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 it’ll 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 it’ll 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 grams — one 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 they’re
small. Hence their name. They’re so hot they also glow in the ultraviolet,
even in X-rays. Weirdly, though, because they’re so small, they’re 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 Herschel — the same man who discovered infrared
light and the planet Uranus — gave 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
of “avoid these objects” list. 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 list
— one 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. We’ll 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 stars’ orbits 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 faster…
fast 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 come… full 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 you’re 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,
it’ll go quietly and without a lot of visible fanfare. Alien astronomers, if they’re out there
in 8 billion years, may not even notice. But more massive stars do make quite the spectacle.
And if they’re 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.
r/titlegore
A red giant star still has the same mass as the original star, but much larger volume. When the Sun giants, the density of it at Earth's orbit will be many orders of magnitude less than the Earth's atmosphere. Over many years, the additional drag causes the planet to inspiral.
blackdwarfsmatter :p