Friday, September 05, 2008

Why I Went to the Mountains, Part 3

Variations in the brightness of a white dwarf
Image Credit: K. Williams/T. Jones/McDonald Observatory

Over the past couple of days, I've slowly been telling the story of my observing run. I hope you'll forgive the length, but I thought it needed some background. Anyway, in my first post I described the Kepler mission, and in the second post, I described asteroseismology and how Kepler may be a useful tool for exploring the interior of stars.

So, enough background. Now for my story. I study white dwarfs, the remains of stars that have completed their life cycles. For most of a star's life, it shines by nuclear fusion. First it fuses hydrogen into helium, and, toward the end, it fuses helium into carbon and oxygen. For most stars, that's it, as their gravity isn't strong enough to do any further nuclear reactions. The star's outer layers puff off in a planetary nebula, laying bare the hot ashes of the dead star's nuclear furnace. This we call a white dwarf, because it is white hot, and because it is tiny (the mass of the sun squeezed into a ball the size of the Earth).

There are many things we don't understand about white dwarfs. For example, while we know that they are mostly made of carbon and oxygen, we don't know how much of each is present. We also don't know how well mixed these elements are in the star. White dwarfs tend to have atmospheres of hydrogen or helium (or, in rare cases, carbon), but we don't know how thick these atmospheres are, or if the thickness varies from star to star. To answer these and other mysteries about white dwarfs, we'd like to be able to probe the interior of these stars. Only we have to do it from dozens of light-years away.

White dwarfs don't make any of their own energy, they simply radiate away the heat they gained back when their parent star was still alive and happily fusing elements. So, over hundreds of millions of years, the white-hot white dwarf will slowly cool off, just like a hot poker withdrawn from a blacksmith's fire will cool from white to orange to red. And, in a specific range of temperatures (about 19,000-21,000 degrees Fahrenheit for white dwarfs with hydrogen atmospheres), the atmospheres become unstable and start to slosh around. That sloshing makes sound waves that penetrate the star, so we can do asteroseismology!

The plot at the top of this post shows the variations in the light from a white dwarf due to this sloshing. Where the atmosphere piles up, it gets hotter, which causes it to glow more brightly. Where the atmosphere is thinning out, it gets cooler and fainter. So, we can "watch" the sloshing of the star by measuring its brightness.

Astronomers have been using this sloshing (which we give the more elegant name of pulsations) to study the interiors of white dwarfs for 25 years. But, like with the sun, our data will become far better and far more useful if we can use a satellite to stare at the same star for years at a time. And that's just what Kepler aims to do!

So, all we have to do is find a pulsating white dwarf in the field of stars that Kepler will be looking at. And that's easier said than done. The Kepler field is full of stars, about 12 million of them! And our best estimates are that there are 2 or 3 pulsating white dwarfs in the Kepler field. Those aren't good odds. But the Kepler people have put a lot of work into cataloging all of those 12 million stars, and our collaborators around the world have used detailed selection criteria to pick out the best candidates. So, our European collaborators have been looking at these stars at their telescopes in the Canary Islands, and I've been sitting in the fog trying to help out from here. Now we just need to cross our fingers and hope to find that needle in the haystack.

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