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  • Hey folks, Phil Plait here. In the last episode of Crash Course Astronomy, I talked about

  • the eventual fate of the Sun, and other low mass stars like it. After a series of expansions and contractions,

  • they blow off their outer layers, become white dwarfs, and fade away over billions of years. The end.

  • Except not so much. First, white dwarfs are pretty awesome objects, and worth investigating.

  • And second, when a star becomes a white dwarf it produces what is, quite simply, one of

  • the most gorgeous objects in space.

  • To recap, when the Sun ages, it undergoes a series of changes in its core. It’s fusing

  • hydrogen into helium now, today, and will eventually fuse helium into carbon, and itll

  • make a dash of oxygen and neon too. But when it runs out of helium to fuse, it’s in trouble.

  • It doesn’t have enough mass to squeeze the carbon nuclei together, so they can’t fuse.

  • Fusion is the Sun’s energy source. Once the core is nearly pure carbon, that power is switched off.

  • By this time, nearly 8 billion years from now, the Sun’s outer layers are gone, expelled

  • by all the shenanigans going on in the core. What’s left of our star is just its core,

  • exposed to the dark of space. Over the next few billion years itll cool and fade to black.

  • That might seem like the end of the story. But I skipped a step, and it’s a beauty.

  • When helium fusion stops, the Sun’s core will have about half the mass of what the

  • Sun does today; the rest will have blown away into space around it. What remains is basically

  • composed of electrons and carbon nuclei, mixed with a small amount of a few other elements.

  • So what kind of an object are we left with here?

  • You may know that like charges repel; electrons have a negative charge and repel each other.

  • The tighter you squeeze them, the stronger that pressure.

  • There’s also a second force, called electron degeneracy pressure. It’s a result of some

  • of the weird rules of quantum mechanics (for you QM nerds, it’s The Pauli Exclusion Principle).

  • This describes how sub-atomic particles behave on teeny scales. One of these rules is that

  • electrons really hate to be squeezed together, above and beyond simple electric repulsion.

  • This resistance is phenomenally strong, and becomes the dominant force in supporting the

  • core of a star once helium fusion stops.

  • By the time this electron degeneracy pressure balances the core’s immense gravity, the

  • core is only about the size of Earth, 1% the original width of the Sun.

  • And it’s called a white dwarf.

  • Listing its characteristics is enough to melt your brain. Ironically, everything about it

  • gets amplified as its size shrinks. It becomes ridiculously dense; a single cubic centimeter

  • of it, the size of a six-sided die, has a mass of a million gramsone metric ton.

  • An ice cream scoop of white dwarf material outweighs 60 cars.

  • Because it’s so dense, the gravity at the surface of a white dwarf screams up, easily

  • topping 100,000 times the Earth’s gravity. If you have a mass of 75 kilos, you’d weigh

  • 7,500 tons if you stood on the surface of a white dwarf.

  • Not that you can. Stand there, I mean. You’d be flattened into a greasy smear.

  • But not for long. Newborn white dwarfs are hot; they can glow at a temperature of upwards

  • of 100,000 degrees Celsius. If you were on the surface, you’d be a vaporized and ionized smear.

  • Their intense heat makes them white, and theyre small. Hence their name.

  • Theyre so hot they also glow in the ultraviolet, even in X-rays. Weirdly, though, because theyre

  • so small, theyre actually quite faint. The closest one to us, Sirius B, can only

  • be seen with a telescope even though it’s nine light years away, one of the ten closest

  • known stars! Over 10,000 white dwarfs have now been found in our galaxy.

  • Still, any gas near a newly formed white dwarf is likely to be affected by the intense radiation pouring out of it.

  • And hey, wait a sec. When a star like the Sun is in its final death throes, it expels

  • its outer layers as a gaseous wind. You don’t suppose…?

  • Yup. By the time that white dwarf forms, the gas blown off hasn’t gotten very far from it,

  • at most a light year or two. That’s plenty close enough to get zapped by the white dwarf radiation,

  • causing that gas to glow in response. What does something like THAT look like?

  • Why, it looks like this.

  • This object is what we call a planetary nebula. It’s a funny name, and like so many other

  • names it’s left over from when these objects were first discovered. The astronomer William

  • Herschelthe same man who discovered infrared light and the planet Uranusgave them

  • that name, because they appeared like small green disks through the eyepiece.

  • The first planetary nebula was discovered in 1764 by the French astronomer Charles Messier,

  • who spent years scanning the skies looking for comets. He kept seeing faint fuzzy objects that he mistook for

  • comets, so he decided to make a catalog of them, a sort ofavoid these objectslist. That list is now a staple

  • of amateur astronomers, because ironically it contains some of the best and brightest objects to observe.

  • Among them is M 27 — the 27th object on Messier’s listone of the biggest planetary nebulae in the sky,

  • and one of my favorites; I love seeing it through my telescope when it’s up high in the summer.

  • Planetary nebulae can be a bit tough to observe; most are small and faint. On film, even with

  • long exposures, they can appear to be little more than disks. For a long time, they weren’t

  • thought to be terribly complicated; when a star becomes a red giant and blows off its

  • outer layers it’s rotating very slowly, so the wind should blow away in a sphere surrounding

  • the star. Many planetaries (as we call them for short), are round and look like soap bubbles,

  • pretty much what you expect when you look at an expanding shell of gas.

  • But with the advent of digital detectors, their fainter structures became clearer, and the

  • true beauty of these phenomenal objects was revealed. Some are elongated. Some have spiral patterns.

  • Some have jets shooting out on either side. Some have delicate tendrils streaming away from them.

  • In fact, only a handful of the hundreds known are actually circular!

  • Clearly, there’s more to planetaries than meets the eye.

  • If the wind from a star blows off in a sphere, how can planetaries come in all these fantastic shapes?

  • It turns out the real situation is more complicated. As usual.

  • When a star is a red giant, it spins slowly, and blows off a dense but slow solar wind.

  • If there’s nothing else happening to the star, then that wind will blow outward in

  • all directions, spherically. However, as those outer layers of the star peel away, they expose

  • the deeper, hotter part of the star. The star starts to blow a much faster, though far less

  • dense wind. That wind catches up with and slams into the slower wind.

  • When that happens, you get that idealized soap bubble nebula.

  • But some stars are binary; two stars that orbit very close together. Well go into

  • detail on them in a later episode, but, if the dying star has a companion, they may circle

  • each other rapidly. That will shape the wind, forcing more of it outward in the plane of

  • the starsorbits due to centrifugal force.

  • The overall shape of the expanding gas is flattened, like a beach ball someone sat on.

  • When the fast wind kicks in, it slams into that stuff in the orbital plane and slows down.

  • But there’s less stuff in the polar direction, up and down out of the plane. It’s easier

  • for the wind to expand in those directions, and it forms huge lobes of material stretching

  • for trillions of kilometers. That’s a very common shape for planetary nebulae.

  • But to explain the shapes we see, the two stars would have to orbit improbably close.

  • Most binaries aren’t that tight. So what can cause these shapes?

  • When I was in graduate school, my Master’s degree advisor, Noam Soker, came up with a

  • nutty idea; maybe the stars had planets, like in our solar system. If the star expanded into

  • a red giant and swallowed them, it would take millions of years or more for the planets

  • to vaporize. And for all that time they’d be orbiting INSIDE the star, moving faster

  • than the star itself. Like using a whisk to beat eggs, the planets inside the star would

  • spin it up, causing it to rotate fasterfast enough to explain the shapes of planetaries.

  • That was in the early 1990s. A few years later, the first exoplanets were found, and we saw

  • that massive planets orbiting very close to their stars were common. I suspect this is why we see

  • so many weird and fantastic shapes in planetaries; their progenitor stars swallowed their planets.

  • So planetary nebulae really may owe their existence to planets! And we comefull circle.

  • The glow in a planetary nebula is due to the hot central white dwarf exciting the gas.

  • Most of the gas is hydrogen, which glows in the red. However, a lot of the gas is oxygen.

  • There’s not nearly as much oxygen as hydrogen, but kilo for kilo oxygen glows more brightly than hydrogen

  • due to the atomic physics involved. This oxygen glows green, giving planetaries their characteristic hue.

  • Funny: When this green glow was first analyzed spectroscopically, astronomers couldn’t

  • identify the responsible element making it. They dubbed it nebulium, but eventually figured

  • out is was just extremely tenuous oxygen.

  • Other colors can be found, too. Nitrogen and sulfur glow red, and oxygen can emit blue

  • as well, all adding to the beauty of these celestial baubles. But these aren’t just

  • pretty pictures: The structure, color, and shape of a planetary nebula tells us about

  • the life of the star that formed it. We learn even more about stellar evolution by studying how stars die.

  • Mind you, the gas in a planetary nebula is still expanding, cruising outward from its

  • initial momentum of being thrown off the star. Eventually, the gas expands so much it thins

  • out, and it stops glowing. That takes a few thousand years, so when you see a planetary

  • nebula youre seeing a very short snapshot of the life, the death, of a star. And that’s

  • why we don’t see many; though there are billions of stars in the galaxy that die this way,

  • this phase is very brief. Enjoy looking at them while you can. And what of the Sun?

  • Will it, one day in the distant future be at the center of a planetary nebula it expels as it dies?

  • Ehh probably not. When the Sun becomes a white dwarf, it most likely won’t be energetic

  • enough to make the surrounding gas glow; most planetaries start off as stars more massive

  • and hotter than the Sun. When our Sun dies, itll go quietly and without a lot of visible fanfare.

  • Alien astronomers, if theyre out there in 8 billion years, may not even notice.

  • But more massive stars do make quite the spectacle. And if theyre really massive, more than

  • about 8 times the Sun’s mass, they really and truly make a scene when they die. They explode.

  • But that’s for next week. Mwuhahahaha.

  • Today you learned that when low mass stars die, they form white dwarfs: incredibly hot

  • and dense objects roughly the size of Earth. They also can form planetary nebulae: huge,

  • intricately detailed objects created when the wind blown from the dying stars is lit

  • up by the central white dwarf. They only last a few millennia. The Sun probably won’t

  • form one, but higher mass stars do.

  • Crash Course Astronomy is produced in association with PBS Digital Studios. Why don’t you

  • head on over there and check out their YouTube channel -- they have lots of great videos

  • there. This episode was written by me, Phil Plait. The script was edited by Blake de Pastino,

  • and our consultant is Dr. Michelle Thaller. It was directed by Nicholas Jenkins, edited

  • by Nicole Sweeney, the sound designer is Michael Aranda, and the graphics team is Thought Café.

Hey folks, Phil Plait here. In the last episode of Crash Course Astronomy, I talked about

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白色矮星と惑星状星雲。クラッシュコース天文学 #30 (White Dwarfs & Planetary Nebulae: Crash Course Astronomy #30)

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