Wednesday, October 15, 2008

Bubble, bubble, toil and trouble: How convection affects stars

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As I mentioned yesterday, this week I'm at a conference in Baltimore on the ages of stars. Many of yesterday's talks centered around calculating the ages of stars from theory. We do this by "building" a star in a computer, letting the computer age the star according to physics, and seeing what happens.

As a general idea, this is a fairly simple concept. You need to tell the computer how massive the star is, what material the star is made out of, and then just give it a list of physics: nuclear reactions, gravity, temperature and pressure laws, and other such stuff. The computer then calculates the life cycle of the star. Essentially, at the core of the star, nuclear fusion converts hydrogen to helium. When the hydrogen runs out, the star begins to die.

But the devil is in the details. One of the biggest issues that we've struggled with for decades is called mixing. Parts of stars undergo a slow roiling, like the material in a lava lamp. Blobs of hot material rise up, cool off, and sink back down. The motion of these blobs stirs up the material in a star, and can bring some fresh fuel down to the nuclear reacting core of a star. The problem is, this motion is very hard for computers to model. We can put simplified versions of mixing into computers, but it's a bit of a guess as to how much.

Physics doesn't tell us how much things will get mixed up by this roiling, because we don't have enough information about the exact conditions in a star. So, we just put a knob on the computer models, and slowly turn it from no mixing to a lot of mixing. When astronomers do this, the age of a given star can change by almost a factor of two. And that's not so great; it's like you asking me how old John Q Public is, and I respond that he's between 40 and 80 years old.

But all is not lost. The mixing does more than just bring some fresh fuel to a star's nuclear furnace. It also subtly changes what the star looks like from the outside. So, the job then falls to people like me, an observer, to measure what large numbers of stars look like and to compare that with the various computer models. And, when we do this with stars, we find that there needs to be some moderate mixing, which makes stars a little older than we thought. So, I could use that to tell you that John Q Public is in his mid to late 60s. That may not be as accurate as you'd like, but it's better than the earlier answer.

This was part of the point of this conference, to bring both theorists and observers together to compare our data and computer models, and to let each other know our newest and best results.

Tomorrow, we talk about my main topic of interest -- white dwarfs.

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