Wednesday, September 03, 2008

Why I Went to the Mountains (Part 2)

The sun rings like a bell
Image Credit: NOAO / NSO

Yesterday I blogged about the Kepler Mission and how it will be looking for Earth-like planets around sun-like stars. But, as I said, Kepler can do more than find planets. It can look at any star and return precision data on exactly how bright that star is as a function of time.

Many stars vary in brightness for many reasons. Our own sun varies its brightness over a period of about a month (the length of time it takes the sun to rotate), as sunspots come into view, rotate across the sun, move out of view behind the sun, and then rotate into view again. The sun also slowly changes brightness over its entire 11 year sunspot cycle. And, sometimes the sun flares, producing extra light from a magnetic storm for a matter of a few seconds or minutes. Other stars also show changes in brightness due to rotation or due to flares and other activity, and Kepler will watch several stars for these variations.

Perhaps the most interesting variations in the sun's brightness, though, take place on five-minute time scales. These variations are the "ringing" of the sun due to sound waves moving around the sun. Because these sound waves can penetrate down into the sun, we can use them to probe the interior of the sun , just like geologists can use seismic waves from earthquakes to study the interior of the Earth. (The picture at the top of this post is a cartoon of sound waves penetrating the sun.) The use of these sound waves is called helioseismology.

If you want to study the interior of the sun using sound waves, you have to be constantly watching the sun. Helioseismologists, the astronomers studying the interior of the sun from these sound waves, have set up a collection of solar telescopes around the Earth (called GONG) to watch the sun as much as possible. But the biggest breakthroughs have come from satellites like SOHO, which watch the sun 24/7. Through a careful study of the tiny variations in the sun's brightness from studies like GONG and SOHO, helioseismologists have been able to confirm many theories about the structure of the sun.

Other stars also change their brightness due to sound waves, just like the sun. Although we can't study them in the same detail as the sun, we can still measure the variations. And, just like the sun, we learn more if we can watch stars for as long as possible. So, a satellite like Kepler is perfect for trying to do seismology of other stars (called asteroseismology).

Tomorrow, I'll talk about the type of asteroseismology my collaborators and I are trying to do with Kepler, and why I need McDonald Observatory now, when we'll have a satellite next year!

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