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What If a Star Exploded Near Earth_ _ The Supernova That Could Change Everything · Boring Science

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Speaker 1: Hello, and welcome to Boring Science to Sleep, a quiet
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Speaker 1: place where science becomes slow, soft, and easy to drift through. Tonight,
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Speaker 1: you're going to gently explore what would happen if a
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Speaker 1: star exploded near Earth, not to learn everything about stellar explosions,
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Speaker 1: and not to worry about what might happen, but simply
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Speaker 1: to notice the story slowly, calmly, one small idea at
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Speaker 1: a time. You'll travel inward, toward the hidden heart of
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Speaker 1: a massive star, where gravity quietly gathers matter together, where
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Speaker 1: tiny particles move through incredible densities, and where an ancient
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Speaker 1: star may eventually release its energy into the darkness around it.
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Speaker 1: You'll also drift outward, across the space between the stars,
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Speaker 1: toward the Earth beneath you, where our atmosphere quietly receives
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Speaker 1: particles and light that have travelled across enormous distances, But
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Speaker 1: before you settle in, if you truly enjoy these calm
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Speaker 1: journeys through interesting facts about science, you're welcome to like
1:05
Speaker 1: this video and subscribe, only if it feels right for you.
1:09
Speaker 1: And when you're ready, let me know in the comments
1:11
Speaker 1: where you're listening from, and what time it is right now.
1:15
Speaker 1: It's always a quiet joy to see how many places
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Speaker 1: and moments gather here together. Now, dim the lights, maybe
1:24
Speaker 1: let a fan hum softly in the background. and allow
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Speaker 1: your body to become a little heavier against whatever is
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Speaker 1: supporting you. There's nothing to do. You are safe. Just
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Speaker 1: listen and drift. And somewhere above you, beyond the roof,
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Speaker 1: beyond the clouds, beyond the familiar blue air of daytime,
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Speaker 1: the stars are quietly continuing their very old work. So,
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Speaker 1: very slowly, let's look toward one of them. You can
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Speaker 1: imagine that you are lying quietly beneath the night sky.
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Speaker 1: perhaps looking through a window, perhaps resting somewhere where the
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Speaker 1: ceiling has disappeared into darkness. And above you, the stars
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Speaker 1: are scattered so far away that their distances become almost
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Speaker 1: impossible to feel. They seem still, almost perfectly still, but
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Speaker 1: they are not still at all. Every star is a
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Speaker 1: place of movement, of heat, of pressure, and of gravity,
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Speaker 1: with countless tiny particles moving through enormous regions of glowing gas.
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Speaker 1: while the whole star quietly holds itself together against the
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Speaker 1: inward pull of its own weight. And most of the time,
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Speaker 1: from where you are, none of this seems to matter.
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Speaker 1: A star simply shines, a small point of light, steady
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Speaker 1: enough to become part of the background of your thoughts.
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Speaker 1: But imagine, just for a little while, that one of
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Speaker 1: those stars was much larger than the sun, much heavier,
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Speaker 1: and much farther along in its life. Imagine that, After
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Speaker 1: millions of years of quietly burning, it reached the final
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Speaker 1: stages of its existence. And imagine that this star was
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Speaker 1: close enough to Earth for its death to become something
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Speaker 1: you could eventually see in the sky. What would happen?
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Speaker 1: The answer begins with something surprisingly ordinary. Gravity. Gravity is
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Speaker 1: always pulling matter inward. Inside a star, enormous amounts of
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Speaker 1: gas are being drawn toward the centre. while the heat
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Speaker 1: and pressure produced deep inside push outward. For a very
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Speaker 1: long time, these two tendencies can remain in balance. The
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Speaker 1: star does not simply collapse, it does not simply fly apart.
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Speaker 1: It stays there, glowing quietly in space. You might picture
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Speaker 1: it as a vast, warm sphere, gently holding itself together,
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Speaker 1: with gravity pressing inward and the energy of its hot
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Speaker 1: interior pressing outward. And for a star like the Sun,
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Speaker 1: this balance can last for billions of years. The Sun, however,
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Speaker 1: is not destined to become a supernova. It does not
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Speaker 1: contain enough mass. This is an important little detail, because
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Speaker 1: when you look up at the Sun, it may seem enormous.
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Speaker 1: It is enormously larger than anything you experience on Earth,
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Speaker 1: of course. But compared with the stars that can end
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Speaker 1: their lives as core collapse supernovae, the Sun is simply
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Speaker 1: not heavy enough. One day, far in the future, the
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Speaker 1: sun will change. It will expand into a red giant,
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Speaker 1: shed its outer layers, and eventually leave behind a small,
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Speaker 1: dense remnant called a white dwarf. But it will not
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Speaker 1: end with the enormous stellar collapse that creates a supernova.
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Speaker 1: A massive star follows a different path. And the word
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Speaker 1: massive is important here. A massive star is not simply
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Speaker 1: a star that looks bigger in the night sky. It
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Speaker 1: is a star carrying many times the mass of the Sun.
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Speaker 1: That extra mass means stronger gravity. Stronger gravity compresses the
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Speaker 1: interior more intensely, and that compression allows the centre of
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Speaker 1: the star to become extraordinarily hot. The hotter the interior becomes,
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Speaker 1: the faster nuclear fusion can proceed. Nuclear fusion sounds complicated,
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Speaker 1: but you can think of it simply as a process
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Speaker 1: in which tiny atomic nuclei join together under enormous heat
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Speaker 1: and pressure, releasing energy as they do so. That energy
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Speaker 1: helps support the star. So a massive star becomes something
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Speaker 1: like a very bright, very busy furnace. It burns its
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Speaker 1: fuel much more rapidly than the sun, and this creates
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Speaker 1: one of the quietest paradoxes in stellar life. The bigger
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Speaker 1: the star, the shorter its life can be. A star
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Speaker 1: many times heavier than the sun can shine with astonishing brightness.
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Speaker 1: but it spends its fuel much more quickly. A star
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Speaker 1: like the Sun can remain in its long, stable phase
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Speaker 1: for billions of years. A star around 20 times the
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Speaker 1: mass of the Sun may spend only around 10 million
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Speaker 1: years in its main stage of life. 10 million years
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Speaker 1: may sound like an impossibly long time to you, and
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Speaker 1: it is. But on the scale of stars, it is
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Speaker 1: remarkably brief. It is as though the largest stars live quickly,
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Speaker 1: they burn brightly, they move through their lives at a
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Speaker 1: faster internal rhythm, and as their lives approach the end,
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Speaker 1: that rhythm becomes faster still. Eventually, the star begins changing
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Speaker 1: the elements in its deepest layers. Hydrogen is gradually transformed
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Speaker 1: into helium. Later, the interior can move through carbon, neon,
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Speaker 1: oxygen and silicon. And eventually, at the centre, iron begins
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Speaker 1: to accumulate. But for now, You do not need to
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Speaker 1: follow every step. You can simply imagine the star slowly
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Speaker 1: changing inside, layer after layer, while from far away it
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Speaker 1: continues to look like an ordinary point of light. That
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Speaker 1: contrast is part of what makes stars so strange. The
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Speaker 1: surface can appear calm, while the interior is moving through
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Speaker 1: enormous changes, and the final stages become remarkably compressed in time.
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Speaker 1: The long hydrogen burning stage can last for most of
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Speaker 1: the star's life. The helium stage may last around a
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Speaker 1: million years. Carbon burning may continue for roughly a thousand years.
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Speaker 1: Later stages involving neon and oxygen can last only a
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Speaker 1: few years, and the final silicon burning stage may last
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Speaker 1: only months. So, as the star approaches its final moment,
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Speaker 1: Millions of years of quiet existence become followed by a
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Speaker 1: much faster sequence of changes. Still, from far away, there
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Speaker 1: may be almost nothing for you to notice. The star
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Speaker 1: simply remains there, a small light, a quiet point. Perhaps
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Speaker 1: you would never know that its interior had begun moving
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Speaker 1: toward its final state. Human beings have watched stars like
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Speaker 1: these for thousands of years. Long before modern telescopes, People
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Speaker 1: noticed when the night sky changed. A new point of
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Speaker 1: light would sometimes appear where no star had been seen before.
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Speaker 1: In the year 1604, Johannes Kepler observed one such event,
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Speaker 1: a bright new object that became known as Kepler's supernova.
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Speaker 1: At the time, a sudden new star in the sky
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Speaker 1: could seem like the appearance of something that had never
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Speaker 1: existed before. But we now understand these events differently. The
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Speaker 1: light was not announcing the birth of a star, it
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Speaker 1: was announcing the death of one, and that is a
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Speaker 1: gentle idea to carry with you tonight. A star can
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Speaker 1: spend millions of years quietly existing, far beyond the reach
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Speaker 1: of your senses, and then, for a short time, its
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Speaker 1: death becomes visible across an enormous distance. The Milky Way
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Speaker 1: contains hundreds of billions of stars, yet supernovae are relatively rare.
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Speaker 1: Only roughly one or two are expected to occur in
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Speaker 1: our galaxy during an average century. So most nights are quiet.
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Speaker 1: Most stars continue shining. Most of the points of light
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Speaker 1: above you will remain exactly what they seem to be,
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Speaker 1: distant suns, quietly burning, with no sudden ending waiting anywhere
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Speaker 1: near you. And if one massive star somewhere in the
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Speaker 1: galaxy is approaching its final moments, its story is still
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Speaker 1: unfolding far away. beyond the darkness that surrounds you. For now,
9:42
Speaker 1: you can simply rest beneath the stars. There is nothing
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Speaker 1: you need to predict, nothing you need to solve. The
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Speaker 1: night remains quiet. And somewhere in that enormous darkness, a
9:53
Speaker 1: massive star continues its slow journey toward a moment when
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Speaker 1: gravity will begin to ask something very different of it.
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Speaker 1: For a little longer, it is still holding itself together.
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Speaker 1: For a little longer, it is still burning. And deep inside,
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Speaker 1: almost unnoticed, the iron at its centre is beginning to matter.
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Speaker 1: For a while, You can simply remain with that distant star.
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Speaker 1: You do not need to hurry toward its ending. It
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Speaker 1: is still there, glowing quietly in the darkness. While inside it,
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Speaker 1: gravity continues its patient pull toward the centre. Gravity is
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Speaker 1: easy to notice on Earth. You feel it whenever your
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Speaker 1: body rests against a bed, whenever a leaf settles onto
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Speaker 1: the ground, whenever rain falls softly through the air. But
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Speaker 1: inside a massive star... Gravity has a much larger task.
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Speaker 1: It is holding together an enormous sphere of gas, a
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Speaker 1: sphere containing many times the material of the Sun, and
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Speaker 1: every part of that star is being gently drawn inward.
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Speaker 1: If gravity were the only thing acting there, the star
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Speaker 1: would simply collapse. But gravity is not alone. Deep inside
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Speaker 1: the star, the temperature is extraordinarily high, and tiny particles
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Speaker 1: are moving through the dense interior, colliding and combining in
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Speaker 1: ways that release energy. That energy creates pressure, and that
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Speaker 1: pressure pushes outward. So, for a very long time, the
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Speaker 1: star exists between two quiet tendencies. Gravity pulls inward, heat
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Speaker 1: and pressure push outward. Neither one completely wins. The star
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Speaker 1: settles into a kind of balance, sometimes called hydrostatic equilibrium,
11:43
Speaker 1: although you do not need to remember the name. You
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Speaker 1: can simply imagine an enormous glowing sphere that is constantly
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Speaker 1: trying to fall inward, while its own hot interior quietly
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Speaker 1: holds it up. Nothing about this balance is perfectly motionless.
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Speaker 1: Inside the star, everything is moving, everything is changing. But
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Speaker 1: the changes happen on such enormous scales of time that,
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Speaker 1: from far away, the star can appear almost perfectly still.
12:13
Speaker 1: And this is where nuclear fusion becomes important. You can
12:16
Speaker 1: think of fusion as a process in which very small
12:20
Speaker 1: atomic nuclei join together under immense heat and pressure. When
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Speaker 1: lighter nuclei join, energy is released. That energy travels through
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Speaker 1: the star, helping to maintain the pressure that supports it.
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Speaker 1: For a massive star, this process begins with hydrogen. Hydrogen
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Speaker 1: is the simplest of the elements. and for a long
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Speaker 1: period of the star's life, its deepest interior gradually transforms
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Speaker 1: hydrogen into helium. As this happens, the star shines. The
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Speaker 1: light you see from such a star is connected to
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Speaker 1: this hidden activity deep within it. You might imagine that
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Speaker 1: every soft photon leaving the surface carries with it a
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Speaker 1: tiny part of a much longer story, beginning deep inside
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Speaker 1: a place you cannot see. But eventually, The hydrogen in
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Speaker 1: the central region becomes depleted. The star changes. Its centre contracts.
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Speaker 1: The temperature rises, and helium becomes the next fuel. Now
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Speaker 1: the star can begin building carbon. The process continues. Not
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Speaker 1: because the star has suddenly become violent, but because gravity
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Speaker 1: keeps gently compressing the interior whenever the previous source of
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Speaker 1: pressure becomes less effective. The centre grows hotter. Another kind
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Speaker 1: of fusion becomes possible. carbon begins to participate. Then, deeper
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Speaker 1: into the star's life, the interior can move through still
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Speaker 1: heavier stages involving neon, oxygen and silicon. You do not
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Speaker 1: need to picture these as separate explosions. They are more
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Speaker 1: like different rooms in the same enormous furnace, each becoming
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Speaker 1: important when the conditions inside become warm and dense enough.
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Speaker 1: And something curious happens when As the star grows older,
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Speaker 1: the stages become shorter, much shorter. The transformation of hydrogen
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Speaker 1: can occupy most of a massive star's lifetime. Helium burning
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Speaker 1: may last around a million years. Carbon burning can last
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Speaker 1: roughly a thousand years. Later stages involving neon and oxygen
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Speaker 1: can last only a few years, and the final silicon
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Speaker 1: burning stage may last only a matter of months. so
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Speaker 1: the star's life begins to feel almost as though its
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Speaker 1: internal clock is speeding up. Millions of years pass quietly,
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Speaker 1: then perhaps a million years, then thousands of years, then years,
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Speaker 1: then months. And yet, throughout much of this process, the
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Speaker 1: star still looks like a star. From a great distance,
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Speaker 1: you might see only a bright point against a black sky.
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Speaker 1: You would not see the layers forming inside. You would
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Speaker 1: not see the increasingly hot centre. You would not see
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Speaker 1: the changing balance between gravity and pressure. You would simply
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Speaker 1: see light. This is one of the gentle mysteries of
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Speaker 1: stellar evolution. The most important changes can happen where no
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Speaker 1: ordinary eye could ever watch them. Deep inside, the star
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Speaker 1: is becoming layered. Different regions can contain different elements, created
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Speaker 1: during different stages of its life. The outer regions may
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Speaker 1: still contain lighter material. Deeper inside, heavier elements have accumulated,
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Speaker 1: and near the centre, the story is moving toward iron.
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Speaker 1: Iron is different from the lighter elements that came before it.
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Speaker 1: For the earlier stages of fusion, combining lighter nuclei can
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Speaker 1: release energy that helps support the star, but once the
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Speaker 1: star builds a core dominated by iron, that particular source
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Speaker 1: of support reaches its limit. Iron does not provide the
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Speaker 1: same useful energy return through fusion. The star has reached
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Speaker 1: a quiet boundary. It has spent much of its life
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Speaker 1: creating energy by joining lighter elements together. Now, at its centre,
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Speaker 1: it has built something that cannot continue that process in
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Speaker 1: the same helpful way. The star has not exploded yet.
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Speaker 1: Nothing needs to happen immediately from your point of view.
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Speaker 1: You can still imagine yourself lying beneath the night sky,
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Speaker 1: looking toward that distant point of light. Perhaps the star
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Speaker 1: seems peaceful, perhaps it looks unchanged, but deep inside, the
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Speaker 1: balance that has held it together for so long is
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Speaker 1: becoming more fragile. Gravity is still there, it has always
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Speaker 1: been there, and now, as the centre becomes filled with
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Speaker 1: iron and the familiar source of energy weakens, gravity begins
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Speaker 1: to have a little more influence. The star responds by
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Speaker 1: compressing its central region. The temperature rises again. The pressure changes.
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Speaker 1: More layers gather around the growing iron core, and the
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Speaker 1: timescale becomes shorter still. A star that once seemed to
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Speaker 1: have all the time in the universe now has very
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Speaker 1: little time left, at least by the standards of stars. Still,
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Speaker 1: you do not need to imagine a sudden alarm. There
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Speaker 1: is no clock ringing in the darkness. There is only
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Speaker 1: the quiet movement of matter, the slow gathering of weight,
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Speaker 1: the gradual transformation of one element into another. The star
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Speaker 1: is approaching a point where its enormous mass will no
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Speaker 1: longer be comfortably supported, and somewhere inside that glowing sphere,
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Speaker 1: the iron core is becoming heavier, denser, and more important
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Speaker 1: with every passing moment. You can let that thought remain soft.
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Speaker 1: A star, carrying its own immense weight, A star that
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Speaker 1: has spent millions of years pushing gently against gravity. A
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Speaker 1: star that has slowly built its way toward iron. And now,
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Speaker 1: almost quietly, gravity is beginning to win. You can stay
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Speaker 1: for a little while with the massive star, just as
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Speaker 1: it is, suspended in the darkness. From a great distance,
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Speaker 1: there is still nothing particularly alarming about it. It shines.
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Speaker 1: Perhaps its light is blue-white, perhaps slightly warmer, depending on
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Speaker 1: the star itself, but from Earth it remains only a
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Speaker 1: distant point, almost motionless against the background of space. And yet,
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Speaker 1: deep inside, the balance has changed. The star has spent
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Speaker 1: much of its life turning lighter elements into heavier ones,
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Speaker 1: creating energy each time its inner furnace found another way
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Speaker 1: to keep gravity at bay. Hydrogen became helium, helium helped
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Speaker 1: create carbon. The deeper furnace moved through carbon, neon, oxygen,
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Speaker 1: and silicon. And now, at the centre, iron has begun
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Speaker 1: to gather. Iron is an important boundary. It is not
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Speaker 1: that iron is somehow dangerous. It is simply that the
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Speaker 1: usual arrangement of the star has reached a limit. For
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Speaker 1: the lighter elements, combining atomic nuclei can release energy. That
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Speaker 1: energy helps keep the interior hot. The heat helps create pressure,
19:30
Speaker 1: and the pressure helps support the star against its own
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Speaker 1: enormous weight. But when the core becomes dominated by iron,
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Speaker 1: there is no longer the same useful energy to be
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Speaker 1: gained by continuing that process. The furnace at the centre
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Speaker 1: has reached the end of the road, and without that
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Speaker 1: energy the balance begins to change. change. Gravity is still
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Speaker 1: pulling inward. The star is still carrying all of its
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Speaker 1: enormous weight, but now there is less support pushing back,
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Speaker 1: so the iron core begins to contract, slowly at first,
20:04
Speaker 1: then faster and faster. You can imagine a vast warm sphere,
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Speaker 1: gradually becoming smaller at its centre, not because anything is
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Speaker 1: pulling it from outside, but because its own weight is
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Speaker 1: becoming too much for the old balance to comfortably support.
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Speaker 1: The core becomes denser, the particles inside it are pressed
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Speaker 1: closer together, the temperature rises to extraordinary levels, and eventually
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Speaker 1: the density becomes so great that even the familiar behaviour
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Speaker 1: of matter begins to change. The electrons– those tiny particles
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Speaker 1: that normally move around atomic nuclei, are forced into an
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Speaker 1: environment where there is almost nowhere left for them to go.
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Speaker 1: They begin to be captured by protons. When an electron
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Speaker 1: meets a proton under these extreme conditions, the two can
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Speaker 1: combine and become a neutron, and a neutrino is released.
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Speaker 1: You can think of this as the star quietly changing
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Speaker 1: the character of its own heart. matter that once contained
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Speaker 1: many protons and electrons becomes increasingly rich in neutrons, and
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Speaker 1: those neutrinos begin streaming away. They are almost ghost-like. They
21:17
Speaker 1: rarely interact with ordinary matter, so they can move outward
21:21
Speaker 1: through the collapsing star far more easily than ordinary light can.
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Speaker 1: For a brief moment, the centre becomes a place of
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Speaker 1: almost unimaginable density. The iron core is which may have
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Speaker 1: been thousands of kilometres across, is compressed into something only
21:38
Speaker 1: a few tens of kilometres across. That is difficult to picture,
21:43
Speaker 1: so you can let the numbers soften. Imagine a region
21:46
Speaker 1: wider than a large country becoming compressed until it is
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Speaker 1: only about the size of a city. The amount of
21:53
Speaker 1: matter has not simply vanished, it has been squeezed. The
21:57
Speaker 1: same enormous weight is now contained inside a much smaller space.
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Speaker 1: The collapse happens incredibly quickly. Matter falls inward at a
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Speaker 1: substantial fraction of the speed of light, roughly one quarter
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Speaker 1: of that ultimate cosmic speed. You do not need to
22:14
Speaker 1: imagine this as a sharp or violent sound. There is
22:18
Speaker 1: no sound travelling through the vacuum toward you. There is
22:21
Speaker 1: only motion, an enormous inward movement happening far away, deep
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Speaker 1: inside a star that whose light has been travelling across
22:30
Speaker 1: space toward you for perhaps thousands of years. And then
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Speaker 1: something happens at the centre. The collapsing material reaches an
22:39
Speaker 1: extraordinarily dense state. A neutron-rich core forms. This new object
22:45
Speaker 1: is called a neutron star. It is astonishingly compact. A
22:50
Speaker 1: neutron star can contain more mass than the Sun, while
22:53
Speaker 1: being only a few tens of kilometres across. You can
22:57
Speaker 1: imagine taking the mass of a star and placing it
23:00
Speaker 1: inside a space comparable to a city. The result would
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Speaker 1: be matter packed so tightly that ordinary ideas of weight
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Speaker 1: and density begin to feel almost meaningless. But the collapse
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Speaker 1: does not simply continue forever. The newly formed neutron star
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Speaker 1: resists further compression. The material falling inward encounters this incredibly
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Speaker 1: dense core. A tremendous pressure wave develops. it moves outward
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Speaker 1: through the star, and this is where the story begins
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Speaker 1: to turn. For a moment, the outer layers of the
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Speaker 1: star are still falling inward. Deep beneath them, the newborn
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Speaker 1: neutron star is forming. Neutrinos are streaming away, matter is
23:43
Speaker 1: becoming incredibly dense, and a vast pressure wave is trying
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Speaker 1: to move outward. The star is no longer quietly burning
23:51
Speaker 1: through its fuel, It is now changing its entire structure
23:55
Speaker 1: in a matter of seconds. And yet, even here, the
23:59
Speaker 1: most remarkable part is still almost invisible. Much of the
24:03
Speaker 1: energy released during the collapse does not become the bright
24:07
Speaker 1: light that you eventually see. Instead, an enormous amount of
24:11
Speaker 1: energy is carried away by neutrinos, those tiny particles that
24:15
Speaker 1: can pass through matter almost as though the matter were
24:18
Speaker 1: barely there. Some of their energy can also be transferred
24:22
Speaker 1: to the material around the newborn neutron star, helping the
24:26
Speaker 1: outward-moving shock gain strength. The exact details of how the
24:30
Speaker 1: explosion develops are complicated, and scientists continue to study the process.
24:36
Speaker 1: But you do not need to hold every detail in
24:38
Speaker 1: your mind. You can simply picture the centre becoming smaller
24:42
Speaker 1: and denser, while something enormous begins pushing outward, inward, outward, Compression. Release.
24:51
Speaker 1: The star has spent its entire life balancing gravity against pressure.
24:56
Speaker 1: Now that balance has become a rapidly changing conversation between
24:59
Speaker 1: the two. And somewhere inside the collapsing heart, a new
25:04
Speaker 1: object has appeared. A neutron star. Tiny compared with the
25:08
Speaker 1: star that created it. Dense beyond ordinary imagination. Still surrounded
25:14
Speaker 1: by the remains of its former self. You might imagine
25:17
Speaker 1: that little remnant sitting in the darkness, only a few
25:21
Speaker 1: tens of kilometres across, while enormous layers of the old
25:25
Speaker 1: star begin moving away from it. The star that once
25:28
Speaker 1: held everything together is becoming something else. Its outer layers
25:33
Speaker 1: are beginning to leave. Its inner core has become something new.
25:37
Speaker 1: And among all the particles moving through this strange moment,
25:41
Speaker 1: the neutrinos are already travelling outward, carrying news of what
25:46
Speaker 1: has happened long before the visible light can escape. For now,
25:50
Speaker 1: you can let the scene become quiet again, a tiny,
25:54
Speaker 1: impossibly dense heart, a great cloud of matter around it,
25:59
Speaker 1: and a stream of nearly invisible particles moving silently into
26:03
Speaker 1: the darkness. The collapse has changed the star forever, but
26:07
Speaker 1: the light of that change has not yet reached you,
26:13
Speaker 1: The star is still changing. Deep inside its former centre,
26:17
Speaker 1: a neutron star has appeared, incredibly small compared with the
26:21
Speaker 1: enormous star that once surrounded it. And around this new,
26:25
Speaker 1: dense heart, the outer layers are still moving. But before
26:29
Speaker 1: you see anything, before the sky becomes brighter, before the
26:33
Speaker 1: light of the explosion begins its long journey toward Earth,
26:37
Speaker 1: something else has already begun to leave the star. Neutrinos.
26:42
Speaker 1: You have already met them briefly, those almost invisible particles
26:46
Speaker 1: created as the collapsing core changes its matter. Now you
26:50
Speaker 1: can simply let yourself drift alongside them. Imagine that you
26:54
Speaker 1: are not made of solid matter for a moment, but
26:57
Speaker 1: that you are somehow able to move effortlessly through everything
27:00
Speaker 1: around you. There is no floor beneath you, no ceiling
27:05
Speaker 1: above you, no resistance. You simply pass through. This is,
27:10
Speaker 1: in a very small way, how a neutrino experiences much
27:14
Speaker 1: of the universe. Neutrinos interact with ordinary matter extraordinarily weakly.
27:21
Speaker 1: They have almost no electrical charge, and they do not
27:24
Speaker 1: behave like ordinary particles that constantly bump into the objects
27:28
Speaker 1: around them. Instead, they can pass through enormous amounts of
27:33
Speaker 1: material without being stopped. Even now, while you are lying
27:37
Speaker 1: quietly wherever you are, Enormous numbers of neutrinos are passing
27:42
Speaker 1: through your body. They move through you without making a sound.
27:46
Speaker 1: You do not feel them. You do not see them.
27:49
Speaker 1: You do not notice them. And then they continue onward.
27:53
Speaker 1: They can pass through the earth itself. They can travel
27:57
Speaker 1: through rock, water, buildings, and your own body, usually without
28:02
Speaker 1: interacting with anything at all. One way to imagine their
28:06
Speaker 1: strange freedoms is to picture a wall so unimaginably thick
28:10
Speaker 1: that it would take an enormous journey just to cross it,
28:13
Speaker 1: and yet a neutrino could pass through that wall with
28:17
Speaker 1: surprisingly little difficulty. The reason is that neutrinos interact through
28:22
Speaker 1: one of the fundamental forces of nature, a very weak
28:25
Speaker 1: interaction compared with the forces that keep ordinary matter together.
28:30
Speaker 1: You do not need to hold on to the name.
28:33
Speaker 1: The important feeling is simply this. Matter is almost transparent
28:37
Speaker 1: to them. But inside a collapsing massive star, the situation
28:41
Speaker 1: becomes different. The centre of the star is no longer
28:44
Speaker 1: ordinary matter. It has been compressed to an extraordinary density.
28:50
Speaker 1: Particles are crowded together far more tightly than they are
28:53
Speaker 1: anywhere in your familiar surroundings. And during the collapse, an
28:58
Speaker 1: enormous number of neutrinos are created. The number is difficult
29:02
Speaker 1: to imagine. It can be described as a number with
29:05
Speaker 1: around 58 digits. Rather than trying to picture every digit,
29:10
Speaker 1: you can simply imagine an ocean of invisible particles, so
29:14
Speaker 1: numerous that ordinary numbers begin to feel almost too small
29:18
Speaker 1: to be useful. They stream outward from the newborn neutron star.
29:23
Speaker 1: Most of them continue into space, and most of their
29:26
Speaker 1: energy leaves with them. This is one of the strangest
29:29
Speaker 1: things about a supernova. When you eventually see the brilliant flash,
29:34
Speaker 1: you may imagine that the light is carrying almost all
29:37
Speaker 1: of the energy. But visible light is only a small
29:40
Speaker 1: part of the story. Most of the energy released during
29:44
Speaker 1: the collapse escapes as neutrinos. The visible glow is the
29:48
Speaker 1: part your eyes can notice. The neutrinos are the much larger,
29:53
Speaker 1: quieter part. They are like an invisible river flowing away
29:57
Speaker 1: from the collapsing star. and because they interact so weakly
30:01
Speaker 1: with matter, they can begin their journey outward from the
30:04
Speaker 1: core before the visible light has escaped from the surface.
30:09
Speaker 1: This difference in timing matters. The explosion itself begins deep
30:13
Speaker 1: inside the star, but the star has a huge amount
30:16
Speaker 1: of material surrounding its core. The outward moving shock has
30:21
Speaker 1: to travel through all of those layers before it can
30:23
Speaker 1: reach the surface. The neutrinos do not have to wait
30:27
Speaker 1: in quite the same way. they can move outward through
30:30
Speaker 1: the dense interior and eventually escape. So if a massive
30:34
Speaker 1: star in our galaxy were to undergo a nearby supernova,
30:38
Speaker 1: one of the first signs might not be a bright
30:41
Speaker 1: new star in the sky. It might be a quiet
30:44
Speaker 1: increase in the number of neutrinos detected by instruments on Earth.
30:48
Speaker 1: For astronomers, that could be an extraordinary moment. A detector
30:53
Speaker 1: buried deep underground or surrounded by a large volume of
30:57
Speaker 1: water or another suitable material, might notice a brief burst
31:01
Speaker 1: of interactions. The particles themselves would still be invisible, but
31:06
Speaker 1: their tiny encounters with matter could leave a measurable trace,
31:11
Speaker 1: and those traces could arrive hours before the visible light.
31:15
Speaker 1: Imagine that. The star has collapsed. The light is still
31:19
Speaker 1: hidden inside the outer layers. The shock is still moving outward.
31:25
Speaker 1: But the neutrinos are already crossing the darkness between the stars.
31:29
Speaker 1: They are travelling toward Earth, quietly, almost unnoticed, and eventually
31:35
Speaker 1: they reach you, not as a feeling, not as a flash,
31:39
Speaker 1: not as a sound, only as an almost imperceptible interaction
31:44
Speaker 1: inside a detector somewhere on the planet. It is a
31:48
Speaker 1: beautiful example of how different kinds of information can travel
31:52
Speaker 1: at different speeds through the same cosmic event. The star
31:55
Speaker 1: has not yet become bright in the way your eyes
31:58
Speaker 1: understand brightness, but its invisible message has already begun. There
32:03
Speaker 1: is another part of this story that is just as important.
32:07
Speaker 1: Although neutrinos usually pass through matter, the enormous density inside
32:11
Speaker 1: the collapsing star means that some of them can interact
32:15
Speaker 1: with the material around the newborn neutron star. a small
32:19
Speaker 1: portion of their energy can be transferred to that surrounding matter.
32:23
Speaker 1: This process, called neutrino heating, can help strengthen the outward
32:27
Speaker 1: movement of the shock. You can imagine it very gently.
32:31
Speaker 1: The collapsing core has created an enormous flow of invisible energy.
32:36
Speaker 1: Most of it escapes. But some of that energy is
32:39
Speaker 1: left behind, little by little, in the material around the core.
32:43
Speaker 1: Enough to matter. enough to help the stalled, outward-moving shock
32:48
Speaker 1: continue its journey. Scientists study this process carefully, because the
32:53
Speaker 1: exact details of how a core-collapse supernova explodes are complex.
32:57
Speaker 1: The neutrinos are important, but they are part of a
33:00
Speaker 1: larger physical story involving the collapsing core, the shock, convection,
33:07
Speaker 1: magnetic effects, and the changing structure of the stellar material.
33:11
Speaker 1: You do not need to carry all of those details
33:14
Speaker 1: with you tonight. you can simply keep the central image.
33:18
Speaker 1: A tiny, dense heart. A huge cloud of falling and
33:22
Speaker 1: moving matter. An invisible flood of neutrinos. And a pressure wave,
33:27
Speaker 1: slowly finding its way outward. The strange thing is that
33:31
Speaker 1: the neutrinos, which usually seem almost too quiet to matter,
33:35
Speaker 1: may be among the first pieces of information to tell
33:38
Speaker 1: the universe that a star has died. And while they
33:41
Speaker 1: continue moving through the darkness, The visible light is still waiting.
33:46
Speaker 1: It has another journey to make. It must cross the
33:49
Speaker 1: star itself. It must pass through layers of gas and matter.
33:54
Speaker 1: It must finally escape into open space. Only then can
33:58
Speaker 1: the brilliant flash begin its much slower journey toward distant worlds.
34:03
Speaker 1: Toward planets. Toward telescopes. Toward your night sky. So for now,
34:09
Speaker 1: you can remain with the invisible particles. You can imagine
34:13
Speaker 1: them passing through the star, passing through space, passing through
34:18
Speaker 1: the earth, and passing through you. No sound, no sensation,
34:23
Speaker 1: just motion, and somewhere far away, before the first visible
34:28
Speaker 1: light has escaped, the universe has already begun whispering that
34:32
Speaker 1: something has changed. The invisible particles have already begun their journey.
34:40
Speaker 1: They have crossed the collapsing star. They have slipped through
34:43
Speaker 1: the darkness. Some have passed through the Earth itself, and
34:47
Speaker 1: a tiny fraction have left traces inside quiet detectors, telling
34:51
Speaker 1: astronomers that somewhere, something enormous has happened. But the visible
34:56
Speaker 1: light is still waiting. Deep inside the star, the shock
35:01
Speaker 1: is moving outward. The outer layers of the star are
35:04
Speaker 1: still there, surrounding the newborn neutron star, and all of
35:08
Speaker 1: that material must now respond to what has happened at
35:11
Speaker 1: the centre. You can imagine the star as an enormous
35:15
Speaker 1: glowing sphere, thousands or even millions of times larger than
35:20
Speaker 1: the small remnant forming at its heart. For most of
35:23
Speaker 1: its life, everything inside that sphere was held together. Now, slowly,
35:29
Speaker 1: the arrangement begins to change. The shock travels outward through
35:33
Speaker 1: the layers. It carries energy with it. It pushes against
35:37
Speaker 1: the material surrounding the core. And eventually, After passing through
35:42
Speaker 1: layer after layer of the former star, it reaches the surface.
35:47
Speaker 1: The star opens into space. Material begins to move outward.
35:52
Speaker 1: Light escapes. And from a distant planet, the event that
35:56
Speaker 1: began deep inside the star finally becomes visible, a supernova.
36:02
Speaker 1: For a short time, the dying star can become extraordinarily bright.
36:07
Speaker 1: It may shine with enough apparent brilliance that to rival
36:10
Speaker 1: the combined light of enormous numbers of other stars in
36:13
Speaker 1: its galaxy. But even here, you do not need to
36:17
Speaker 1: imagine a sudden explosion like something from a movie. There
36:21
Speaker 1: is no sound travelling across the vacuum of space. There
36:24
Speaker 1: is no expanding sphere of fire racing toward Earth like
36:29
Speaker 1: a wall. Instead, there is light. There is radiation. There
36:34
Speaker 1: are particles. There is matter moving outward through space. and
36:39
Speaker 1: there is an enormous amount of time for all of
36:41
Speaker 1: these things to spread apart. The star that once held
36:45
Speaker 1: itself together is becoming a cloud. Its outer layers are
36:49
Speaker 1: being carried away. Elements created during the star's life are
36:54
Speaker 1: being released into the space between the stars. Some of
36:58
Speaker 1: these elements were formed deep inside the star. Others can
37:02
Speaker 1: be created during the explosion itself. and some of this
37:06
Speaker 1: material may eventually become part of something that does not
37:09
Speaker 1: yet exist. Another star, another planet, perhaps another world, but
37:15
Speaker 1: not every massive star follows exactly the same path. Some
37:20
Speaker 1: very massive stars may collapse so strongly that they form
37:24
Speaker 1: black holes without producing the kind of bright supernova you
37:27
Speaker 1: might expect. The details depend on the star's mass, its
37:32
Speaker 1: internal structure, how much material it loses during its life,
37:36
Speaker 1: and the conditions inside its collapsing core. So when you
37:40
Speaker 1: imagine a massive star reaching its final moment, there is
37:43
Speaker 1: not always a single ending. Sometimes a brilliant supernova leaves
37:49
Speaker 1: behind a neutron star. Sometimes the core collapses further and
37:54
Speaker 1: forms a black hole. And sometimes the explosion itself may
37:58
Speaker 1: be weaker or difficult to observe. There is also another
38:02
Speaker 1: route to a stellar explosion. It begins not with a
38:06
Speaker 1: massive star collapsing, but with a much smaller and very
38:10
Speaker 1: dense object called a white dwarf. A white dwarf is
38:14
Speaker 1: the compact leftover core of a star that has already
38:18
Speaker 1: completed its ordinary life. Now imagine that white dwarf living
38:22
Speaker 1: in a system with another star. The two stars share
38:26
Speaker 1: the same gravitational neighbourhood. Over time, Material from the companion
38:31
Speaker 1: can fall toward the white dwarf. Little by little, matter
38:35
Speaker 1: gathers there. Eventually, under the right conditions, the white dwarf
38:40
Speaker 1: can reach a critical state, and a runaway thermonuclear reaction
38:45
Speaker 1: can destroy it in an enormous explosion. This kind of
38:49
Speaker 1: event is known as a Type Ia supernova. It is
38:53
Speaker 1: different from the core collapse event we have been following,
38:56
Speaker 1: different beginnings. different physical pathway. Yet the same broad cosmic
39:02
Speaker 1: theme remains. A star changes. Its old structure cannot continue forever,
39:08
Speaker 1: and matter is released back into space. There is another
39:12
Speaker 1: gentle detail hidden inside supernova explosions. They are not always
39:17
Speaker 1: perfectly symmetrical. You might imagine a star exploding equally in
39:22
Speaker 1: every direction, like a perfect sphere, but real explosions can
39:27
Speaker 1: be uneven One region may push more strongly than another.
39:31
Speaker 1: One side may release more energy. Material may flow outward
39:36
Speaker 1: in complicated patterns. And when the explosion is uneven, the
39:40
Speaker 1: newborn neutron star can receive a push in the opposite direction.
39:45
Speaker 1: It is almost as though the star's final breath is
39:48
Speaker 1: slightly stronger on one side. The result can be remarkable.
39:53
Speaker 1: A neutron star, only a few tens of kilometres across,
39:57
Speaker 1: can be launched through space at enormous speed. One observed
40:02
Speaker 1: neutron star has been measured moving at roughly 1600 kilometres
40:06
Speaker 1: per second. You can soften that number in your imagination.
40:11
Speaker 1: More than a thousand kilometres every second. a tiny stellar
40:15
Speaker 1: remnant moving through the galaxy faster than almost anything you
40:19
Speaker 1: encounter in ordinary life. And yet, once again, there is
40:23
Speaker 1: no sound, no sensation, just motion through an enormous darkness.
40:29
Speaker 1: The neutron star continues travelling. Around it, the remains of
40:34
Speaker 1: the original star continue expanding. The cloud grows larger, the
40:39
Speaker 1: material becomes thinner, the light begins to fade. What was
40:44
Speaker 1: once one enormous star gradually becomes a wide, delicate structure
40:49
Speaker 1: moving through space. This is called a supernova remnant, and
40:54
Speaker 1: one of the most famous examples began with an explosion
40:57
Speaker 1: observed on Earth nearly a thousand years ago. In the
41:01
Speaker 1: year 1054, observers recorded a remarkably bright new object in
41:06
Speaker 1: the sky. It remained visible for a considerable period before
41:11
Speaker 1: gradually fading away. Today, we understand that event as the
41:15
Speaker 1: supernova that created the Crab Nebula. The Crab Nebula is
41:21
Speaker 1: not a single solid object. It is an expanding cloud
41:25
Speaker 1: of material, glowing faintly as different particles and magnetic processes
41:30
Speaker 1: continue to shape it. Its remnant now stretches across roughly
41:35
Speaker 1: 11 light-years. Imagine that distance. Light itself needs about 11
41:40
Speaker 1: years to cross from one side to the other. And
41:44
Speaker 1: yet this enormous structure began with one star. One star
41:48
Speaker 1: that once had a life of its own. One star
41:51
Speaker 1: that once seemed permanent. Now its material is drifting outward,
41:56
Speaker 1: becoming part of the galaxy around it. This is perhaps
41:59
Speaker 1: one of the quietest things about a supernova. The explosion
42:03
Speaker 1: is temporary. The remnant is slow. The star may shine
42:07
Speaker 1: brilliantly for a while, but the material it leaves behind
42:11
Speaker 1: can remain visible for thousands of years, sometimes much longer.
42:15
Speaker 1: The death of the star becomes a long, fading presence.
42:19
Speaker 1: And eventually, even that presence becomes thinner. The material continues outward.
42:25
Speaker 1: The light becomes softer. The cloud blends into the surrounding
42:30
Speaker 1: interstellar space. Nothing has truly disappeared. The star has simply
42:35
Speaker 1: changed form. Its core has become something incredibly dense. Its
42:40
Speaker 1: outer layers have become a drifting cloud. Its elements have
42:44
Speaker 1: begun a new journey, and all of it is moving
42:47
Speaker 1: quietly through the galaxy. For now, you can remain with
42:51
Speaker 1: that expanding cloud. You can imagine its edges becoming wider
42:56
Speaker 1: and fainter, its particles spreading farther apart, its light slowly
43:01
Speaker 1: softening against the darkness. The star that once seemed to
43:05
Speaker 1: be one thing has become many things. A compact remnant,
43:10
Speaker 1: a cloud of gas, a trail of elements, a little
43:14
Speaker 1: more material added to the enormous space between the stars.
43:19
Speaker 1: And somewhere far beyond that expanding cloud, there is another planet,
43:23
Speaker 1: perhaps another world, waiting beneath another sky. Because a supernova
43:29
Speaker 1: does not only send something away, it also changes the
43:33
Speaker 1: space around it, and when the explosion is close enough,
43:37
Speaker 1: that changed space can eventually reach worlds that are quietly
43:41
Speaker 1: watching from far away. The expanding cloud of the supernova
43:47
Speaker 1: continues to move through space. Its particles drift outward, its
43:52
Speaker 1: light gradually becomes softer, and far away, the galaxy continues turning,
43:59
Speaker 1: carrying stars and planets through the darkness. In a slow movement,
44:04
Speaker 1: that is almost impossible to notice. For most of the
44:07
Speaker 1: distance between a supernova and Earth, there is nothing for
44:11
Speaker 1: you to see. Space is extraordinarily empty. A few particles here,
44:16
Speaker 1: a little dust there, a thin scattering of gas between
44:20
Speaker 1: the stars. And yet, if a stellar explosion happened close
44:25
Speaker 1: enough to Earth, some of what it released could eventually
44:28
Speaker 1: reach our planet. Not as a wall of fire, not
44:31
Speaker 1: as a sound. but as radiation and high-energy particles moving
44:36
Speaker 1: quietly through space. Distance would matter enormously. A supernova thousands
44:42
Speaker 1: of light-years away could be spectacular to look at, while
44:45
Speaker 1: remaining essentially harmless to life on Earth. A much closer
44:50
Speaker 1: explosion would be different. There is no single magical distance
44:54
Speaker 1: where a supernova suddenly becomes dangerous, because the effects depend
44:58
Speaker 1: on many things, including the energy of the explosion, the
45:02
Speaker 1: type of star, the direction and amount of radiation, and
45:06
Speaker 1: how Earth's atmosphere responds. So rather than imagining one precise boundary,
45:12
Speaker 1: you can imagine a gradual change. The closer the explosion,
45:16
Speaker 1: the stronger its influence could become. And between the distant
45:20
Speaker 1: star and you stands something remarkably thin, the atmosphere. You
45:25
Speaker 1: rarely think about it. You breathe it. You move through it.
45:29
Speaker 1: You feel it when a breeze passes over your skin
45:32
Speaker 1: or when cool air enters a room through a window.
45:35
Speaker 1: But from the perspective of space, the atmosphere is a
45:39
Speaker 1: delicate layer surrounding the planet. It is our first great shield.
45:44
Speaker 1: High above you, molecules of air are constantly moving, colliding, separating,
45:51
Speaker 1: and rearranging themselves. Most of the radiation and particles arriving
45:56
Speaker 1: from ordinary space never reach the ground unchanged. The atmosphere
46:02
Speaker 1: absorbs some, scatters some, transforms some into other particles, and
46:08
Speaker 1: quietly protects the living world below. But a nearby supernova
46:13
Speaker 1: could change the amount of high-energy radiation arriving from space.
46:17
Speaker 1: One important part of this radiation comes in the form
46:21
Speaker 1: of cosmic rays. Despite their name, cosmic rays are not
46:25
Speaker 1: simply rays of light. Many are fast-moving particles. especially atomic
46:30
Speaker 1: nuclei such as protons and helium nuclei. They can travel
46:35
Speaker 1: across enormous distances through space. When some of these particles
46:40
Speaker 1: encounter Earth's atmosphere, they collide with molecules high above the ground,
46:45
Speaker 1: and then something interesting happens. The original particle may not
46:49
Speaker 1: reach you. Instead, its energy can be transformed into a
46:54
Speaker 1: cascade of new particles. One collision creates others. Those create more,
47:00
Speaker 1: and the atmosphere becomes a place where energy from deep
47:03
Speaker 1: space is gradually transformed as it moves downward. Among the
47:08
Speaker 1: particles that can be created are muons. A muon is
47:12
Speaker 1: somewhat like a heavier relative of the electron. It is tiny,
47:17
Speaker 1: it carries an electric charge, and unlike many other particles
47:21
Speaker 1: created high in the atmosphere, muons can travel surprisingly far
47:26
Speaker 1: before disappearing. Some can reach the Earth's surface. Some can
47:30
Speaker 1: even travel through significant amounts of water. So if a
47:34
Speaker 1: nearby supernova increased the cosmic ray environment around Earth, the
47:39
Speaker 1: atmosphere could become the beginning of a longer chain of changes.
47:43
Speaker 1: Particles from space, collisions high above you, new particles, more
47:49
Speaker 1: movement through the atmosphere, and eventually, a greater number of
47:53
Speaker 1: certain particles reaching the surface. This does not mean that
47:57
Speaker 1: every nearby supernova would produce the same effect. The details matter.
48:02
Speaker 1: The distance matters. The duration matters. The energy carried by
48:07
Speaker 1: the cosmic rays matters. And the way those particles interact
48:11
Speaker 1: with Earth's atmosphere matters. But one of the concerns scientists
48:15
Speaker 1: study is what intense cosmic radiation could do to atmospheric chemistry.
48:22
Speaker 1: High-energy radiation can change molecules in the upper atmosphere. It
48:26
Speaker 1: can create reactive forms of nitrogen and oxygen. These compounds
48:31
Speaker 1: can participate in chemical reactions that affect ozone, and ozone
48:35
Speaker 1: is important because it absorbs much of the sun's ultraviolet radiation.
48:40
Speaker 1: You can think of the ozone layer as part of
48:43
Speaker 1: the planet's invisible protective covering. It is not a solid roof.
48:48
Speaker 1: You would never feel it. You cannot see it from
48:50
Speaker 1: the ground. but its chemistry helps prevent a significant amount
48:55
Speaker 1: of the sun's most biologically damaging ultraviolet radiation from reaching
49:00
Speaker 1: Earth's surface. If a nearby cosmic event reduced the amount
49:04
Speaker 1: of ozone in the atmosphere, more ultraviolet radiation could reach
49:09
Speaker 1: the ground. And that would change the radiation environment experienced
49:13
Speaker 1: by living things– plants, animals, microorganisms– everything living near the
49:20
Speaker 1: surface would exist beneath a slightly different sky. The important
49:24
Speaker 1: thing is that this process would not necessarily happen all
49:28
Speaker 1: at once. The initial radiation from the explosion could arrive first.
49:33
Speaker 1: The atmosphere could respond afterward. Cosmic rays could continue arriving
49:37
Speaker 1: for a much longer period. The chemistry of the atmosphere
49:41
Speaker 1: could take time to change, and the biological consequences, if
49:45
Speaker 1: significant enough, could unfold gradually. This is another reason the
49:50
Speaker 1: story of a nearby supernova is less like a sudden
49:53
Speaker 1: disaster and more like a slow change in the environment.
49:58
Speaker 1: The universe would not need to shout. It could simply
50:01
Speaker 1: alter the number of particles arriving from above, and Earth's
50:05
Speaker 1: atmosphere would quietly respond. You can imagine yourself beneath that
50:09
Speaker 1: atmosphere now. You are lying still. The air around you
50:13
Speaker 1: is calm. Perhaps you can feel the gentle movement of
50:17
Speaker 1: a fan. Perhaps the room is cool. Perhaps there is
50:21
Speaker 1: almost no sound at all. Above you, however, even during
50:25
Speaker 1: this quiet moment, the atmosphere is busy. Molecules are moving.
50:30
Speaker 1: Particles are arriving from space. Energy is being absorbed and transformed.
50:36
Speaker 1: The planet is constantly receiving a faint stream of material
50:39
Speaker 1: from the universe. Most of it is harmless. Most of
50:43
Speaker 1: it never reaches you in any meaningful way. The atmosphere
50:47
Speaker 1: takes care of much of it without asking you to notice.
50:50
Speaker 1: And this is one of the comforting things about Earth's
50:53
Speaker 1: place in space. The planet is not simply sitting exposed
50:57
Speaker 1: in an empty universe. It has layers of protection. An atmosphere.
51:02
Speaker 1: A magnetic field. The ozone layer. and an enormous distance
51:07
Speaker 1: from most of the energetic events that occur among the stars.
51:11
Speaker 1: Even the supernovae that have happened in our galaxy throughout
51:14
Speaker 1: human history have overwhelmingly been far enough away to remain
51:19
Speaker 1: objects of observation rather than causes of concern. So when
51:23
Speaker 1: you imagine a star exploding near Earth, you do not
51:26
Speaker 1: need to imagine the sky suddenly falling. Instead, imagine something subtler,
51:32
Speaker 1: a change in the invisible environment above you. a little
51:35
Speaker 1: more energy arriving from space, a little more activity in
51:39
Speaker 1: the upper atmosphere, particles moving through layers of air, some
51:44
Speaker 1: of them disappearing high above the ground, others continuing downward,
51:49
Speaker 1: and among them, tiny muons quietly making their way toward
51:53
Speaker 1: the surface. You would not hear them, you would not
51:56
Speaker 1: feel them, you would simply continue resting beneath the sky,
52:01
Speaker 1: and somewhere above you, the atmosphere would continue doing what
52:05
Speaker 1: it has always done, quietly standing between life and the
52:09
Speaker 1: larger universe. But Earth has another remarkable way of remembering
52:13
Speaker 1: what happens beyond its atmosphere. Not only through the sky,
52:17
Speaker 1: not only through telescopes, sometimes the memory of a star
52:22
Speaker 1: can settle gently into the ground beneath the ocean, and
52:25
Speaker 1: those tiny traces can remain there for millions of years.
52:31
Speaker 1: The atmosphere above you is quiet again. Air moves gently
52:35
Speaker 1: around the planet. Particles arrive from space and disappear into
52:40
Speaker 1: the layers above. And beneath all of this, beneath the
52:43
Speaker 1: clouds and oceans and continents, Earth is keeping its own
52:47
Speaker 1: quiet record. You can imagine yourself descending very slowly now,
52:52
Speaker 1: away from the night sky, away from the thin blue atmosphere,
52:56
Speaker 1: down toward the deep Pacific Ocean. The surface of the
53:00
Speaker 1: water is far above you, Sunlight has long since faded.
53:04
Speaker 1: The water becomes darker and colder as you drift downward.
53:08
Speaker 1: There is almost nothing to see, only the slow movement
53:11
Speaker 1: of the ocean around you. And beneath you, layer after
53:15
Speaker 1: layer of sediment is resting on the seafloor. Tiny particles
53:19
Speaker 1: have been falling there for millions of years. Dust. Mineral grains.
53:25
Speaker 1: Fragments of microscopic life. Material carried gently through the ocean.
53:30
Speaker 1: Year after year, century after century, the layers accumulate. Very slowly.
53:37
Speaker 1: So slowly that a single human lifetime seems almost motionless
53:41
Speaker 1: beside them. But if you could somehow look closely enough,
53:45
Speaker 1: these layers would tell you stories about the world that
53:49
Speaker 1: existed long before you were here. And some of those
53:52
Speaker 1: stories may have come from the stars. Scientists have found
53:55
Speaker 1: an unusual form of iron in deep ocean sediments. It
54:00
Speaker 1: is called iron-60. You do not need to picture the
54:03
Speaker 1: symbol used by scientists to write its name. You can
54:07
Speaker 1: simply hear it as iron-60. It is a radioactive form
54:11
Speaker 1: of iron with a particular arrangement of tiny particles inside
54:15
Speaker 1: its atoms. What makes it interesting is not simply that
54:20
Speaker 1: it exists, it is where it came from. Iron-60 can
54:24
Speaker 1: be produced in massive stars and released when those stars die.
54:29
Speaker 1: When a nearby supernova sends material outward, tiny particles of
54:33
Speaker 1: that material can eventually travel through interstellar space. Some may
54:38
Speaker 1: find their way toward Earth. Some may settle into the atmosphere.
54:42
Speaker 1: Some may eventually become part of ocean sediments. And millions
54:46
Speaker 1: of years later, scientists can discover them. You can imagine
54:51
Speaker 1: one tiny grain resting quietly on the ocean floor. It
54:55
Speaker 1: looks like nothing. It is almost nothing. And yet …
54:59
Speaker 1: Inside it may be a microscopic trace of a star
55:02
Speaker 1: that disappeared millions of years ago. The star itself is gone.
55:07
Speaker 1: Its light has passed. Its outer layers have spread through space.
55:12
Speaker 1: But one small piece of its material has remained. Iron-60
55:16
Speaker 1: is especially useful because it is radioactive. Radioactive atoms change
55:21
Speaker 1: slowly over time. They do not all change at once. Instead,
55:26
Speaker 1: a portion changes... then another portion, and another, following a
55:30
Speaker 1: predictable pattern. The time required for roughly half of a
55:34
Speaker 1: radioactive sample to transform is called its half-life. For iron-60,
55:40
Speaker 1: that timescale is around 2.6 million years. That makes it
55:45
Speaker 1: useful as a kind of cosmic clock. Not a clock
55:48
Speaker 1: with hands, not a clock you can hear ticking, a
55:52
Speaker 1: clock hidden inside atoms. By measuring how much iron-60 remains,
55:57
Speaker 1: Scientists can begin to estimate when the material arrived, and
56:01
Speaker 1: when traces of it appear in sediments from different periods,
56:05
Speaker 1: a much older story begins to emerge. The evidence suggests
56:10
Speaker 1: that material from nearby stellar explosions reached Earth during a
56:14
Speaker 1: period roughly a few million years ago. One estimate places
56:19
Speaker 1: one of these possible supernova events at around 150 light-years away.
56:25
Speaker 1: That is still an enormous distance. Light itself needs about
56:30
Speaker 1: 150 years to cross it. For you, it would be
56:34
Speaker 1: unimaginably far. But compared with the size of the Milky Way,
56:39
Speaker 1: it is relatively nearby. And this is where the story
56:42
Speaker 1: becomes especially interesting. The same sediments have also been associated
56:47
Speaker 1: with another radioactive element called manganese-53. Again, you do not
56:53
Speaker 1: need to remember the chemical notations. Simply think of it
56:56
Speaker 1: as another tiny radioactive trace that can help scientists investigate
57:01
Speaker 1: the history of material arriving from space. Taken together, these
57:06
Speaker 1: isotopes provide evidence consistent with nearby stellar explosions. They do
57:11
Speaker 1: not give us a perfect photograph of the star. They
57:14
Speaker 1: do not tell us every detail of the explosion. And
57:17
Speaker 1: they do not allow scientists to point to one exact
57:20
Speaker 1: star and say, with complete certainty, that this was the source. Instead,
57:26
Speaker 1: they are clues, very small clues, the kind of clues
57:30
Speaker 1: that become meaningful only when they are collected carefully. And
57:35
Speaker 1: because these events happened millions of years ago, there is
57:38
Speaker 1: still uncertainty about their exact timing, distance, and effects on life.
57:44
Speaker 1: Some researchers have wondered whether increased cosmic radiation from a
57:48
Speaker 1: nearby supernova could have contributed to changes in Earth's environment
57:52
Speaker 1: during roughly the same period. One possible pathway involves cosmic
57:57
Speaker 1: rays entering the atmosphere, producing secondary particles such as muons,
58:03
Speaker 1: and increasing radiation exposure at the surface. But this remains
58:07
Speaker 1: a hypothesis. The isotopes tell us that material associated with
58:12
Speaker 1: stellar explosions reached Earth. They do not by themselves prove
58:17
Speaker 1: that a supernova caused an extinction or major ecological transition.
58:21
Speaker 1: That distinction matters. Science often works this way. A tiny
58:26
Speaker 1: piece of evidence does not arrive carrying a complete explanation. Instead,
58:31
Speaker 1: it becomes one quiet piece of a much larger picture.
58:36
Speaker 1: And there is another part of that picture surrounding our
58:38
Speaker 1: solar system. You can imagine rising slowly again, leaving the
58:43
Speaker 1: dark ocean behind, passing through the atmosphere, and returning to
58:47
Speaker 1: the region between the stars. The solar system appears to
58:51
Speaker 1: sit inside an enormous region of unusually thin interstellar material.
58:56
Speaker 1: This region is known as the local bubble. It is
58:59
Speaker 1: a large, low-density cavity surrounding the Sun and many nearby stars.
59:05
Speaker 1: Compared with the ordinary interstellar environment, there is much less
59:09
Speaker 1: gap inside this region. Scientists have proposed that repeated supernovae
59:15
Speaker 1: may have helped create it. Long ago, several massive stars
59:19
Speaker 1: may have ended their lives in our stellar neighbourhood. Their
59:22
Speaker 1: explosions would have sent waves of energy and matter outward.
59:27
Speaker 1: Over time, Repeated explosions could have pushed much of the
59:31
Speaker 1: surrounding gas away, leaving behind a large, sparse region. So
59:36
Speaker 1: the space around you may not be empty by accident.
59:40
Speaker 1: It may partly reflect the history of stars that lived
59:43
Speaker 1: and died before the Sun was born. The solar system
59:46
Speaker 1: itself is only a small island inside this much larger structure,
59:51
Speaker 1: and the remarkable thing is that the evidence is not
59:53
Speaker 1: necessarily something dramatic. There is no giant scar across the sky.
59:59
Speaker 1: No glowing mark remains where those ancient stars once existed. Instead,
1:00:05
Speaker 1: there are subtle traces. A radioactive atom in an ocean sediment.
1:00:10
Speaker 1: A faint pattern in the surrounding interstellar gas. A large
1:00:14
Speaker 1: region of unusually thin space. Tiny pieces of evidence spread
1:00:20
Speaker 1: across enormous stretches of time. You can let yourself rest
1:00:24
Speaker 1: with that image. Deep beneath the Pacific, a layer of
1:00:28
Speaker 1: sediment holds an atom of iron. Above it, the ocean
1:00:32
Speaker 1: moves gently. Above the ocean, the atmosphere continues its quiet work.
1:00:38
Speaker 1: Above the atmosphere, the solar system drifts through a sparse
1:00:42
Speaker 1: region of space. And far beyond it, the galaxy continues
1:00:46
Speaker 1: to carry countless stars along its slow journey. Some stars
1:00:50
Speaker 1: are being born. Some are ageing. Some have already disappeared.
1:00:55
Speaker 1: And every so often, one of them leaves a trace
1:00:58
Speaker 1: that can travel farther than its light, farther than its
1:01:01
Speaker 1: expanding cloud, and farther than the lifetime of any human being.
1:01:06
Speaker 1: A tiny atom can remember, not perfectly, not completely, but
1:01:11
Speaker 1: enough to tell you that the stars have reached Earth before.
1:01:15
Speaker 1: And perhaps the most interesting question is not only what
1:01:18
Speaker 1: a dying star can do to a world that already exists,
1:01:22
Speaker 1: It is what that same dying star might leave behind
1:01:26
Speaker 1: for a world that has not yet been born. For
1:01:31
Speaker 1: a moment, you can remain inside the darkness between the stars.
1:01:35
Speaker 1: There is almost nothing around you. A little gas, a
1:01:39
Speaker 1: little dust, a few wandering particles, and enormous distances in
1:01:44
Speaker 1: every direction. It can seem as though space is empty,
1:01:47
Speaker 1: but it is not completely empty. The material between the
1:01:51
Speaker 1: stars is slowly moving, slowly mixing, slowly gathering, and sometimes,
1:01:57
Speaker 1: after long periods of quiet, beginning the process that will
1:02:01
Speaker 1: create another star. This is where the story of a
1:02:04
Speaker 1: supernova begins to become something gentler, because a dying star
1:02:09
Speaker 1: does not only send energy outward, it also sends material.
1:02:14
Speaker 1: During its life, a massive star has created new elements
1:02:18
Speaker 1: inside itself. Some of the atoms that make up the
1:02:21
Speaker 1: star were already present when it was born. Others were
1:02:25
Speaker 1: built gradually inside its hot interior. And when the star dies,
1:02:30
Speaker 1: some of this material is returned to space. The explosion
1:02:34
Speaker 1: becomes a kind of cosmic recycling. A star is born
1:02:38
Speaker 1: from older material. It lives. It changes that material. And
1:02:44
Speaker 1: when it dies... Some of what it created becomes part
1:02:47
Speaker 1: of the environment from which other stars may eventually form.
1:02:51
Speaker 1: You can imagine a vast cloud drifting through the galaxy.
1:02:56
Speaker 1: It is cold compared with the interior of a star.
1:02:59
Speaker 1: It is thin. It is quiet. But inside this cloud
1:03:04
Speaker 1: are countless particles, each moving gently, each carrying a tiny
1:03:09
Speaker 1: part of the history of the galaxy. Some atoms are
1:03:12
Speaker 1: very simple. Others are heavier. Some may have been created
1:03:16
Speaker 1: inside stars that disappeared long before the Sun existed. And
1:03:21
Speaker 1: among these particles, there can be material from supernova explosions.
1:03:26
Speaker 1: This is one reason stellar explosions matter so much to
1:03:29
Speaker 1: the chemistry of the universe. The early universe was dominated
1:03:33
Speaker 1: mostly by the lightest elements. Hydrogen was abundant. Helium was abundant.
1:03:40
Speaker 1: There were only small amounts of heavier elements. But generations
1:03:44
Speaker 1: of stars gradually changed that. Stars became places where new
1:03:48
Speaker 1: elements could be created. Then stellar winds and explosions returned
1:03:53
Speaker 1: some of that material to space. Later, stars formed from
1:03:57
Speaker 1: this increasingly enriched environment. And eventually, much later, one of
1:04:02
Speaker 1: those stars was the sun. Around the young sun, a
1:04:06
Speaker 1: disk of gas and dust began to rotate. Tiny grains collided.
1:04:12
Speaker 1: Some stuck together. Larger pieces formed. Those pieces gathered into
1:04:17
Speaker 1: still larger bodies. Over enormous stretches of time, planets emerged.
1:04:23
Speaker 1: Earth became one of them. And the atoms that eventually
1:04:26
Speaker 1: became part of Earth did not all begin with Earth.
1:04:30
Speaker 1: They came from an older cosmic history. Some were forged
1:04:34
Speaker 1: inside ancient stars. Some were released when those stars died.
1:04:39
Speaker 1: Some became part of the cloud. from which the solar
1:04:42
Speaker 1: system formed, and some eventually became part of you. You
1:04:47
Speaker 1: can let that thought become very simple. The atoms in
1:04:50
Speaker 1: your body have a history older than your planet. Some
1:04:53
Speaker 1: of them are older than the sun. They travelled through
1:04:56
Speaker 1: space before becoming part of the world beneath you. They
1:05:00
Speaker 1: may have passed through stars. They may have been scattered
1:05:03
Speaker 1: through stellar winds. They may have rested inside clouds of
1:05:07
Speaker 1: gas and dust for millions or billions of years. And eventually,
1:05:12
Speaker 1: they became part of a planet. Perhaps part of a stone.
1:05:16
Speaker 1: Perhaps part of an ocean. Perhaps part of a leaf.
1:05:20
Speaker 1: Perhaps part of you. This is not a metaphor in
1:05:23
Speaker 1: the usual sense. It is a quiet description of cosmic history.
1:05:28
Speaker 1: The material of Earth came from an older universe. And
1:05:32
Speaker 1: some of that material was created by stars. There is
1:05:35
Speaker 1: another possibility that makes nearby supernovae even more interesting. A
1:05:40
Speaker 1: stellar explosion may sometimes help disturb a nearby cloud of
1:05:44
Speaker 1: gas and dust. The expanding shock from a supernova can
1:05:49
Speaker 1: move through interstellar space. When it reaches a dense cloud,
1:05:53
Speaker 1: it can compress parts of that cloud. And if the
1:05:56
Speaker 1: conditions are right, the compression can help gravity begin to
1:06:00
Speaker 1: gather the material more strongly. The cloud slowly collapses. It
1:06:05
Speaker 1: becomes denser. It begins to rotate. The centre becomes warmer.
1:06:11
Speaker 1: And eventually, a new star can begin to form. You
1:06:14
Speaker 1: can imagine this happening very slowly. No sudden beginning. No
1:06:19
Speaker 1: dramatic moment. Just a cloud becoming slightly denser. Then slightly
1:06:24
Speaker 1: denser again. Dust drifting inward. Gas gathering. Gravity quietly collecting
1:06:31
Speaker 1: what was already there. Perhaps this happened in the environment
1:06:34
Speaker 1: where our own sun was born. Scientists have found evidence
1:06:38
Speaker 1: in meteorites that the young solar system contained certain short-lived
1:06:42
Speaker 1: radioactive materials. Some of these materials could have been supplied
1:06:47
Speaker 1: by a nearby massive star or supernova shortly before the
1:06:51
Speaker 1: solar system formed. The evidence is complex and scientists continue
1:06:56
Speaker 1: to discuss exactly how those materials arrived. But the broader
1:07:00
Speaker 1: picture is beautiful. The solar system may have formed in
1:07:04
Speaker 1: a region that had recently been touched by the lives
1:07:07
Speaker 1: and deaths of massive stars. The same kind of event
1:07:11
Speaker 1: that could threaten life on an established planet may also
1:07:14
Speaker 1: have helped provide material to a cloud from which a
1:07:18
Speaker 1: new planetary system could emerge. Destruction and creation are not
1:07:23
Speaker 1: always separate stories in the universe. Sometimes they are different
1:07:27
Speaker 1: moments in the same cycle. A star burns, a star
1:07:32
Speaker 1: grows old, A star releases its material. That material enters
1:07:37
Speaker 1: the space between the stars. The material mixes. A cloud gathers.
1:07:42
Speaker 1: Gravity draws it inward. A new star appears. Around it,
1:07:47
Speaker 1: planets form. And eventually, on one small planet, chemistry becomes
1:07:52
Speaker 1: complicated enough for life to emerge. You do not need
1:07:56
Speaker 1: to imagine the universe as having a plan. There is
1:07:59
Speaker 1: no need for a destination. These things can simply happen
1:08:03
Speaker 1: through the ordinary laws of physics, repeated across enormous periods
1:08:07
Speaker 1: of time. Stars form. Stars live. Stars die. Their material moves.
1:08:14
Speaker 1: And stars form again. The galaxy is continually recycling itself.
1:08:20
Speaker 1: The space around you is not merely an empty background.
1:08:23
Speaker 1: It is a place where matter is always travelling. Some
1:08:27
Speaker 1: particles are moving away from old stars. Others are gathering
1:08:31
Speaker 1: toward new ones. Some have been part of worlds that
1:08:35
Speaker 1: disappeared long ago. Others are waiting inside cold clouds for
1:08:40
Speaker 1: a future that may be billions of years away. And
1:08:43
Speaker 1: somewhere in this enormous process, the sun appeared. Then earth.
1:08:49
Speaker 1: Then oceans. Then the long, unfolding history of life. So
1:08:53
Speaker 1: when you return to the original question, when you quietly
1:08:57
Speaker 1: ask yourself, what would happen if a star exploded near Earth?
1:09:01
Speaker 1: The answer becomes more than a story about danger. Yes,
1:09:06
Speaker 1: a sufficiently close stellar explosion could change the radiation environment
1:09:10
Speaker 1: around our planet. It could disturb the atmosphere. It could
1:09:14
Speaker 1: alter the chemistry of the ozone layer. Its particles could
1:09:18
Speaker 1: reach Earth, Its light could brighten the sky. But the
1:09:22
Speaker 1: deeper story is much older. Stars have been exploding long
1:09:27
Speaker 1: before Earth existed. Their deaths helped scatter the elements from
1:09:31
Speaker 1: which later generations of stars and planets were built. Some
1:09:35
Speaker 1: ancient supernovae may have passed close enough to leave traces
1:09:38
Speaker 1: on Earth. Some may have helped shape the interstellar neighbourhood
1:09:42
Speaker 1: around the solar system. And some of the material released
1:09:45
Speaker 1: by ancient stars may have eventually become part of the
1:09:49
Speaker 1: world where you are resting now. You can let yourself
1:09:52
Speaker 1: drift with that idea. You are beneath a quiet atmosphere.
1:09:57
Speaker 1: Above you the stars continue shining. Far beyond them clouds
1:10:02
Speaker 1: of gas and dust continue moving. Somewhere a star is
1:10:06
Speaker 1: being born. Somewhere else another is slowly approaching its end.
1:10:11
Speaker 1: And between them Matter continues its patient journey. Nothing needs
1:10:16
Speaker 1: to happen tonight. There is nothing for you to watch,
1:10:20
Speaker 1: nothing for you to solve. You are safe, you can
1:10:23
Speaker 1: simply listen and drift. Because the universe has been carrying
1:10:28
Speaker 1: this quiet cycle for billions of years, stars becoming clouds,
1:10:33
Speaker 1: clouds becoming stars, old elements becoming new worlds, and perhaps,
1:10:39
Speaker 1: very quietly, the atoms around you are still carrying the
1:10:43
Speaker 1: memory of stars that exploded long before anyone was here
1:10:47
Speaker 1: to see them. And so, our journey drifts quietly to
1:10:54
Speaker 1: its end. For a little while, you have travelled through
1:10:57
Speaker 1: the life of a massive star, from its long and
1:11:01
Speaker 1: gentle balance between gravity and pressure, through the quiet building
1:11:05
Speaker 1: of heavier elements. toward the moment when an iron heart
1:11:09
Speaker 1: could no longer support the enormous weight above it. You
1:11:13
Speaker 1: have drifted alongside neutrinos as they escaped the collapsing star,
1:11:17
Speaker 1: passing almost silently through matter, carrying their invisible message across
1:11:22
Speaker 1: the darkness before the visible light could arrive. You have watched,
1:11:27
Speaker 1: in imagination, as the star released its outer layers into space,
1:11:32
Speaker 1: leaving behind a dense neutron star, or perhaps... under different circumstances,
1:11:38
Speaker 1: a black hole. You have wandered through the faint traces
1:11:42
Speaker 1: left in Earth's ancient sediments, tiny atoms resting quietly beneath
1:11:47
Speaker 1: an old ocean, carrying evidence that stellar explosions may once
1:11:51
Speaker 1: have passed relatively close to our planet. And you have returned, slowly,
1:11:57
Speaker 1: to the beginning of another story. The material of an
1:12:01
Speaker 1: old star becoming part of a cloud, a cloud becoming
1:12:04
Speaker 1: a new star, dust gathering into planets, ancient elements becoming oceans, rocks,
1:12:11
Speaker 1: living things, and eventually you. So perhaps a star's death
1:12:15
Speaker 1: is never simply an ending. Perhaps it is also a
1:12:18
Speaker 1: quiet change of form. The light fades. The cloud expands.
1:12:24
Speaker 1: The particles continue drifting. And somewhere, much later, those same
1:12:29
Speaker 1: particles may become part of something entirely new. You do
1:12:33
Speaker 1: not need to think about any of this now. There
1:12:36
Speaker 1: is nothing to solve. There is nowhere you need to go.
1:12:39
Speaker 1: You are safe. Just listen and drift. Above you, the
1:12:44
Speaker 1: atmosphere continues its quiet work. Beyond it, the planets continue
1:12:49
Speaker 1: their slow movements. Farther still, the stars continue shining. Some
1:12:55
Speaker 1: are young, some are old. Some are quietly changing inside.
1:13:00
Speaker 1: And somewhere, in the enormous darkness, a massive star appears.
1:13:05
Speaker 1: may be approaching the end of its own long journey.
1:13:08
Speaker 1: But tonight, you are here, resting beneath the same universe
1:13:12
Speaker 1: that has existed through countless generations of stars. The night
1:13:17
Speaker 1: around you can remain quiet, the room can become softer,
1:13:21
Speaker 1: your breathing can become slower, and the distant stars can
1:13:25
Speaker 1: simply continue without asking anything from you. Whether you're still
1:13:30
Speaker 1: listening beneath the glow of your screen, or already floating
1:13:33
Speaker 1: somewhere between dreams and distant stars, thank you for travelling
1:13:37
Speaker 1: here tonight, through silent stellar interiors, invisible particles, ancient oceans,
1:13:44
Speaker 1: and the quiet processes that connect dying stars with newly
1:13:48
Speaker 1: forming worlds. If this wandering brought you a little calm,
1:13:52
Speaker 1: you're welcome to subscribe, or share it with someone who
1:13:55
Speaker 1: could use a gentler night beneath the cosmos. For now,
1:13:59
Speaker 1: rest well. The stars will keep burning. The planets will
1:14:03
Speaker 1: keep turning, the galaxies will keep drifting, and the universe
1:14:08
Speaker 1: will carry on softly around you while you sleep. You
1:14:11
Speaker 1: can let the questions become quieter now. Let the stars
1:14:15
Speaker 1: remain distant, let the darkness remain gentle, and somewhere beyond
1:14:19
Speaker 1: the edge of what you can see, matter continues its
1:14:22
Speaker 1: slow journey through space, carrying the memory of ancient stars,
1:14:27
Speaker 1: gathering patiently into things that have not yet begun. You
1:14:30
Speaker 1: do not need to follow it. You can simply rest,
1:14:34
Speaker 1: and let the universe continue without you for a while, quietly, slowly, softly.