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The Interstellar Medium – The Stuff Between the Stars · Talking Science

0:00
So when you look up at a, you know, a really
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pristine dark night sky, your brain is basically
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falling for one of the oldest optical illusions
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in the universe. Oh, absolutely. We're wired
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for it. Right. Because our eyes are just immediately
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drawn to those brilliant piercing dots of starlight.
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And since those stars are so overwhelmingly bright,
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our brains just automatically fill in all the
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spaces between them as this total void. You know,
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just vast, inky, empty black gaps. Yeah, we just
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assume there's nothing there. Exactly. Yeah.
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But the central premise of our deep dive today,
0:30
based on some really fascinating source material
0:32
on astrophysics, is that this intuitive picture
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is, well, it's almost completely wrong. It really
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requires a total rewiring of how you visualize
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the galaxy. Yeah. So today, our mission is to
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shift our focus entirely away from the glowing
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stars. We're going to uncover this incredibly
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complex, invisible drama happening in the exact
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spaces we assumed were empty. This sprawling
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ecosystem that astronomers call the interstellar
0:57
medium. and it's such a great topic because that
1:01
supposed empty space is actually this dynamic,
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highly structured soup of gas and dust. I mean,
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it's not just a boring black backdrop. Right.
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It is the raw material for stellar birth. It's
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the sprawling graveyard for dead stars. And it's
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basically the main character in the galaxy's
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ongoing story. OK, but to really grasp this,
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we have to kind of unpack a weird paradox about
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just how thin this soup actually is, because
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it really challenges our basic definition of
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a vacuum. It does. Yeah. Like right now in the
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room you're sitting in listening to this, you
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are surrounded by roughly 100 billion billion.
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billion molecules per cubic meter. Which is a
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very dense, chaotic swarm of air. Right. But
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if you transport yourself to an average patch
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of the interstellar medium, you're looking at
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what, maybe one single atom per cubic centimeter?
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Sometimes even less, which is an emptiness that
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completely exceeds the most advanced laboratory
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grade vacuum chambers we can build here on Earth.
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Yeah. Inside our most pristine terrestrial vacuums,
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there are still thousands of molecules pinging
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around in that same cubic centimeter. Out between
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the stars, the dense... Okay, so let me push
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back on this a bit for you listening. Because
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if the interstellar medium is literally a harder
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vacuum than a pristine physics lab vacuum chamber
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on Earth, calling it a soup or an ecosystem feels
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almost counterintuitive. I can totally see why
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you'd say that. Like, why doesn't that just mean
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space is, for all practical purposes, empty?
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Well, the resolution to that paradox really comes
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down to the sheer staggering scale of the universe.
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Yes, the gas is extraordinarily dilute on a local
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level. But when you take that ultra hard vacuum
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and you spread it across trillions of kilometers
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in every possible direction, all that mass aggregates
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to a monumental degree. So it adds up. Exactly.
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Thinness multiplied by vastness generates enormous
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substance. When you tally up all those single
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atoms per cubic centimeter across the entire
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Milky Way, it accounts for anywhere from 10 to
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15 percent of all the ordinary matter in the
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entire galactic disk. 10 to 15 percent. That's
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a massive fraction just floating around out there
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in the dark. It is a massive reservoir of material
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just stretched to its absolute physical limit.
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Which naturally makes you wonder, like, what
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exactly is floating around in there? Because
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I know it's not a uniform fog. No, not at all.
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By mass, it is overwhelmingly hydrogen, about
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74%. Hydrogen is, you know, the simplest, lightest
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element from the Big Bang. Right, the basic building
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block. Yeah. And then you have helium making
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up about 24%. Okay. But then there's this remaining
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tiny fraction, often just 1 % or 2%, which consists
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of heavier elements, or what astronomers confusingly
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just call metals, along with solid interstellar
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dust. Let's definitely clarify the dust part.
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Because when you hear the word dust, you naturally
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think of like the lint and skin cells drifting
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through a sunbeam in your living room. Oh, sure.
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Yeah. Earth dust is mostly biological. Exactly.
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But interstellar dust is entirely different,
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right? We're talking about microscopic solid
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grains of carbon and silicate minerals, basically
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tiny specks of soot and rock. often smaller than
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a speck of smoke. Yeah, they're incredibly tiny.
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And this mixture of gas and solid grains isn't
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just sitting statically in the dark. The interstellar
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medium actually has intense space weather. Space
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weather. I love that term. It's highly turbulent.
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It's fractured into distinct phases based on
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some really extreme variations in temperature
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and density. You have coronal gas pockets that
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are scorching hot, warm atomic clouds, and then
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these freezing cold molecular regions. See, okay.
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Let's talk about the physics of that extreme
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heat, because I feel like this detail in the
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source material really trips people up. The million
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degree gas. Yes. There are pockets where the
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gas is blasted to over a million degrees by dead
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stars, but it's basically a vacuum. So if you
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were in a spacesuit floating through a million
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degree gas cloud, you wouldn't vaporize. No,
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you would actually freeze to death. Wait, really?
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How can something be a million degrees, but you
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freeze? Well, it comes down to the difference
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between kinetic temperature and thermal heat
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transfer. In physics, temperature is really just
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a measurement of how fast the particles are moving.
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Right. So when a dead star like a supernova explodes,
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the shockwave slams into that very thin hydrogen
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and accelerates those few atoms to unbelievable
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speeds because they are moving so fast their
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temperature is technically a million degrees
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but heat transfer like actually feeling hot yeah
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requires collisions right precisely if you put
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your hand in a hot oven on earth billions of
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dense air molecules are instantly smashing into
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your skin transferring all that kinetic energy
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right it's dense yeah but in that million degree
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coronal gas in space there are so few atoms around
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that the rate of collisions with your space suit
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would be virtually zero oh wow the particle are
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moving incredibly fast, but there just isn't
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enough mass to transfer any real heat to you,
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so it would feel incredibly cold. That is so
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counterintuitive, but it makes perfect sense.
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But, you know, while those hot, fast -moving
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particles are super fascinating, the most important
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work in the galaxy actually happens in the exact
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opposite conditions. Yes, the cold pockets. Right.
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The vital mechanisms of the galaxy rely on this
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gas cooling down, slowing down, and clumping
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together into visible structures called nebulae.
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Exactly. When the gas cools, its internal pressure
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drops. In the warmer regions, the particles are
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bouncing around so fast they resist the inward
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pull of gravity. But in the densest, most frigid
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pockets where temperatures drop to just like
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10 or 20 degrees above absolute zero, that resistance
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fades. And these specific cold pockets are the
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molecular clouds, right? Yeah, they're called
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molecular clouds because it's finally calm and
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cold enough for individual hydrogen atoms to
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actually pair up and bond into molecular hydrogen
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or H2. And this is where gravity really starts
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to win the tug of war against the gas trying
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to spread out. Exactly. It crosses a threshold
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called the jeans mass, and the cloud starts collapsing
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under its own immense weight. It fragments into
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these smaller, really dense cores. And here is
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where the source material had an analogy I absolutely
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loved. Because as these clumps shrink under their
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own weight, they pull in material, and they start
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spinning faster and heating up, which is exactly
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like an ice skater pulling their arms in during
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a spin. It is the exact same physics, conservation
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of angular momentum. Right. As that material
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falls inward... Any tiny bit of rotation gets
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wildly amplified. The core spins faster, it flattens
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into a disk, and the friction and gravitational
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compression at the center cause the temperature
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to just skyrocket. Until it hits that ultimate
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threshold, right? Yes. The heat and pressure
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become so extreme that the hydrogen nuclei are
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literally forced together. They fuse into helium,
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and that releases a massive burst of energy.
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And boom, a new star is turned on. Exactly. That
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fusion ignition is stunning. And because these
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molecular clouds are so huge, they don't just
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make one star. They trigger massive bursts of
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star formation, creating hundreds or thousands
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of stars all at once. Like the Orion Nebula,
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which you can even see with the naked eye. Yeah,
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Orion is a perfect example of an emission nebula.
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The crazy intense ultraviolet radiation from
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those hot newborn stars strips the electrons
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from the leftover hydrogen gas around them. And
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that makes it glow from within. Right, it basically
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fluoresces brightly in the night sky. But you
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also have dark nebulae like the Coalsack near
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the Southern Cross, where the dust is just so
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dense it acts like a silhouette, completely blocking
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the light from stars behind it. Okay, so the
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interstellar medium is essentially the galaxy's
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giant fuel tank. Without it, the galaxy just
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ages and goes quiet. But if it was just a one
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-way trip, you know, a one -way fuel tank, the
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Milky Way would have run out of gas billions
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of years ago. Oh, entirely. The universe would
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be completely dark by now. So it's got to be
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a two -way street. It is. The interstellar medium
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is a highly efficient recycling bin. Stars don't
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just hoard their mass forever. Sun -like stars
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will gently puff off their outer layers as they
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age, returning a lot of gas. And massive stars,
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well, they end their lives in catastrophic supernovae
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that violently blast their material back into
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the void. And the really crucial part is that
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the material they return is not the same as the
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material they started with. No, it is chemically
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transformed. Because inside the star, it's basically
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a giant nuclear forge. It spends its whole life
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crushing lighter elements into heavier ones,
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hydrogen into helium, helium into carbon, oxygen,
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all the way up to iron. And iron is where the
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star hits a wall. Right, because fusing iron
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takes more energy than it gives off. Exactly.
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The outward pressure stops, gravity instantly
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wins, the core collapses, and the rebound triggers
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the supernova blast. And this is where it gets
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really interesting for you listening right now.
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Because that supernova blast... has so much kinetic
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force and heat that it forges even heavier elements
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gold platinum uranium and flings them all out
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at tremendous speed seeding the dilute gas clouds
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with all this incredibly rich complex material
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right and this isn't just poetry the iron in
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your blood right now the calcium in your bones
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the rock under your feet Literally all of it
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was forged in the core of a long -dead star.
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It was flung out into the interstellar medium,
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swept up into a cold molecular cloud, and eventually
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built our sun and the Earth. You are quite literally
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made of recycled stardust. It just bridges that
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gap between abstract astrophysics and human biology
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so perfectly. But, okay, if all these elements
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and tiny dust grains are out there building solar
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systems, how do astronomers actually map them?
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Because we said earlier, they're mostly invisible
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and incredibly spread out. It's a huge challenge.
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Astrophysicists have to rely on some really clever
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observational tricks. One of the main ways is
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observing how this dust alters the light of distant
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stars. We call it extinction, which just means
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the dust grains physically block and scatter
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the light, making the star look a lot dimmer
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than it actually is. But it doesn't block all
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light equally. Exactly. There's a phenomenon
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called interstellar reddening. Those tiny dust
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grains are perfectly sized to block short -wavelength
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blue light, but they let the longer -wavelength
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red light pass right through. So wait, this is
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exactly the same underlying physics as a sunset
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here on Earth, isn't it? It is the exact same
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mechanism, Rayleigh scattering. Yeah, because
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when the sun is setting, the light passes through
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a thicker slice of Earth's atmosphere. The air
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and dust scatter all the blue light away, leaving
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only the red and orange to reach our eyes. But
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in space... It's happening over trillions of
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kilometers. Yeah. And astronomers take this annoying
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interference and use it to their advantage. How
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so? Well, they look at star clusters where they
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know what the true color and brightness of the
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star should be. Then they measure how dimmed
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and reddened the light actually is when it reaches
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our telescopes. By working backward, they can
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map out the 3D structure and density of those
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invisible dust clouds. That is so clever. But
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that only covers the dust. How do they see the
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cold hydrogen gas itself? Because that doesn't
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emit visible light, right? Right. For the cold
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gas, they have to shift out of visible light
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completely and use radio telescopes. And they
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rely on this beautiful little quirk of quantum
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mechanics called the 21 centimeter emission line.
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OK, lay this quantum quirk on us. So in a cold,
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dark cloud of atomic hydrogen, you just have
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a single proton and a single electron. Both particles
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have a quantum property called spin which creates
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a tiny magnetic field. Usually they are aligned
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parallel to each other. Which is a slightly higher
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energy state. Exactly. But as we know, nature
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always wants to be in the lowest possible energy
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state. So very rarely, like once every few million
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years for a given atom, the electron will spontaneously
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flip its spin to be opposite to the proton. And
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when it drops down to that lower energy state,
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it shoots out a faint radio photon with a wavelength
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of exactly 21 centimeters. But wait, if it only
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happens once every few million years for one
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atom, how on earth do we detect it? That sounds
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like nothing. It does sound like nothing. But
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you have to remember the vast scale we talked
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about earlier. Trillions of miles of dilute gas.
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So across that immense volume, there are constantly
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enough atoms doing this little quantum flip at
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any given millisecond to produce this steady,
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faint radio hum. Ah, so they just tune their
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radio telescopes to 21 centimeters and listen
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to the hum to map the atomic hydrogen. Exactly.
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But of course, there's a catch. There's always
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a catch in astrophysics. Naturally, that technique
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completely fails when you look at the really
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dense, cold molecular clouds where stars are
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actually born. Oh, because the... The hydrogen
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atoms have bonded together into H2 molecules.
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Yes. A hydrogen molecule is perfectly symmetrical.
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It doesn't have a permanent dipole moment, which
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means it can't emit those radio signals at those
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frigid temperatures. It just goes completely
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dark. So you have these massive critical clouds
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that are totally invisible. How do they find
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them? They use tracer molecules. They basically
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look for a stand -in. And the best one is carbon
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monoxide, or CO. Why carbon monoxide? Because
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it's an asymmetrical molecule. One carbon, one
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oxygen. It has a per - So even in the freezing
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cold, if it bumps into anything, it easily gets
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excited and emits a very bright detectable radio
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signal. So they basically map the glowing carbon
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monoxide and just safely assume the invisible
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molecular hydrogen is right there with it. Precisely.
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Okay. So having mapped all of this out, the red
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and starlight, the 21 centimeter flips, the carbon
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monoxide tracers. astronomers realized something
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highly specific and frankly a little ironic about
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where our own solar system is located in this
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cosmic fog. Yes, the local bubble. Right. Explain
14:42
the local bubble. So we aren't actually drifting
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through an average patch of the interstellar
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medium right now. Our sun is currently cruising
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through a massive, sparse, hot cavity called
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the Local Bubble. And this cavity was basically
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carved out millions of years ago by ancient supernovae,
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right? Yeah, a cluster of exploding stars acted
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like a cosmic snowplow, just sweeping all the
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dense gas and dust outward and leaving behind
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this cleared -out void. And plus, our sun has
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its own magnetic bubble, the heliosphere, which
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pushes outward and shields our planets from even
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more of whatever interstellar material is left.
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Exactly. We are heavy. So the irony for you listening
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is that space feels so empty and abstract to
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us with these crystal clear views of distant
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galaxies, specifically because we are basically
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coasting through a cleared out cosmic cul -de
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-sac. It's a huge observational bias. If we live
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somewhere else in the galaxy, like inside a dense
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molecular cloud, the night sky would look totally
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different. The dust would block the stars. You'd
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see dark lanes of soot everywhere. Our whole
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philosophy of a vast transparent universe is
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totally dependent on this one little clear patch
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we happen to be in. It really is. And if you
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zoom out to the whole galaxy, you see that this
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whole ecosystem is shaped by even more invisible
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forces. It's not just gravity and supernovae.
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What else is at play? Well, there are weak magnetic
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fields threading through the dust and cosmic
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rays, which are basically protons moving at near
16:07
light speed. Right. These forces actually push
16:10
and organize the interstellar medium, compressing
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it into those massive sweeping spiral arms that
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give the Milky Way its shape. And I should add,
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this isn't a closed textbook. Exactly how these
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clouds collapse or how dust grains grow in the
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dark are huge active mysteries that keep astronomers
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busy today. It's just so dynamic. But as we wrap
16:32
up, we need to bring it all back to that recycling
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cycle. Right. Because we said the interstellar
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medium is a two -way street, right? The fuel
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tank. Stars are born, they die, they return enriched
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gas to the void. Yes, but it is a lossy process.
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Right, it's not a perfect one -to -one exchange.
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Not at all, because every time a star dies, it
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leaves a dense core behind a white dwarf, a neutron
16:52
star, or a black hole. And the mass locked inside
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those remnants is permanently removed from the
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ecosystem. It can never be used to build a new
16:59
star. Wow, so every star that forms permanently
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locks away some of that gas. Yes. Which means
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eventually, billions and billions of years from
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now, the fuel will just run out. It has to. The
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gas reserves will thin out past the point of
17:12
no return, star formation will completely halt,
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and the Milky Way will just enter this fading
17:17
twilight. It just fundamentally changes how you
17:19
look at the sky. So I want to leave you with
17:21
one final thought to ponder. The next time you
17:24
look up at a clear night sky, try to hold both
17:28
realities in your head. Those bright dots aren't
17:31
permanent and the black gaps between them aren't
17:33
empty. They are brimming with glowing hydrogen
17:36
and dark soot. Exactly. They are the recycled
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ashes of ancient stars waiting to be reborn,
17:42
but knowing that the fuel is finite. What will
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the universe look like in the deep future when
17:48
the last star finally dies out and the night
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sky is finally truly nothing but empty gaps?
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Something to think about the next time you're
17:54
stargazing.