White Dwarfs

Where a star ends up at the end of its life depends on the mass, or amount of matter, it was born with. Stars that have a lot of mass may end their lives as black holes or pulsars. Low and medium mass stars will become something called a white dwarf.

Medium mass stars, like our sun, live by burning the hydrogen that dwells within their cores, turning it into helium. This is what our sun is doing now. The heat the sun generates by its internal combustion creates pressure inside. In another 5 billion years, the sun will have used up all the hydrogen in its core.

This situation in a star is similar to a pressure cooker. Heating something in a sealed container causes a build up in pressure. The same thing happens in the sun. Although the sun may not strictly be a sealed container, gravity causes it to act like one, pulling the star inward, while the pressure created by the hot gas in the core pushes to get out. The balance between pressure and gravity is very delicate.

When the sun runs out of hydrogen to burn, gravity temporarily tips the balance, and the star starts to collapse. But compacting a star causes it to heat up again and it is able burn what little hydrogen remains in a shell wrapped around its core.

January 15, 1996, Hubble Space Telescope Captures First Direct Image of a Star, A. Dupree (CfA) and NASA.

This burning shell of hydrogen will give our sun the energy to expand again. When this happens, our sun will become a red giant; it will be so big that Mercury will be completely swallowed! When a star gets bigger, its heat spreads out making its overall temperature cooler. But over time, the core temperature of our red giant sun will increase again until it's finally hot enough to burn all the stored up helium it created in its former incarnation. Eventually, it will transform the helium into carbon and other heavier elements. The sun will only spend one billion years as a red giant, as opposed to the nearly 10 billion it spent busily burning hydrogen.

Low-mass stars, because they contain so little matter, soon run out of things to burn. When they run out of hydrogen, they will never be able to get hot enough to get big and burn helium. Instead, they contract, get very small, and become what we call a red dwarf. The red dwarf will keep shrinking and the smaller it gets, the more it heats up. But it still will not heat up enough to become a red giant; never quite able to reignite itself, it will, instead, become a white dwarf. But more about that in a minute.

We already know that medium mass stars, like our sun, become red giants. But what happens after that? Well, our red giant sun is still eating up helium and cranking out carbon. But when it's finished its helium, it isn't quite hot enough to be able to burn the carbon it created. What now?

Our sun isn't hot enough to ignite the carbon it its core, so the only thing it can do is succumb to gravity again. When the core of the star contracts, it causes a release of energy that makes the envelope of the star expand. Now the star has become an even bigger giant than before! Our sun's radius has become larger than Earth's orbit! The sun is not very stable at this point because thermal pulsations are causing it to lose mass; this happens until finally the star finally blows its outer envelope off in a gust of superwind. This leaves behind a hot core surrounded by an expanding shell of gas that looks a bit like a planet when viewed through a telescope. Thus these shells of gas were given the name: planetary nebula! They are still called this for historical reasons, although we now know they have nothing to do with planets. There are some planetary nebulae that can be viewed through a small backyard telescope, although it is not possible to resolve the small hot star that was formerly the core of a red giant. You'd need a much larger telescope for that!

Planetary nebulae seem to mark the transition of a medium mass star from red giant to white dwarf. Stars that are comparable in mass to our sun will become white dwarfs within 75,000 years of blowing their envelopes. Eventually they, like our sun, will cool down, radiating heat into space, fading into black lumps of carbon. It has taken 10 billion years, but our sun has reached the end of the line and quietly become a black dwarf.

White dwarfs can tell us important things about the age of the universe. If we can estimate the time it takes for a white dwarf to cool into a black dwarf, that would give us a lower limit on the age of the universe and our galaxy. Because it takes billions of years for white dwarfs to cool, we don't think the universe is old enough yet for many, if any, white dwarfs to have become black dwarfs. This is why we want to learn more about white dwarfs. They could be an important key to understanding our universe.

A teaspoonful of white dwarf matter would weigh 5.5 tons on earth - as much as an elephant!

Because a white dwarf is no longer able to create internal pressure, gravity unopposedly crushes it down until even the very electrons that make up a white dwarfs atoms are mashed together. In normal circumstances, identical electrons (those with the same "spin") are not allowed to occupy the same energy level. Since there are only two ways an electron can spin, only two electrons can occupy a single energy level. This is what's know in physics as the Pauli Exclusion Principle. And in a normal gas, this isn't a problem; there aren't enough electrons floating around to completely fill up all the energy levels. But in a white dwarf, all of its electrons are forced close together; soon all the energy levels in its atoms are filled up with electrons. Well, if all the energy levels are filled, and it is impossible to put more than two electrons in each level, than our white dwarf has become degenerate. For gravity to compress the white dwarf anymore, it must force electrons where they cannot go. Once a star is degenerate, gravity cannot compress it any more because quantum mechanics tells us there is no more available space to be taken up. So our white dwarf survives, not by internal combustion, but by quantum mechanical principles that prevent its complete collapse.

Degenerate matter has other unusual properties; for example, the more massive a white dwarf is, the smaller it is! This is because the more mass a white dwarf has, the more its electrons must squeeze together to maintain enough outward pressure to support the extra mass. There is a limit on the amount of mass a white dwarf can have, however. It was found by Subrahmanyan Chandrasekhar to be 1.4 times the mass of our sun, and is is call the Chandrasekhar limit after its discoverer.

With a surface gravity of 100,000 times that of the earth, the atmosphere of a white dwarf is very strange. The heavier atoms in its atmosphere sink and the lighter ones remain at the surface. Some white dwarfs have almost pure hydrogen or helium atmospheres, the lightest of elements. Also, the very strong gravity pulls the atmosphere close around it in a very thin layer, that, if were it on earth, would be lower than the tops of our skyscrapers!

Underneath the atmosphere, scientists believe there is a 50 km thick crust, the bottom of which is a crystalline lattice of carbon and oxygen atoms. One might make the comparison between a cool carbon/oxygen white dwarf and a diamond! (After all, a diamond is just crystalized carbon!)

There are several ways to observe white dwarf stars. The first white dwarf ever to be discovered was found because it is a companion star to Sirius, a bright star near the constellation Orion. In 1844, astronomer Friedrich Bessel noticed that Sirius had a slight back and forth motion, as if it were being orbited by an unseen object. In 1863, this mysterious object was finally resolved by optician Alvan Clark and it was found to be a white dwarf. This pair are now referred to as Sirius A and B, B, being the white dwarf. The orbital period of this system is about 50 years. Since white dwarfs are very small and thus very hard to detect, binary systems are a helpful way to locate them. As with the Sirius system, if a star seems to have some sort of unexplained motion, we may find that the single star is really a multiple system. Upon close inspection we may find that it has a white dwarf companion.

The arrow is pointing to white dwarf, Sirius B.

The Hubble Space Telescope, with its 2.4 meter mirror and advanced optics, has been successful at viewing white dwarfs with its Wide Field and Planetary Camera. In August of 1997, this camera observed more than 75 white dwarfs in the globular cluster M4. These white dwarfs were so faint that the brightest of them was no more luminous than a 100 watt light bulb seen at the moon's distance, NASA said in a press release. M4 is located 7,000 light years away, but is the nearest globular cluster to Earth. It is also approximately 14 billion years old, which is why so many of its stars are near the end of their lives.

Optical mirrors are not the only way to view white dwarfs. The white dwarf HZ 43 was observed by the X-ray satellite ROSAT. In most single stars that are X-ray sources, the X-ray emission comes from the surrounding corona of 1 - 10 million degree gas. However, in a white dwarf that has outer layers with no heavy elements, the observed X-rays most likely come from its deep interior. A white dwarfs helium and hydrogen outer layers make the star essentially transparent to the X-rays that are emitted by the much hotter inner layers.

The elephant design is by Silk Oak Studios.

This file was last modified on 18 October, 1997