner. It's a region
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of spacetime so curved that all future paths
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point inward. At its center, classical general
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relativity predicts a singularity, a point of
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infinite density and zero volume. But infinities
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in physics are usually a sign that the theory
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has been pushed beyond its valid domain. Most
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physicists suspect that a future theory of quantum
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gravity will replace the singularity with something
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finite, though we don't yet know what. We tend
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to group black holes into categories based on
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mass. Stellar mass black holes form from collapsing
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massive stars. These typically range from about
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3 to perhaps 50 times the mass of the Sun. But
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these objects are only tens of kilometers across,
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and that's astonishingly compact. Then there
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are intermediate -mass black holes, which likely
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range from hundreds to hundreds of thousands
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of solar masses. Evidence for these is still
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emerging, but gravitational wave detections and
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observations of dense star clusters do suggest
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they exist. And finally, there are supermassive
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black holes. Every large galaxy we've studied
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appears to host one at its center. The one in
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our galaxy, Sagittarius A, has a mass of about
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4 million suns. In the far future of the universe,
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after stars have burned out, after white dwarfs
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have cooled, and after neutron stars have decayed
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or collapsed, black holes may be the last major
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structures left. This hypothetical scenario is
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sometimes called the black hole era of the universe.
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And it's so far in the future that it makes the
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sun's 5 billion remaining years look like a brief
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candle flicker. At some point, even black holes
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sputter out. In the weird quantum fields of a
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black hole, some particles manage to escape.
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You've maybe heard this term before. Hawking
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radiation. because it was in fact theorized by
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physicist Stephen Hawking. Over time, and I do
15:52
mean a very long time, the black hole will slowly
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lose mass. We're talking time scales that are
15:59
slow even by astronomical standards. We need
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to shift our perspective by powers of 10 for
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it to even make sense. For example, the evaporation
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of a stellar mass black hole could take years
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on the order of 10 to the 67th power. That's
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a 1 with 67 zeros behind it. A supermassive black
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hole may take 10 to the 100 years or more to
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die. By comparison, the current age of the universe
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in years is only about 10 to the 10th power.
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Black holes aren't eternal, but for all practical
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astrophysical purposes they might as well be.
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As they lose mass their temperature increases
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and near the very end of their evaporation they
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would radiate intensely and vanish in a final
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burst of high energy radiation. This final phase
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has of course never been observed and remains
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theoretical. This brings me back around to where
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we started this episode. pondering the dread
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of our own sun's death. Our sun feels foundational,
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yet it is temporary. But its transformation lies
17:13
so far in the future that it functions more as
17:15
a philosophical boundary than an impending event.
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Humanity has only existed for a tiny fraction
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of the sun's lifespan. If the sun were a 70 -year
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-old human, our entire species would have only
17:30
appeared a few hours ago. When its time comes,
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the Sun will not rage. It will compress, ignite
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new layers, expand, shed, and settle into a dense
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white ember held together by the quantum rules
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that make the universe coherent at its smallest
17:48
scales. In the meantime, it's humbling to realize
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that everything heavier than helium in your body
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was forged in stars that lived and died before
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ours was even born. Stellar deaths made rocky
18:02
planets possible. It made chemistry complex.
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And it made us. After a quick break, we'll be
18:24
back with this week's Sky Report. Stay with us.
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Welcome back. This week's sky offers a rich mix
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of wandering worlds and familiar lunar phases,
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perfect for both binocular stargazers and people
18:51
simply stepping outside after dinner. As the
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sun sets, start by finding the moon. This week
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it's rotating into view as a thin waxing crescent
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early in the week. By February 24th, the moon
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reaches first quarter. appearing half illuminated
19:08
and hanging high in the southern sky around sunset.
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By week's end, it grows into a bright, waxing
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gibbous rising earlier each evening and dominating
19:18
the early night. This week is especially good
19:22
for planets because a slow -building planetary
19:25
parade is unfolding in the evening sky. Six planets,
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Mercury, Venus, Jupiter, Saturn, Uranus, and
19:33
Neptune, will appear along the same stretch of
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sky shortly after sunset. From Earth's perspective,
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they trace a broad arc along the ecliptic that
19:43
can be seen in a single sweep with dark skies
19:46
and clear horizons. Start your evening soon after
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sunset by looking low toward the western horizon.
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There, Venus shines brightest among the inner
19:57
planets, a brilliant beacon just above the horizon.
20:01
Not far below it is Mercury, still a challenge
20:04
because it sits low and close to the Sun's glare,
20:07
but visible if you can catch it while the sky
20:10
is still dusky and the horizon is clear. Saturn
20:14
also lingers in this direction, a softer yellow
20:17
point that will benefit from binoculars or a
20:20
small telescope to cut through the twilight.
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Toward the east and southeast as night deepens,
20:26
you'll see Jupiter, the brightest star in the
20:29
evening sky this month. It rises early and stays
20:33
high well into the night, far easier to spot
20:36
than Mercury, Venus, or even Saturn. In a telescope,
20:40
Jupiter's cloud bands and its large moons make
20:43
for stunning details if the air is steady. The
20:46
outer ice giants, Uranus and Neptune are also
20:50
part of this extended parade, located between
20:53
the brighter worlds. They're too faint to see
20:56
with the naked eye. Binoculars or a telescope
20:59
will help you tease them out against the star
21:01
fields. The best collective view of this six
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-planet parade builds toward February 28th, when
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all six planets are visible in the same post
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-sunset window roughly 30 to 90 minutes after
21:15
sundown. Start with the western horizon for Venus,
21:19
Mercury, and Saturn, then sweep eastward to catch
21:22
Jupiter higher in the sky. Uranus and Neptune
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will be between them, subtle, but there. Finally,
21:30
even with the moon brightening later in the week,
21:32
don't miss a couple of quieter sky -watching
21:35
opportunities earlier in the period. On the evening
21:38
of February 23, the moon moves through the northern
21:42
part of the Pleiades star cluster, subtly occulting
21:46
stars as it passes. It makes a fine pairing for
21:50
binoculars or a wide -field telescope. Next week
21:53
we'll return to our book club covering chapters
21:56
6 and 7 in Nightwatch. These chapters cover deep
22:01
sky objects and the planets of our solar system.
22:04
And like the previous chapters, make excellent
22:07
companion readings for this podcast. That's going
22:14
to do it for this week. If you found this episode
22:16
interesting, please share it with a friend who
22:18
might enjoy it. The easiest way to do that is
22:21
by sending folks to our website, StarTrails .Show.
22:26
And if you'd like to support the show, use the
22:28
link on the site to buy me a coffee. That really
22:31
helps. Be sure to follow Star Trails on Blue
22:35
Sky and YouTube. Links are in the show notes.
22:38
Until we meet again beneath the stars, clear
22:41
skies everyone.