Monday, March 16, 2009

Making a wild star

As I mentioned yesterday, I'm spending this week at a conference called "Wild Stars in the Old West", where we are talking about a star system known as cataclysmic variables. These are objects where a white dwarf is orbiting so close to another star that the white dwarf's gravity can pull material off of its companion. This process produces an ever-changing amount of heat and light, which we see on Earth as a single star that appears to flicker like crazy.

But where do these exotic things come from?

It all starts with the fact that half of the stars you see in the night sky are not single stars, like our sun, but often systems of two or more stars. The stars can be far apart, hundreds of times the distance that the Earth is from the Sun, or they can be close together, only about ten million miles apart (remember that the Earth is about 93 million miles from the Sun), or anywhere in between.

Stars live by nuclear fusion, turning hydrogen into helium. When a star runs out of hydrogen, it begins to bloat up into a red giant star. Our own sun, as a red giant, will swell up from its current 900 thousand mile diameter to about 150 million miles across; that's big enough to swallow Mercury and Venus!

If a star like the sun has a companion several billion miles away, the companion star will happily go about its business, continuing in its orbit and pretending not to notice that its sibling is swelling up like a balloon. But if the dying star's companion is within 100 million miles, it will soon find itself inside its sibling. You might think that would mean a fast and messy end for the companion star, sort of like the Blob absorbing a poor innocent bystander. But no!

The dying star is not gaining weight or mass, it is simply expanding. When you take a star like the sun and spread it out over a volume the size of Earth's orbit, its density drops by a factor of a million. So, while the star looks big and imposing, it's really quite fluffy and tenuous. The companion star, meanwhile, is still pretty dense. So, the companion star can actually continue to orbit inside the outermost layers of the red giant star!

This doesn't mean the companion star is completely unscathed; it does get heated up a bit, and even though the red giant star is fairly fluffy, the companion does encounter some small but persistent wind resistance. This friction causes the companion star to slowly spiral even closer to the core of the red giant star.

There are two possible outcomes. One is that the companion star spirals all the way in to the center of the red giant, where it merges with the red giant's core and loses its own identity. Later, when the red giant sheds its outer layers as a planetary nebula, it will leave behind a single white dwarf star, leaving little, if any, evidence that the companion ever existed.

However, if the companion doesn't spiral all the way in to the center, the red giant can still go through its planetary nebula phase, leaving behind a white dwarf with a scarred, but surviving, companion in a very tight orbit (In fact, the companion star often helps to eject this material, almost as if it is trying to avoid being swallowed alive). In many cases, the companion star is only a couple of million miles (or less) away from the white dwarf, completing an orbit in just a few hours (remember, the Earth takes a whole year to go around the sun). At those small distances, the strong gravity of the white dwarf can begin to steal material from its companion, and we get a cataclysmic variable.

Anyway, most of today's discussions were on these companion stars (most are tiny red dwarf stars) and how the orbits of these companions change as their outer layers are slowly sucked away by the white dwarf (first the orbits get smaller, then they get bigger. Don't ask me why -- it's complicated stellar physics that I don't feel like explaining tonight.)

Tomorrow I present my results, so I'll tell you about my findings then.

1 comment:

  1. This article has been added to the Astronomy Link List.

    ReplyDelete