What are white dwarfs? (Astronomy)

By | December 6, 2019


The universe is dotted with a TREMENDOUS number
of stars, each shining brightly from the energy of nuclear fusion. At any given moment, new stars are born, while
others begin to die. The fate of a dying star depends on its mass. The largest stars will become neutron stars
or black holes. But the vast majority of the stars will become
hot balls of glowing matter that will shine for billions of years as they slowly cool
down and disappear from view. These are white dwarfs. Stars shine because of nuclear fusion. It takes a LOT of energy to fuse two atoms
together. This is why large bodies, like stars, fuse
matter, while smaller objects, like planets, do not. Stars are so large that gravity squeezes their
matter together tightly enough for fusion to occur. This fusion mostly occurs in the core of the
star, but at times it can also take place in layers surrounding the core called shells. As the star ages, fusion happens in different
parts of the star and produces different elements. This process is called stellar evolution, and the path it takes depends on the star’s initial mass. During this process, the size, temperature
and composition change. But for every star, a point is reached where
the pull of gravity is no longer strong enough to keep fusion going. For most stars, once fusion stops, that’s it. Because fusion is no longer generating an
outward pressure, the star contracts under the force of gravity to about the size of
the Earth. But the lights don’t go out right away;
all that matter is still extremely hot! And hot things glow. So now you have a small ball of white-hot
matter floating in cold space. We call this a white dwarf. As the years pass, white dwarfs cool down
and begin to dim. After all, space is very cold – close to absolute
zero! So how long do you think it will take the
white dwarf to cool down to the point that it goes dark? One year? A thousand years?? A million years??? It’s believed it will take TRILLIONS of years for white dwarfs to stop glowing. That’s longer than the current age of the universe! But one day, somewhere in space, the lights
will go out in a white dwarf. At that point all that’s left is a dead
ball of dark matter which we call a black dwarf. Although white dwarfs are small compared to
burning stars, they are still quite massive. Their mass can be as high as 1.4 solar masses,
a value called the Chandrasekhar Limit. Because of their large mass, gravity very
tightly compresses the stellar matter. But at some point you run up against the Pauli
Exclusion principle. This idea from quantum mechanics says, basically,
that two identical particles cannot occupy the same place at the same time. For white dwarfs, the pressure from gravity
has stripped all the electrons from the atoms. Gravity then packs the electrons so closely
together that the Pauli exclusion principle kicks in and stops the white dwarf from shrinking
any further. This is called degenerate electron pressure. White dwarfs are about the size of an Earth-like
planet and shine with only a fraction of the brightness of a star. So how do you find and observe them? One way is to look for stars with an unexplained
back-and-forth motion. This suggests a massive companion. If you can’t see the companion,
it’s probably a black hole. But oftentimes, telescopes are able to see
a small, dim neighbor – a white dwarf. In fact, the brightest star in the night sky,
Sirius, has a white dwarf companion which we call Sirius B. There’s another way to find white dwarfs,
and it’s due to a curious phenomenon which occurs near the end for some stars. Earlier we mentioned that fusion can occur
not just in the core, but in shells around the core. When a star begins to die,
fusion may stop in the core. That’s because the core is now made of heavier
elements and there isn’t a strong enough compression to fuse them. But a shell of helium may start to burn, in
an event called a Helium Shell Flash. This creates a shock wave called a thermal
pulse, which blows a lot of the star’s matter into space. This expanding sphere of matter glows from
the star creating a beautiful display called a planetary nebula. The name is a bit misleading. It has nothing to do with planets. The name was chosen hundreds
of years ago by William Herschel because he thought they looked like planets. Regardless of the name, when you see a planetary
nebula in space, there’s a good chance you’ll find a white dwarf in the center. It is estimated that more than 90% of the
stars in our galaxy will end up as white dwarfs. So as time passes and the billions and billions
of stars in the Milky Way begin to burn out and fade from view, you can rest assured that
for more than a trillion years, you’ll still be able to find a warm,
glowing white dwarf to call home until you can find a more permanent location. While white dwarfs will shine long after
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