Wednesday, June 27, 2007

Stellar Evolution: Variables and Astroseismology

So far in this series on stellar evolution, we’ve talked about things that we don’t really observe happening in real time. Rather, models show that things happen over time, and we have a series of stills to compare it to because, typically, things take a very long time.

But not always.

Sometimes things can happen very quickly. And if our models are to really be any good, they must be able to explain these events too.

Perhaps the most common set of events that we can observe on short timescales is that of variable stars. This is a very large grouping, including stars that change their brightness in less than an hour to a year or so. These wonderful stars are extremely important to astronomy. Certain types are used to measure distances and many can be used to test our models of stellar evolution.

In astronomy, there are three main types of variables that are often discussed. Regular variables are stars that change their brightness in a regular fashion. Irregular ones are, well, irregular. The third type is known as cataclysmic variables. This group includes stars which explode as novae or supernovae.

For the first two groups, there are a very large number of sub classes, typically named for the first star of the type identified as variable. For example, it was long ago known that the star Delta Cephei varied in a regular manner. Thus, stars that were discovered later which varied in a similar manner were labeled Cepheids.

Cepheids are especially important in astronomy because they’re wonderful distance indicators (their period is related to the average brightness). However, as with everything else I’ve discussed in this series of posts, models better be able to account for why these stars are changing like this.

It turns out that there’s a region on the H-R diagram where models tell us stars should become somewhat prone to instabilities. It’s aptly named the instability strip. Cepheids fall right smack in the middle of it and many of the other types do as well.

As the star evolves across the H-R diagram heading towards the Red Giant Branch, some layers of the star become more opaque than normal (less light is getting through). Since light is what carries the energy out of the star, this creates a dam, blocking the energy which causes an increase in temperature and pressure. This buildup pushes these layers outwards. When it expands, it cools, and the primary cause of the blockage (ionized electrons) is removed. But now the star is overextended and begins to collapse again. The collapse causes the temperature to increase again, reionizing the electrons and beginning the process again.

That’s the simple description but works pretty well for the better-behaved Cepheids. For the rest, many patterns can be accounted for by adding more layers of ionization causing different pulsations as well as shock fronts and other mechanisms.

So for at least one type of variable star we do a pretty good job modeling what’s going on. And when our models can jive with observations, that means it’s a good model.

Meanwhile, some variable stars can be used the other way around. Instead of matching the model to the star as we’ve been talking about here, we can use the stars to make the model.

Here on Earth we’re all quite familiar with the practice of seismology which uses shock waves traveling through the Earth and bouncing off the various layers to figure out what sort of stuff is on the inside (sadly, it’s not a cream filling). This works because seismic activity such as the crashing of tectonic plates can create strong shock waves of various sorts.

It turns out that stars do the same thing although through different mechanisms. I’m not going to bother discussion the mechanisms because that’s beside the point. The point is that stars vibrate and by studying those vibrations, we can look inside stars.

And guess what! What we find matches pretty well with our models. One particular class of variables that is commonly used for this practice (known as astroseismology) is known as Delta Scuti stars. But really any star that vibrates and we can get accurate data on it’s radial velocity to an accuracy of a few meters per second is susceptible to this method. In fact, it was originally performed on the sun via helioseismology.

So, yet again, models and observations fit hand in hand, giving us a clear picture of how stars work and evolve. In my next post on stellar evolution, I plan to look at the last stages of a star’s life, which is one of the most important to us because, as Carl Sagan put it, “We are star stuff” and the heavy materials that comprise our bodies had to be cooked up in these late phases. That’s really the last topic I planned to cover on stellar evolution so I’ll probably make one more post as a grand summary and conclusion.

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