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How Empty Is the Universe_ Really · Boring Science

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Speaker 1: Hello, and welcome to Boring Science to Sleep, a quiet
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place where science becomes slow, soft, and easy to drift through. Tonight,
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you're going to gently explore the distant worlds and enormous
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spaces beyond our familiar sky, not to learn everything, but
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simply to notice it slowly, calmly, one quiet idea at
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a time. But before you settle in, if you truly
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enjoy these calm journeys through interesting facts about science and
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the universe, You're welcome to like this video and subscribe,
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only if it feels right for you. And when you're ready,
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let me know in the comments where you're listening from
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and what time it is right now. It's always a
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quiet joy to see how many places and moments gather
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here together. Now, dim the lights, perhaps let a fan
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hum softly in the background, allow your body to become
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comfortable and let your thoughts become a little lighter. You
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are safe, there is nothing to do you can simply
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listen and drift. For a moment, you can remain exactly
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where you are, beneath the familiar sky, on the familiar earth,
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while somewhere far above you, the universe continues its slow
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and almost silent movement. And gradually, without needing to hurry,
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you can imagine yourself rising, not quickly, not dramatically, just
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gently enough to leave the ground behind. Below you, the
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room becomes smaller, then the building, then the streets and lights,
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until the whole landscape begins to soften into a quiet
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pattern beneath you. The earth appears, blue oceans, pale clouds,
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dark continents, a thin atmosphere glowing softly around the edge.
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From here, the planet does not look busy anymore. The
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roads disappear, the buildings disappear, the borders between countries disappear.
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You can simply see a small world turning slowly in
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the darkness, carrying everything you know, everything familiar, everything that
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has ever felt like home. And for a little while,
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you can stay there. You don't need to leave immediately.
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You can simply watch the earth turning beneath you, quietly
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illuminated by the sun. Then you begin to move farther away.
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The moon passes behind you. The earth becomes smaller. The
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sun remains bright, But even the Sun begins to look
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different as the distance grows. You are entering the solar
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system from the outside, looking back toward the place where
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all of its familiar planets travel. There are worlds here
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that you know by name. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
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But the planets themselves occupy only a small inner portion
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of the enormous space belonging to the Sun. Beyond Neptune,
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the solar system does not simply end. It becomes quieter,
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much quieter. There are distant worlds of ice and rock,
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small frozen bodies moving through deep darkness, remnants from the
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early history of the planetary system. Some travel on long,
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slow paths around the sun. Some are so distant that
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sunlight has become only a faint suggestion. And farther still,
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Scientists expect an enormous, loosely scattered population of icy objects
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surrounding the solar system in a distant region known as
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the Oort Cloud. You can think of it not as
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a solid shell and not as a wall, but as
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an unimaginably large and gentle outskirts where small frozen objects
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may remain separated by enormous amounts of empty space. The
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solar system stretches much farther than the neat diagram you
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may remember from school. The planets are only the familiar centre.
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Beyond them, there is distance, then more distance, and then
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still more. Even the farthest known regions of our solar
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system are tiny compared with the space between stars. You
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continue outward. The sun becomes smaller. Its light becomes softer.
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The planets disappear one by one behind you. until they
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are no longer visible at all. And now, something changes.
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You are no longer simply travelling through the neighbourhood of planets.
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You are beginning to enter interstellar space, the great region
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between one star system and another. There is no obvious doorway,
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no boundary that you can see, no sign telling you
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that you have crossed from one place into another. The
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transition is almost impossibly gentle. The sun simply becomes another
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star behind you, and the darkness around you grows. If
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you could measure this distance using the language of light,
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you would use a light year, which is simply the
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distance that light travels during one year. It sounds like
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a measurement of time, but it is really a measurement
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of distance. And light moves very quickly, yet even light
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needs years to cross the space between some stars. That
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is how wide the quiet becomes. You continue drifting, The
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sun is now only one point among many. Around you,
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other stars begin to stand out. Some are brighter, some
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are fainter, some are larger than the sun, some are
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much smaller, and some are so close by cosmic standards
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that they belong to our immediate stellar neighbourhood. One of
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them is Proxima Centauri, a small red star more than
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four light-years away, four years for light. and much, much
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longer for anything moving at the speeds available to you today.
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You do not need to imagine crossing that distance. You
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only need to notice it. A star can be relatively close,
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and still be separated from you by an ocean of darkness.
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That is one of the quiet truths of space. Things
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can be near each other, and yet unimaginably far apart.
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The sun behind you. Proxima Centauri somewhere ahead. and between
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them almost nothing. Not absolutely nothing. There are tiny particles
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of gas and dust. There are faint magnetic fields. There
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are traces of ancient material drifting between stars. But compared
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with the spaces we know on Earth, the emptiness is immense.
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And as you continue outward, you may begin to notice
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that the darkness itself has become part of the journey.
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There is no need to fill it. There is nothing missing.
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The space between the stars is simply there, quiet and patient,
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holding the distant lights apart. You are safe. There is
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nothing to do. The sun can remain behind you. The
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stars can remain far ahead. And for now, you can
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simply float between them, listening to the quiet distance, while
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somewhere in that darkness, another small star waits to become visible.
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The small star ahead of you is Proxima Centauri, glowing
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faintly against the darkness, and as you drift toward it,
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you begin to notice something quietly remarkable. It is not
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a bright star like the Sun. It is smaller, cooler,
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and much fainter, a red dwarf star, one of the
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many small stars that make up much of the stellar
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population around us. Its light has a softer, deeper colour,
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and from the darkness between the stars, it almost seems
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less like a distant sun and more like a tiny
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ember floating in an enormous room. You do not need
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to hurry toward it. You can simply watch it grow,
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very slowly, as the darkness between you and the star
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becomes a little smaller. Proxima Centauri belongs to the same
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broad stellar neighbourhood as two other stars, Alpha Centauri A
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and Alpha Centauri B. Together, the three stars form our
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nearest stellar system, a small gathering of suns, compared with
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the enormous number of stars spread throughout the Milky Way.
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And somewhere around Proxima Centauri, there are worlds. One of
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them is known as Proxima Centauri b. The name is technical,
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but the world itself is easier to imagine. It is
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a planet moving around a small red star, travelling through
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a darkness that is very different from the darkness around Earth.
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Because Proxima Centauri is so much cooler and dimmer than
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the Sun, the planet needs to orbit much closer to
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its star to receive a similar amount of warmth. And
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that closeness creates a strange possibility. Over immense stretches of time,
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the gravitational pull of the star may have slowed the
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planet's rotation until one side continually faces the star, while
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the other side continually turns away. Astronomers call this tidal locking,
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but you can simply imagine a world that has slowly
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settled into one particular orientation, one face always turned toward
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its distant red sun. On one side, the star would
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remain in almost the same part of the sky. On
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the other side, darkness would stretch across the surface. Between
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these two regions there could be a broad transition. a
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place where permanent day and permanent night meet. Whether Proxima
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Centauri b actually has an atmosphere, and what that atmosphere
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might be like, remains uncertain. The star itself can produce
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powerful bursts of activity, releasing energetic radiation into the space
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around it. So even though the planet receives a potentially
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interesting amount of warmth, that does not automatically mean it
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is comfortable, or Earth-like, or even able to hold onto
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an atmosphere for a long time. This is one of
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the quiet lessons of distant worlds. A planet can be
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in a place where temperatures seem promising, and still be
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very different from Earth. Distance from a star is only
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one part of the story. The atmosphere matters. The star matters.
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The planet's history matters. Even the way the planet turns
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can matter. And so you continue drifting, without needing to
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decide what this distant world is like. You simply let
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it remain a possibility in the darkness. Farther away, another
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nearby star waits. Barnard's Star is an old, small red dwarf,
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moving through the sky relatively quickly compared with many of
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the stars around it. It is only a few light
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years from the sun, yet that simple phrase, a few
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light years, still represents an enormous distance. The light reaching
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you from Barnard's Star began its journey years before you
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saw it, and around this old star, astronomers have found
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evidence for a planet. It is thought to be a
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cold world, much farther from its star than Proxima Centauri
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b is from its own. There have even been ideas
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that such a planet might hide warmth deep beneath its surface,
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perhaps in an underground environment where pressure and internal heat
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behave very differently from anything you experience on Earth. Again,
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you do not need to imagine oceans beneath the ground.
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You only need to let the possibility drift quietly through
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your thoughts. A cold world, A distant star, a little
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warmth hidden somewhere beneath the surface. And then, almost without noticing,
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you move toward another familiar point of light. Sirius. From Earth,
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Sirius is the brightest star in our night sky. It
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appears bright and almost solitary, but it is actually part
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of a binary system. There are two stars there. Sirius
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A is the brighter one, while Sirius B is a
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much smaller white dwarf. A white dwarf is what remains
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after a star has passed through much of its life
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and released its outer layers, leaving behind a compact stellar core.
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The word white makes it sound gentle, and in a
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way the light can be beautiful, but the object itself
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is remarkably dense. A great deal of stellar material has
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been compressed into a tiny volume, leaving behind a remnant
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that is much smaller than an ordinary star. You can
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imagine the two stars together. one bright and familiar, the
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other small and compact, both moving quietly through the darkness.
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Nothing about the scene needs to be dramatic. Stars can change,
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stars can age, stars can leave behind remnants, and still,
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from far away, the whole process looks almost motionless. That
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is one of the strange comforts of astronomy. Events that
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would be enormous on a human scale can become almost
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invisible when stretched across millions or billions of years. You
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can watch them without needing to see them happen. And
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as you drift farther through this nearby stellar neighbourhood, more
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worlds begin to appear in your imagination. Some are rocky.
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Some may be wrapped in thick atmospheres. Some may be frozen.
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Some may be warmer. Some may have moons. Some may
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have nothing resembling the familiar landscapes of Earth. The stars
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themselves are different too. Small red dwarfs, bright stars, old stars,
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dense stellar remnants, and around them, planets following their own
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quiet paths. The more stars you encounter, the less useful
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it becomes to think of planets as variations of one
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single pattern. There is no single way for a world
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to exist. There are countless arrangements of gravity, light, temperature, atmosphere,
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and time. And somewhere ahead, beyond these nearest stars, the
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number of known worlds begins to grow. Not just a few,
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not just dozens, but thousands of planets discovered around other stars,
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each one offering another small reminder that the universe does
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not need to repeat the same design. For now, though,
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you can remain among the nearest stars. The sun is
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far behind you. Proxima Centauri is becoming smaller again, Barnard's
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star fades into the surrounding darkness. Sirius remains a bright
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point somewhere behind and to one side. And ahead, there
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are still more worlds waiting quietly in the distance. You
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do not need to visit all of them. There is
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no destination you must reach. You can simply drift onward,
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knowing that beyond these familiar neighbouring stars are planets where
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oceans may hide beneath ice, where rock may glow beneath
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permanent daylight, where even the atmosphere itself can become part
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of the story. The darkness ahead is still very large,
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and somewhere within it another kind of world is beginning
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to appear. The darkness ahead is still very large, and
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somewhere within it another kind of world is beginning to appear.
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You have already passed the nearest stars, the quiet red
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dwarfs and the brighter points of light, but now you
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can begin to notice something even more interesting. The stars
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are not alone. Many of them have worlds moving quietly
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around them, just as Earth moves around the Sun. These
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worlds are called exoplanets, a simple word for planets that
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belong to other stars. You do not see most of
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them directly. They are usually much too small and too faint,
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especially beside the brilliant stars they orbit. Instead, you notice
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their presence through tiny changes. A planet may pass in
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front of its star, causing the star's light to become
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just slightly dimmer, or the gravity of an unseen planet
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may make its star move almost imperceptibly, back and forth,
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as though the two are gently circling an invisible balance point.
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These changes are small, very small, and yet, from across
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the darkness between stars, they can reveal another world. As
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you drift onward, one of these worlds begins to take
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shape in your imagination. It is called GJ 1214b. The
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name is rather dry, almost like an entry in a catalogue,
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but the world itself may be anything but ordinary. It
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has been studied as a possible ocean-rich planet, a world
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where water may make up a substantial part of its
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interior and and this is where water begins to behave
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in ways that feel almost unfamiliar. On Earth, water is
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usually something you know as a liquid, a gentle flow
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across a surface, or as ice when temperatures fall low enough.
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But deep inside a large water-rich planet, pressure can become
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so enormous that water is forced into completely different arrangements.
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It can become dense forms of ice, have little resemblance
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to the ice floating in a glass of water. The
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strange part is that these forms do not necessarily require
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extreme cold. They can be created by pressure. Water can
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be squeezed so tightly that its molecules settle into structures
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that simply do not occur in ordinary environments on Earth.
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You can imagine descending slowly into such a world. There
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is no need to picture a hard boundary between ocean
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and ice. Instead, The deeper you go, the greater the
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pressure becomes, and the familiar behaviour of water gradually gives
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way to something more unusual. Liquid, then denser material, then
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forms of ice created not by winter, but by enormous pressure.
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The world becomes a quiet experiment in physics, performed naturally,
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without a laboratory, without a machine, simply because the planet
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is large enough and heavy enough to squeeze its own interior.
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And somewhere beyond this water-rich world is another planet with
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an entirely different story, K2141b. It orbits extremely close to
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its star, so close that a complete year there passes
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in only a matter of hours. The planet is probably
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tidally locked, meaning that it may always show the same
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face to its star. One side remains beneath almost continuous starlight.
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the other side remains turned away. And between these two
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extremes is a world of remarkable contrast. The day side
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can become hot enough for rock to melt, not just
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a little warmer than the rocks beneath your feet on
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a summer afternoon, hot enough that the surface may become
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a vast ocean of molten rock, a lava ocean. You
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can imagine the landscape from very far away, without trying
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to stand upon it, a dark world, one side glowing
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beneath its star, the surface softened into molten material, the
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opposite side falling into deep cold, and above the hot
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region some of the material from the surface may rise
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into the atmosphere. Rock itself can be part of the air.
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Minerals can vaporise under the heat, rising from the glowing
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surface and moving through the thin atmosphere. Then, as these
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materials travel toward the cooler side of the planet, they
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can begin to condense again. The atmosphere becomes part of
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a slow planetary circulation. Material rises, it moves, it cools,
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it falls, and eventually some of it may return toward
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the hotter region, where it can melt or vaporise once more.
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The entire planet becomes a quiet cycle of heat and cooling,
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a world with no familiar ocean of water, but perhaps
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an ocean of rock, a world where the weather may
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be made partly from minerals and a world where the
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difference between ground and sky becomes less certain. And yet,
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even here, the same simple rules are still at work.
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Gravity holds the planet together. The star provides energy. The
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planet absorbs some of that energy. Heat moves from one
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region toward another. Material changes its state. Nothing needs to
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be forced into place. The planet simply follows the conditions
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it has been given. And that is perhaps one of
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the most beautiful things about these distant worlds. They do
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not need to resemble Earth in order to make sense.
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They only need gravity, matter, energy and time. Change the
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distance from the star and the temperature changes. Change the
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size of the planet and the pressure changes. Change the
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atmosphere and heat moves differently. Change the rotation and the
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pattern of day and night changes. And slowly... a completely
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different world appears. Somewhere, water may cover a planet. Somewhere else,
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water may be trapped beneath immense pressure. Somewhere, rock may
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glow like a furnace. Somewhere, an atmosphere may carry vaporized
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minerals through the sky. And farther away, there are worlds
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that we have not yet seen clearly enough to understand.
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You can let those possibilities remain in the distance. There
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is no need to name them all, There is no
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need to remember every planet. You can simply notice how
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many different ways a world can exist. The universe does
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not seem to have one preferred design. It has variations.
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Endless variations. Small worlds and large worlds. Warm worlds and
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frozen worlds. Worlds wrapped in thick atmospheres. Worlds perhaps stripped
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almost bare. Worlds where oceans may lie beneath ice. worlds
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where oceans may glow with heat, and each one is
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quietly moving through space, following its own path around its
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own star. For a moment, you can drift beside them.
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You do not need to land. You do not need
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to understand everything below the clouds. You can simply watch
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the distant star overhead, the faint curve of a world
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beneath it, and the slow movement of heat across a
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surface that no human has ever touched. Then, as you
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continue outward, the planets begin to fade behind you once more.
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The stars remain. The darkness remains. And beyond these strange worlds,
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another story is waiting, not about planets this time, but
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about the stars themselves and the long lives they quietly
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carry through the darkness. The strange worlds behind you are
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beginning to fade into the darkness, and as you continue
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drifting outward... your attention slowly moves away from the planets
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and back toward the stars that gave them their light.
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For a moment, you can simply watch them, small points
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of light, some young, some old, some bright and restless,
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others quiet and fading. From a great distance, they all
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seem almost the same. But each star carries its own history.
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A star does not simply appear and remain unchanged forever.
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It begins somewhere, usually deep inside a cold cloud of
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gas and dust, where enormous amounts of material are gathered
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together in the darkness. Very slowly, gravity begins to draw
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some of that material inward. There is no sudden moment
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that you would see from far away. The cloud simply
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becomes a little denser, then denser again, the material gathers,
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the centre becomes warmer, and eventually, a young star begins
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to take shape. You can imagine this as a very
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slow gathering of light, almost as though the darkness itself
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were quietly folding inward. Around the young star, clouds of
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dust and gas may still remain. The star is not
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finished becoming itself. It is still surrounded by the material
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from which it was born. And somewhere in the nearby
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darkness is a young star called AP Columbae. It is
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only a small part of the enormous stellar population of
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the Milky Way, but it offers scientists a glimpse into
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the early stages of a star's life. One of the
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clues to its youth comes from lithium. Lithium is a
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light chemical element, but inside a young star it can
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tell a quiet story about age. As stars mature, their
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internal conditions change, and the amount of lithium that remains
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can help astronomers recognise a star that has only recently
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begun its long journey. AP Columbiae is also an active star.
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Its brightness can change. Its surface can release material. Its
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young energy can create powerful bursts that would make the
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space around it a restless place for any nearby planet.
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But from far away, all of this still looks almost peaceful.
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A small star, a faint glow, a little flicker of brightness.
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and then darkness again. You can let the young star
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remain behind you as you continue. The universe does not
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hurry its stars. A young star may spend enormous stretches
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of time changing very slowly, while the planets and clouds
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around it move through their own quiet histories. Eventually the
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star begins to age. Hydrogen deep inside it is gradually
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used as fuel. The star changes from within. Its inner
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regions become different. its outer layers begin to expand, and
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after enough time, a star that once looked relatively ordinary
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can become something enormous, a red giant. Ahead of you
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now is one such star, CW Leonis. It is hundreds
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of light-years away, and it is so large that if
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you could place it where the Sun is now, its
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outer layers would extend far beyond the inner planets. But
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size alone does not tell the whole story. CW Leonis
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is an ageing star. Its outer layers have become cool
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and extended compared with the hotter interior, giving the star
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its deep reddish appearance. And the star does not remain
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perfectly still. It gently expands and contracts. Its brightness changes
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over long periods. The material in its outer regions rises
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and falls, responding to changes deep within the star. You
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can imagine the entire enormous sphere breathing very slowly, not
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like a living creature. There is no heartbeat, no breath,
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only the gradual movement of stellar material, expanding and contracting
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over periods so long that a human lifetime becomes almost
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invisible beside them. When the star expands, its outer layers
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become thinner. When those layers contract, they become denser again.
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The cycle continues. slowly, quietly, again and again, and sometimes
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material from the outer layers escapes into space. This is
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one of the most beautiful parts of an aging star.
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It does not simply disappear when its life changes, it
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gives something back. Gas and dust begin to surround the star.
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A vast cloud slowly forms around it, extending outward into
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the darkness. Within this material are different chemical ingredients, including carbon, oxygen, water,
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and ammonia. You do not need to picture these as
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separate objects floating around like tiny labels. Instead, imagine a
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faint veil of material spreading outward from the star, carrying
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the remains of its long history into space. Some of
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that material may eventually become part of other things, a
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future cloud, a future star, a future planet. Perhaps, after
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enough time, material released by an ageing star can become
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part of a completely different system, far from the place
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where it began. The universe has a quiet habit of
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recycling its ingredients. Stars gather material. Stars transform material. And
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stars eventually return some of that material to space. The
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darkness between the stars is therefore not completely empty. It
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contains traces of old stars, dust, gas, atoms that have
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travelled through stellar interiors, molecules formed in the cool outer
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regions of dying stars, tiny pieces of ancient history drifting
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between distant suns. And as you float beside CW Leonis,
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the enormous red star seems less like an ending and
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more like a transition. Its outer layers continue moving outward.
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Its surrounding cloud becomes larger. The star itself continues changing.
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Far in the future, its remaining core will become a
30:01
much smaller and denser object, while the material it released
30:05
will continue travelling outward. The star will become quieter, its
30:10
surrounding cloud will slowly mix with the space around it,
30:14
and eventually the material will become difficult to distinguish from
30:18
the wider interstellar medium. But the atoms will remain, the
30:23
carbon will remain, the oxygen will remain, the other elements
30:27
will remain. They will simply move somewhere else. Perhaps into
30:32
another cloud. Perhaps into another star. Perhaps into another world.
30:38
You can let that thought rest gently for a moment.
30:41
Nothing in the universe needs to remain exactly where it began.
30:44
A star can become a cloud. A cloud can become
30:48
a star. A star can create the ingredients for planets.
30:52
And planets can carry those ingredients for billions of years.
30:56
The story keeps moving. Even when everything appears still, you
31:01
drift a little farther from CW Leonis. Its red light
31:05
becomes smaller. Its surrounding cloud fades into the darkness. The
31:10
young star you passed earlier is now far behind you.
31:14
Between them lies an enormous stretch of space, filled with
31:18
the quiet remains of countless stellar lives. And somewhere in
31:22
that darkness, there are stars that have become much more
31:25
compact than CW Leonis. Stars compressed into astonishingly small remnants,
31:31
spinning through space and sending out precisely timed pulses of light.
31:36
For now, you can simply let that distant rhythm remain
31:40
ahead of you. A faint pulse, then darkness, another pulse,
31:45
then darkness again. And slowly, you begin to drift toward it.
31:52
A faint pulse, then darkness, another pulse. then darkness again.
31:58
As you drift toward that distant rhythm, you begin to
32:01
approach one of the quiet remnants left behind, when a
32:05
massive star reaches the end of its ordinary life. The
32:09
object itself is small, much smaller than the star that
32:13
came before it, but its density is almost impossible to imagine.
32:18
This is a neutron star. A neutron star is what
32:21
can remain after the core of a massive star has
32:24
been compressed enormously, leaving most of the former star's material
32:29
packed into a sphere only a few tens of kilometres across.
32:34
From far away, it might look like almost nothing. A
32:38
tiny point, a faint source of light, and yet, hidden
32:43
inside that small object is an extraordinary amount of matter.
32:47
You do not need to picture the pressure. You do
32:49
not need to imagine what it would feel like to
32:51
stand there. There is nowhere to stand. You can simply
32:55
let the contrast remain in your mind. A star that
32:59
once stretched across an enormous distance has become a tiny,
33:02
dense remnant, turning quietly in the darkness. Some neutron stars
33:08
rotate very quickly, and some send beams of radiation outward
33:13
from their magnetic regions. As the star turns, these beams
33:18
sweep across space. If one of them happens to pass
33:21
in your direction, you see a brief signal. Then the
33:25
star turns. The light disappears. Then it returns. Again. And again.
33:32
These objects are called pulsars. The name comes from the
33:36
regular pulses they produce, but the underlying process is simply
33:40
a rotating stellar remnant carrying beams of energy through space.
33:45
You can imagine yourself floating far away, watching the rhythm.
33:50
A pulse. Darkness. A pulse. Darkness. The timing can be
33:55
extraordinarily precise. Some pulsars behave almost like natural clocks, keeping
34:00
their rhythm over long periods of time. And around at
34:04
least one pulsar, astronomers have found something even more unusual.
34:09
A planet. It is a world moving around the remains
34:12
of a dead star. The planet is thought to be
34:15
extremely dense, and its environment would be very different from
34:19
anything familiar on Earth. Its star is no longer a warm,
34:23
stable sun like ours. Instead, it is a compact remnant,
34:28
spinning through space and sending radiation outward. You can simply
34:32
let the little system remain in the distance, a dark planet,
34:36
a tiny star, a regular pulse moving outward through the
34:40
surrounding space. It is another reminder that planets can survive,
34:45
or perhaps form, in places very different from the gentle
34:49
environment around our own sun. And as you drift away
34:53
from the pulsar, the pulses become softer, the rhythm fades,
34:57
the darkness grows wider. Ahead, there is another kind of object.
35:03
This time, there is no visible surface, no solid world,
35:08
no glowing star, only gravity. You are approaching a black hole.
35:13
The name can sound dramatic, But a black hole is
35:16
not a hole in space in the ordinary sense. It
35:19
is a region where matter has become so deeply compressed
35:23
that gravity creates an extreme boundary around it. Once something
35:27
crosses that boundary, even light cannot return to the outside.
35:32
This boundary is called the event horizon. But you do
35:36
not need to imagine a physical wall. There is no
35:39
surface you would touch. It is simply a point of
35:42
no return in the geometry of space and time. From
35:46
far away, the black hole itself may be dark, but
35:50
its surroundings can be bright. Gas and dust can fall inward,
35:54
moving faster and faster as they respond to the intense gravity.
35:59
Before reaching the black hole, some of this material can
36:03
form a wide, glowing disk around it. This is an
36:07
accretion disk. The word sounds technical, but the idea is simple.
36:13
Material gathers around the darkness, it moves, it becomes hot,
36:17
it shines. And because the black hole itself cannot be
36:21
seen directly, the light from this surrounding material can become
36:25
one of the ways we know the black hole is there.
36:28
There is another quiet clue as well. Gravity can bend light.
36:33
If a massive invisible object passes between you and a
36:35
more distant source of light, the path of that light
36:39
can curve around the object. The distant light may appear shifted,
36:43
stretched or distorted. This effect is called gravitational lensing. It
36:48
is another way the invisible can leave a visible trace.
36:52
And this idea becomes especially important when you look toward
36:55
the centre of your own galaxy. There, hidden among an
37:00
extraordinarily dense region of stars, is a supermassive black hole
37:05
called Sagittarius A star. It contains roughly four million times
37:11
as much mass as our Sun, yet from our position
37:14
in the Milky Way, it is not something you see
37:17
shining in the night sky. Instead, astronomers learn about it
37:21
by watching what happens around it. Stars orbit nearby. Gas
37:26
moves through the central region. Light travels through the crowded environment.
37:32
The motions of those stars reveal the presence of something
37:35
enormously massive at the centre. You can imagine pulling back slowly.
37:40
The black hole becomes smaller and The stars around it
37:43
become more numerous. The crowded centre of the galaxy begins
37:46
to appear, and then the centre itself becomes part of
37:50
something much larger, the Milky Way. The black hole is
37:54
not the galaxy. It is only one object inside it.
37:58
A remarkable object, certainly. But still only one part of
38:02
a much greater structure. You have moved from the death
38:05
of a star to the remains of its core. From
38:09
a neutron star to a pulsar. from a pulsar to
38:13
a black hole. And now, without needing to hurry, you
38:17
can begin to turn around. Behind you are the planets
38:21
you passed earlier. Farther behind them are the nearby stars.
38:26
Farther still are the young stars, the ageing giants, the
38:29
faint remnants, and the distant worlds orbiting other suns. All
38:34
of them are part of the same enormous galaxy, the
38:37
Milky Way. And as you begin to drift away from
38:41
Sagittarius A star, the crowded centre slowly becomes less crowded.
38:46
The stars spread apart, the darkness between them becomes wider,
38:51
the central glow begins to fade behind you. You are
38:54
moving outward now, away from the deepest region of the galaxy.
38:58
And for the first time, you can begin to sense
39:01
that the Milky Way is not simply a collection of stars,
39:05
it has a shape. It has a structure. It has
39:08
a vast outer region surrounding its bright inner disk. The
39:12
individual stars are becoming less important, the galaxy itself is
39:16
beginning to appear, and somewhere ahead, beyond the crowded centre,
39:21
the great spiral shape of the Milky Way is waiting
39:25
to unfold. The Crowded Centre of the Milky Way is
39:31
slowly fading behind you. The stars around Sagittarius A star
39:35
begin to spread apart. The bright central region becomes softer,
39:40
less crowded, and more distant, and as you continue drifting outward,
39:46
something changes in the way you see the galaxy. You
39:49
are no longer looking at individual stars, you are beginning
39:53
to see the shape that holds them. The Milky Way
39:56
is an enormous spiral galaxy, with a broad disk of stars,
40:01
gas and dust extending outward from its central region. At
40:05
the very heart is a dense concentration of stars. Around
40:09
that centre is a long structure of stars, stretching through
40:13
the inner galaxy like a quiet bar of light, and
40:16
from this central region, the galaxy gradually opens outward. The
40:21
stars become arranged in broad, curved patterns. These are the
40:26
spiral arms. They are not solid arms, like the arms
40:30
of something you could touch. They are enormous regions where
40:34
stars and clouds of gas become more concentrated, creating the
40:38
familiar spiral shape that you might see when imagining our
40:42
galaxy from far above. You can slowly rise above the
40:46
galactic disk now, Below you, the Milky Way begins to
40:50
resemble a vast whirl of faint light. The centre glows
40:54
more brightly, the outer regions become thinner, and along the
40:58
spiral arms, clouds of gas and dust gather in places
41:02
where new stars can begin to form. Some of the
41:05
brightest stars in the galaxy are young. They have not
41:09
been shining for very long, compared with the age of
41:12
the Milky Way. Around them are clouds where future stars
41:15
may still be gathering. The galaxy is therefore not simply
41:19
an old collection of stars. It is still changing, still forming,
41:24
still recycling its material, and somewhere along one quieter region
41:28
of one of these spiral arms is your home. The
41:31
solar system is not at the centre of the Milky Way.
41:34
It is not close to the outermost edge either. It
41:38
lies within a smaller region, often called the Orion Spur,
41:42
a quieter branch between larger spiral structures. From here, The
41:47
Sun is only one star among an enormous number of others.
41:51
And Earth is only one small planet around that star.
41:55
You can pause here for a moment. Imagine looking down
41:58
toward the solar system from above. You would not see
42:02
Earth as a bright blue world. You probably would not
42:05
see the planets at all. The entire solar system would
42:08
be reduced to a tiny point. And even that point
42:12
would be almost impossible to distinguish among the surrounding stars.
42:16
The Milky Way is simply too large. Its scale is
42:20
difficult to hold in the mind. So you continue outward.
42:24
The spiral arms begin to blend together. The bright disk
42:28
stretches beneath you. And then you notice something strange. Some
42:32
parts of the galaxy appear dark. Long, irregular bands of
42:36
darkness cross the glowing background of stars. But this darkness
42:41
is not empty. It is made partly from dust. Tiny
42:44
grains of material gather in enormous clouds, blocking some of
42:48
the visible light coming from stars behind them. This dark
42:52
region is sometimes called the Great Rift. From Earth it
42:55
can appear as a long, shadowy lane running through the
42:58
Milky Way. But if you could look at it in
43:01
other kinds of light, the darkness would begin to change.
43:05
Radio waves can reveal material hidden inside. Infrared light can
43:10
pass through some of the dust. and show stars that
43:13
visible light cannot easily reach. So once again, the universe
43:17
reminds you that darkness does not always mean emptiness. Sometimes
43:22
darkness is simply something standing between you and the light.
43:26
You drift beyond the bright disk now. The stars become fewer.
43:31
The spiral pattern becomes harder to see. The Milky Way
43:35
begins to lose its familiar shape. Around the galaxy is
43:39
a much larger and much fainter region. the galactic halo. Here,
43:44
stars are scattered more sparsely than in the bright disk.
43:48
Some are extremely old. Some gather in dense, ancient groups
43:52
called globular clusters, small spherical collections of stars that have
43:57
remained together for billions of years. You can imagine one
44:01
of these clusters from a distance. Hundreds of thousands of
44:05
stars gathered into a faint sphere, not crowded in the
44:08
way the galactic centre is crowded. but bound together across
44:12
enormous distances. Some of the stars inside these clusters are
44:17
among the oldest stars in the galaxy. They carry memories
44:21
of a much earlier Milky Way, from a time when
44:24
the galaxy itself looked different. And now, as you move
44:28
even farther outward, the visible stars become increasingly sparse. The
44:34
bright disk is far behind you. The halo stretches around you.
44:38
There are stars here, but there is much more space
44:41
between them, and this is where another invisible part of
44:44
the galaxy begins to become important. Something appears to be
44:49
surrounding the Milky Way, extending far beyond the stars we
44:53
can see. We cannot observe it directly through ordinary light.
44:58
We know it is there because of the way the
45:00
visible stars move. The stars near the outer regions of
45:04
the galaxy are orbit as though they are being held
45:07
by much more mass than the stars, gas and dust
45:11
we can see. Astronomers call this unseen material dark matter.
45:16
The name is simple. The idea is less simple. It
45:21
does not mean ordinary darkness. It does not mean a
45:24
cloud blocking the stars. It means something whose presence is
45:27
inferred from gravity, even though it does not appear to
45:31
shine like ordinary matter. You can imagine the galaxy as
45:34
a vast luminous disk, surrounded by something almost completely invisible.
45:40
The stars are the light you can see. The unseen
45:43
mass is known through the way that light moves, and
45:46
the farther you travel from the bright centre, the more
45:49
this invisible structure becomes part of the story. The Milky
45:53
Way is therefore larger than the stars alone, much larger.
45:58
The bright spiral disk is only the part that your
46:00
eyes would recognise. Beyond it lies the faint halo. Beyond
46:05
the halo lies an even greater region of space. And
46:09
somewhere far above and below the galactic centre are enormous
46:13
structures that seem to preserve traces of powerful activity from
46:17
the distant past. They are known as the Fermi Bubbles,
46:21
vast regions extending outward from the centre of the Milky Way.
46:26
Their exact origin remains uncertain. Perhaps they were created by
46:30
ancient activity around the central black hole. Perhaps by powerful
46:35
bursts of star formation. Whatever their origin, they are like
46:39
a faint memory written into the space around the galaxy.
46:43
You can let that mystery remain unresolved. You do not
46:47
need to solve it tonight. You simply drift farther away.
46:52
The Milky Way now stretches behind you as a broad
46:55
glowing disk. The sun is somewhere inside it. Earth is
46:59
somewhere around the sun, and all the stars you have
47:02
visited are now part of one enormous pattern. From this distance,
47:07
individual worlds no longer matter. Individual stars begin to disappear
47:12
into the whole. The galaxy becomes a single shape, a
47:16
soft spiral of light surrounded by darkness. And as you
47:21
continue moving outward, even that spiral will begin to look smaller,
47:26
because beyond the Milky Way, There are other galaxies, many others.
47:31
And soon, the quiet journey will carry you away from
47:34
your own galaxy, toward the neighbouring island of stars, known
47:39
as Andromeda. The Milky Way is becoming smaller behind you.
47:45
Its spiral shape is still visible, but the details have
47:49
begun to disappear. The bright centre is now only a
47:53
soft glow. The spiral arms have become delicate curves. The
47:58
stars that once seemed countless are now blending into a
48:02
single distant pattern of light. And slowly, gently, you continue outward.
48:08
There is no boundary around the Milky Way, no edge
48:11
that you suddenly cross. The galaxy simply becomes smaller as
48:16
the space between you and it becomes larger. Behind you
48:20
is your home galaxy. Ahead of you is another. At first,
48:25
it is only a faint patch of light. a soft
48:28
glow against the darkness. But as you drift closer, the
48:32
shape begins to emerge. A broad disk, a brighter centre,
48:37
curving arms, a galaxy turning quietly through space. This is Andromeda.
48:44
It is the nearest major galaxy to the Milky Way,
48:47
lying roughly two and a half million light-years away. That
48:51
distance is difficult to imagine, so you do not need
48:54
to measure it. Instead, Imagine light leaving the Milky Way
48:59
and travelling outward for more than two million years before
49:03
finally reaching Andromeda. Or, imagine light leaving Andromeda and beginning
49:09
a journey toward Earth long before human civilisation had taken
49:14
its modern shape. When you look toward Andromeda, you are
49:17
not seeing it exactly as it is now. You are
49:20
seeing ancient light, light that has been travelling through darkness
49:24
for an enormous length of time. The galaxy you see
49:28
is a message from the distant past, arriving quietly after
49:33
a journey that began millions of years ago. Andromeda itself
49:37
is enormous. Its disk stretches across a region of space
49:41
much wider than the distance from one edge of our
49:43
familiar solar system to the other. Within that disk are
49:47
vast numbers of stars. Some are young, some are ancient,
49:52
some are gathered in bright clusters. Others are scattered quietly
49:56
through the galaxy. There are clouds of gas and dust.
50:00
There are regions where new stars are forming. There are
50:04
older regions, where the galaxy's earlier history is preserved in
50:07
the light of ancient stars. From a distance, Andromeda looks peaceful.
50:13
But like the Milky Way, it has a long history
50:15
of change. Galaxies do not always grow alone. They can meet.
50:21
They can pass close to one another. They can pull
50:24
on one another with gravity. And sometimes, over immense stretches
50:28
of time, they can merge. Andromeda carries traces of such encounters.
50:34
Around it are smaller satellite galaxies, small companions held nearby
50:39
by gravity. Among them are M32 and M110. They are
50:44
much smaller than Andromeda itself, but they belong to its
50:48
wider family, and some of the structures around Andromeda may
50:52
be the remains of galaxies that came too close in
50:55
the distant past. Stars can be pulled away, gas can
51:00
be stretched into long streams, The shape of a galaxy
51:03
can slowly change. The process is not like two solid
51:07
objects crashing together. There is far too much empty space
51:11
between individual stars. Instead, a galactic encounter is something much
51:17
slower and more delicate. Two enormous collections of stars pass
51:21
through one another. Their gravity reshapes their paths. Gas clouds interact.
51:28
New regions of star formation may appear. and over millions
51:32
or billions of years, the two galaxies can gradually become one.
51:36
Andromeda itself appears to preserve memories of such encounters. You
51:41
can imagine faint streams of stars extending away from the
51:44
main disk, almost like traces left behind by something that
51:48
passed through long ago. The galaxy does not keep its
51:52
history in a written record. It keeps it in motion,
51:56
in the paths of its stars, in the shape of
51:59
its clouds. In the faint structures surrounding its bright centre,
52:04
even the centre of Andromeda is unusual. Astronomers have observed
52:08
a strange double structure there, with two concentrations of stars
52:13
that do not appear perfectly centred in the same way.
52:16
The exact origin of this arrangement remains uncertain. It may
52:20
be connected to an ancient interaction with another galaxy. It
52:24
may reflect the complicated movement of stars around the massive
52:28
object at Andromeda's centre. Whatever the explanation, you can let
52:33
the uncertainty remain. A galaxy can be beautiful without being
52:37
completely understood. Andromeda continues turning. Its stars continue moving. Its
52:44
smaller companions continue orbiting. Its ancient light continues travelling outward.
52:50
And somewhere within that enormous galaxy, new stars are still
52:54
being born. You can drift past its outer regions now.
52:58
And as you move farther away... Another change begins. Andromeda
53:03
is no longer the only galaxy in view. The Milky
53:06
Way is behind you. Andromeda is nearby, and somewhere between
53:11
and around them are many smaller galaxies, faint companions scattered
53:15
through the darkness. Another larger spiral galaxy begins to appear,
53:20
the Triangulum Galaxy. It is smaller than both the Milky
53:24
Way and Andromeda, but it is still a vast collection
53:27
of stars, gas... dust and planets. Together, these galaxies form
53:34
part of a much larger neighbourhood. The Milky Way, Andromeda, Triangulum,
53:40
and many smaller galaxies. They are not simply arranged randomly.
53:45
Gravity connects them. They move through the same broad region
53:49
of space, influencing one another across enormous distances. Astronomers call
53:55
this neighbourhood the Local Group. The name sounds almost modest. Local. Group.
54:01
But from the perspective of Earth, this local neighbourhood is
54:05
already unimaginably large. It stretches across millions of light years,
54:10
and within it are dozens upon dozens of galaxies, most
54:14
of them much smaller and fainter than the great spirals
54:17
you can see more easily. Some are tiny dwarf galaxies.
54:22
Some contain relatively few stars. Some have been shaped by
54:26
their larger neighbours. Others may be remnants of ancient encounters,
54:31
and yet they all belong to the same broad gravitational family.
54:35
You can now slowly turn around. The Milky Way is
54:39
one island of stars. Andromeda is another. Triangulum is another.
54:45
Small galaxies drift around them like faint points of light.
54:49
The solar system is somewhere inside the Milky Way, too
54:52
small to distinguish from this distance. Earth is smaller still.
54:56
and you are farther away than you have ever been
54:59
in this journey. The universe is beginning to change scale. Earlier,
55:04
the stars seemed enormous. Then the Milky Way seemed enormous.
55:08
Then Andromeda seemed enormous. Now even these galaxies are becoming
55:14
pieces of something larger. The local group itself is beginning
55:18
to look small. And beyond it, the darkness is no
55:22
longer completely empty. Farther away are other groups. other clusters,
55:28
other galaxies, other enormous arrangements of matter. You can see
55:33
them beginning to gather along distant regions of light. The
55:37
familiar island of the local group is slowly fading behind you.
55:41
And ahead lies something even larger, something that cannot be
55:46
understood by looking at one galaxy at a time. A
55:49
wider pattern is beginning to emerge. The galaxies are not
55:53
scattered randomly. They gather. They stretch. They form long, faint
55:59
structures across space. And between those structures are regions where
56:03
very few galaxies can be found. As you drift toward
56:07
that wider pattern, the spaces between the galaxies begin to
56:10
matter as much as the galaxies themselves. The local group
56:17
is slowly becoming smaller behind you. The Milky Way is
56:21
now only one faint spiral among the darkness. Andromeda is another.
56:26
Triangulum is smaller still. The many little galaxies around them
56:30
are becoming difficult to distinguish, until the whole neighbourhood begins
56:34
to look like a handful of soft points of light.
56:38
But as you drift farther away, you begin to notice
56:41
something unexpected. The galaxies are not scattered evenly through space.
56:46
Some regions are more crowded. Others are almost empty. And
56:50
when you pull back far enough, the separate galaxies begin
56:54
to form patterns. Groups gather into larger groups. Clusters of
56:59
galaxies gather together, and between them, long regions of space
57:04
stretch outward, carrying galaxies along enormous structures that are almost
57:08
impossible to imagine from within. You are beginning to see
57:12
the large-scale structure of the universe. At first, it may
57:16
look almost random, tiny points, faint patches, little clusters of light,
57:23
but gradually, a pattern begins to emerge. Some galaxies lie
57:28
along long, thin regions. Others gather where several of these
57:32
regions meet, and between them are enormous areas containing far
57:38
fewer galaxies. Astronomers often describe these long structures as filaments.
57:44
The word is simple. The scale is not. A filament
57:48
can extend across immense distances, carrying galaxies through a vast
57:52
cosmic landscape. You can imagine stepping farther and farther away
57:57
from the local group until the individual galaxies begin to
58:01
disappear and only the larger arrangement remains. The galaxies become points.
58:07
The points become threads. The threads begin to connect. And
58:11
the universe starts to resemble a delicate pattern of light
58:14
and darkness. This is sometimes called the cosmic web. It
58:19
is not a physical web hanging in space. There are
58:22
no threads you could touch. Instead, it is a way
58:25
of describing how matter has gathered across the universe over
58:29
enormous stretches of time. Gravity slowly pulls matter together. Regions
58:35
that begin slightly denser can attract more material. Gas gathers.
58:40
Galaxies form. Galaxies gather into groups and clusters. And the
58:45
spaces between these denser regions become increasingly empty. The process
58:50
is incredibly slow. so slow that even the lifetime of
58:54
a star is only a brief moment compared with the
58:57
history of the cosmic structure around it. You can simply
59:01
watch the pattern from a distance. Light here. Darkness there.
59:06
Another group of galaxies farther away. Another phantom filament extending
59:11
beyond it. And then, between these structures, a region where
59:16
almost nothing seems to be happening. A void. Cosmic voids
59:20
are among the quietest places in the universe. They are
59:24
not completely empty. There can still be thin gas. There
59:28
can still be isolated galaxies. There can still be particles
59:31
and traces of matter moving through them. But compared with
59:35
the crowded regions of galaxy clusters, a void contains remarkably little.
59:41
You are moving toward one now. The galaxies behind you
59:44
become fewer. The bright clusters become distant. The long structures
59:49
of the cosmic web begin to fade, and the darkness
59:53
between them becomes wider. One of the nearby examples is
59:57
known as the local void. It lies close to the
1:00:01
larger structures containing our own local group, yet it contains
1:00:05
relatively few galaxies. From the inside, it would not necessarily
1:00:10
look like a dramatic empty chamber. There would be no
1:00:13
visible wall separating the void from the surrounding universe. There
1:00:17
would simply be more and more distance between the galaxies.
1:00:21
A galaxy here, a galaxy much farther away. then a
1:00:25
long stretch of darkness, then perhaps another faint galaxy. The
1:00:31
emptiness grows gradually, and that gradualness is part of what
1:00:35
makes it so difficult to imagine. You might expect space
1:00:39
to be filled evenly with stars and galaxies, but the
1:00:42
universe does not arrange itself that way. It has crowded
1:00:46
places and quiet places, bright regions and dark regions, places
1:00:53
where gravity has gathered enormous amounts of matter, and places
1:00:57
where comparatively little matter has gathered. As you drift farther
1:01:01
into this quiet region, you may notice a small galaxy
1:01:05
far away. It is known as NGC 6503. Its name
1:01:11
is simply a catalogue name, useful to astronomers but not
1:01:15
especially meaningful to you tonight. What matters is its settings.
1:01:20
This small galaxy lies in a relatively isolated region of space,
1:01:25
surrounded by an environment much less crowded than the dense
1:01:28
neighbourhoods of many other galaxies. And perhaps that makes it
1:01:32
a fitting companion for the final part of this journey.
1:01:36
A small galaxy, a huge surrounding emptiness, light travelling quietly
1:01:41
across the darkness. Even in a region that seems almost empty,
1:01:45
a galaxy can continue to shine. Its stars continue moving.
1:01:51
Its gas continues changing. Its light continues travelling outward. And perhaps,
1:01:57
somewhere inside that galaxy, there are worlds orbiting distant stars,
1:02:02
just as Earth orbits the Sun. You can let that
1:02:05
thought remain very small. You do not need to follow it,
1:02:10
because now you are pulling back again. Farther and farther.
1:02:14
The local void becomes smaller. The local group becomes smaller.
1:02:19
The Milky Way becomes smaller. Andromeda becomes smaller. The galaxies
1:02:24
become points. The filaments become faint patterns. The cosmic web
1:02:29
becomes something almost too large to recognise. And beyond all
1:02:33
of this is the observable universe, the portion of the
1:02:36
cosmos whose light has had enough time to reach us
1:02:40
since the universe became transparent to light. You do not
1:02:44
need to imagine its edge. There is no wall waiting there.
1:02:48
It is simply the limit of what we can observe
1:02:50
from our position in space and time. Beyond what we
1:02:54
can see, there may be much more, but tonight you
1:02:57
do not need to travel there. You can simply remain here,
1:03:01
floating in the darkness between the galaxies. The journey that
1:03:05
began beside Earth has carried you outward through the solar system,
1:03:10
past the distant icy regions, between nearby stars, past strange
1:03:15
worlds of water and rock. beyond young stars and ageing giants,
1:03:20
past pulsars and the dark presence of black holes, through
1:03:24
the Milky Way, beyond Andromeda, out into the local group,
1:03:29
and finally toward the enormous spaces between galaxies, you have
1:03:34
travelled farther and farther away, and with every step outward
1:03:38
there has been more space, more darkness, more distance, until
1:03:43
the universe begins to feel less like a collection of
1:03:46
objects and more like a quiet balance between light and emptiness.
1:03:51
A galaxy here, a galaxy there, a faint filament stretching
1:03:56
across the distance, a great void between them. And everywhere,
1:04:01
enormous amounts of space. You can let that idea become
1:04:05
softer now. You do not need to understand the whole universe.
1:04:09
You do not need to remember the names. You do
1:04:12
not need to hold on to the distances. The stars
1:04:15
can continue moving without you watching them. The galaxies can
1:04:20
continue turning. The cosmic web can remain where it is.
1:04:24
The distant voids can remain quiet. And you can simply rest.
1:04:29
The darkness around you is not something missing. It is
1:04:32
simply space. Wide, quiet, unhurried. Far behind you, the Milky
1:04:38
Way continues to shine. And somewhere inside that distant spiral...
1:04:42
The sun continues its slow journey around the galaxy. Earth
1:04:47
continues turning. The oceans continue moving. Night continues passing into morning.
1:04:54
And for now, you can let yourself become still. You
1:04:57
are safe. There is nothing to do, nothing to solve,
1:05:01
nothing you need to reach. The universe is already moving
1:05:05
without effort. The stars are already shining without asking to
1:05:09
be noticed. And the great spaces between them are already
1:05:13
holding everything quietly apart. So you can simply drift here
1:05:17
for a while. Perhaps there are galaxies farther away than
1:05:21
you can imagine. Perhaps there are still larger structures beyond
1:05:25
the ones we have mapped. Perhaps there are spaces so
1:05:28
wide that even the idea of distance begins to lose
1:05:31
its meaning. You do not need to follow them tonight.
1:05:35
You can let the questions remain open. You can let
1:05:37
the darkness remain wide. And somewhere... Very far away, a
1:05:42
small point of light continues travelling through the quiet.