More on Stellar Evolution

A few months ago, I had a rather lengthy post concerning stellar evolution. However, that post was mainly presented as a response to a rather absurd set of creationist arguments.

But instead of always doing things in response, I think it’s good to occasionally be more pro-active and cover topics in a more educational manner. So in this series of post I intend to first lay out the basic picture of stellar evolution, from the main sequence to death, and later, get into how this theory is derived and supported.

The first thing that I think is important to point out, is that, despite the equivocation of creationists, stellar evolution has absolutely nothing to do with biological evolution. It’s an entirely different theory based upon an entirely different body of evidence. Stars don’t have inheritable characteristics, and even if they did, they don’t reproduce, so the basics of biological evolution just don’t make any sense here.

Instead, “evolution” in the stellar sense refers to the colloquial definition in which it just means a change over time. But what sorts of changes? Well, changes could include such things as temperature, pressure, chemical composition, and a whole array of other features, such as mass, size, luminosity, most of which are determined by the first three.

Astronomers know these features change, albeit very slowly (usually).

I’ll discuss the rather sudden changes later on, but more important, is to begin to explain how we know stars are evolving even when we claim the timescales are hundreds of millions, to billions of years (far too long to witness in the entire course of human history), and the changes nearly imperceptible.

Perhaps the largest reason we know stars evolve, is because stars shine.

This seems like a very confusing statement at first, but if you think about it a bit more, it makes sense. A star shining means that it’s giving off energy. For stars like our sun, they’re giving off a lot of energy. The sun gives off just under 4 x 10²⁶ Joules of energy per second.

In 2000, the US consumed 1 x 10²⁰ Joules of energy. This means that, in 1 second, the sun generates enough energy to last the US 4 million years. That’s a lot of energy.

And it’s got to come from somewhere.

To figure out where, we start off by looking at what’s available. By looking at the spectra of the sun, we know it’s about 75% hydrogen, 24% helium, and 1% everything else (if you need a refresher on what spectra are, go here). This rules out a lot of possibilities of where the energy comes from right off the bat.

Early ideas suggested that the sun was a ball of fire. However, fire requires the presence of oxygen. Thus, that’s right out. Additionally, there’s just not enough chemical energy available to sustain the sun for the amount of time we’ve known the solar system to exist.

The sun shrinking under its own gravity and exchanging gravitational potential energy for other forms was another possibility. But the sun’s not shrinking fast enough to account for the energy generation of the sun and again, the timetable doesn’t fit.

So astronomy required something that generated a lot of energy using hydrogen and could be sustained for billions of years. Fusion fits the bill perfectly. It also fits well because, when physicists realized that fusion will emit neutrinos and we went looking for them and found them (although only 1/3 the predicted number originally. For more on the missing neutrino problem, go here).

Now that we know that fusion is the source of energy generation for the sun and other stars, we can start to ask what the consequences are.

One consequence that fusion has is that it converts hydrogen into helium. This means that there’s going to have to be a change in chemical composition. Why? Because it shines. If it didn’t, there would be no need for fusion to change the chemical composition and there wouldn’t be any evolution.

Another side effect of this, is that, give it long enough, and you’ll use up all your hydrogen. When that happens, it means you’re not going to have much energy generation anymore. This doesn’t mean the star stops shining all the sudden. It takes light a long time to work its way to the surface and escape due to the fact that it’s going to be scattered off lots of particles along the way. But this does mean that the brightness is going to change. Again, the star evolves.

Aside from sustaining the luminosity of the star, the out flowing radiation has another important effect: It provides pressure to keep the star from collapsing in on itself. With this pressure gone, the star will inevitably contract. This happens relatively slowly, but due to the large mass of stars, and the large radius, this can generate a lot of energy.

And it does. This energy primarily goes into the interior of the star (since that’s what’s being squished). The extra energy heats the interior until some of the hydrogen outside of the core can begin fusion (known as H-shell burning). This supports the star somewhat, but doesn’t quite halt the slow contraction.

Rather, the core continues getting squeezed, and heated, until the point at which it gets hot enough that the Helium atoms can begin fusing. Again, the star has a strong energy source supporting it. This extra energy pushes back against the matter on top of it, causing the star to poof up into a red giant. It eventually stabilizes, and becomes something like a normal star again.

Eventually, the helium too runs out. If the star is massive enough, more cycles of contraction, shells of fusion, and new forms of fusion in the core will occur. If not, the star will go through one last bit of the cycle, where the outer layers are pushed out and released as a planetary nebula.

For those more massive stars, the cycles can continue until iron is built up in the core. This is one of the points where things happen fast.

Out of all the forms of fusion, Hydrogen is the most efficient. Each successive cycle is less so. By the time you hit iron, you’re barely getting energy out of it. With iron, you get none at all.

Once iron builds up to a critical mass in the core, it very suddenly collapses in on itself. Several suns worth of mass crashes down, releasing more energy in that act, than the rest of the entire galaxy it lives in. This is known as a supernova.

So that’s the basic story of stellar evolution. But at this point that’s all it is. In my next post, I’ll get more into how this theory of stellar evolution is all determined, both by mathematical modeling and comparisons of those models to direct observations.