Astronomical Data 3b. – Main sequence turnoff

In my last Astronomical Data post, I discussed a wonderfully important tool in astronomy known as the HR diagram. We saw how it gave a wonderful relationship between temperature (or color, or mass) and the luminosity of the star. Additionally, it could also help determine the radius of a star.

So all in all, it’s a pretty nifty little tool. But at the end of my post, I pointed out that, if a HR diagram was composed for a cluster instead of for a number of random, unrelated stars, it gave could also give us the age!

To explain this, I’ll start off by giving away the underlying reason: Massive stars evolve faster. This runs a bit counter to what one might expect.

Most people are aware that stars convert hydrogen to helium in their cores. So you’d expect with more massive stars, there should be a lot more hydrogen to have to use up and those little bitty red stars down in the bottom right hand corner of the HR diagram should burn out pretty quickly.

However, the opposite is actually true. Massive stars are the rock legends of astronomy: live fast, die young, and go out in a blaze. Meanwhile, their diminutive little brothers will putter along for billions of years.

So what causes massive stars to die out so quickly?

The reason is that fusion occurs more rapidly at higher temperatures and at higher densities. Massive stars have both of these quantities in excess. Their mass squeezes the core to immense pressures and temperatures.

If you think about it, this should make sense now. If fusion is caused by two hydrogen nuclei ramming into one another fast enough to stick (ie, fuse) then there’s going to be a hell of a lot more collisions in a tightly packed stellar nuclei. Additionally, when it’s really hot, atoms bounce around a lot more and with greater momentum.

Taking these two together, it’s not a huge challenge to see why massive stars burn their fuel and die faster.

But what does that have to do with the HR diagram?

If you remember in the last post, I pointed out that very special place called the “main sequence” on which stars spend the vast majority of their life. As stars begin to exhaust their fuel, they swell up and become giants.

So take a look at that HR diagram I posted before and see exactly what’s going on here. We can see the main sequence running from the upper left to the lower right across the diagram. The giants appear in the upper right. As the star swells up, it moves across the diagram heading up and to the right.

But remember that more massive stars do this before less massive ones.

Keep that in mind for a minute, as we’ll come back to it. For now, we need to discuss the other half of the situation: the cluster.

Clusters are very special little things. They’re accumulations of stars that are all gravitationally bound. They’re like a mini galaxy in a way. However, they have some very special properties which is what makes them special: They all formed from the same cloud at the same time.

That means for all intents and purposes, the stars will all be the same except for one property: Their mass.

When a cluster forms, there will be some of those massive stars we discussed earlier, and some medium ones, and some runts of the litter. Thus, if it’s a nice young cluster, all stars should be on the main sequence but scattered along the whole thing.

But give that cluster a million years and things start happening. Those giant massive stars will expend all their fuel and start drifting off the main sequence. But remember where massive stars are on the HR diagram? They’re to the upper left.

What this means is that, as stars swell up to giants, it will start at the upper left of the main sequence and slowly work its way down to the less massive stars.

Thus, by seeing where stars are turning off the main sequence, we can determine age!

This method is commonly referred to as “main sequence turnoff” and is one of the few indicators of age available in astronomy. It’s certainly the most reliable.

At this point, I’m thinking spectroscopy sounds like a very good next topic. There’s a number of other topics I’d like to discuss and don’t technically require an understanding of spectroscopy, but since spectroscopy predates many of them, I think it will help put things in a historical context which can be a good thing.