Story of a Star Cluster


Meet M13, one of my favorite globular star clusters.

M13, also known as Messier 13 or the Hercules Cluster, is found—surprise surprise—in the constellation Hercules in the northern hemisphere.

The really cool thing about star clusters is that they look just as spectacular through a telescope as they do in a good image—that is, on a clear, dark night with good seeing conditions.

So…why am I showing you a picture of a star cluster? (Besides the fact that they’re gorgeous?)

Well…after all the talk I’ve done of stellar evolution, I know what you’re going to ask me next…how the heck do we know all this?

That’s a very good question—and one that star clusters can answer.

First…what the heck are star clusters?

In order to answer this, we should make a distinction between open clusters and globular clusters.

What’s the first thing you notice about these two types of clusters?

If you said that the globular cluster (on the left) looks like a very compact ball of stars, and the open clusters (on the right) look far more “open” and spread out—well, you’re right.

Globular and open clusters probably formed in very different ways—ways that I’ll be delving into one of these days. But one thing they have in common is that all their stars formed at about the same time, from about the same materials.

This means that if you look at any one star cluster, all the stars are roughly the same age, and have roughly the same composition.

That makes star clusters a powerful tool for exploring stellar evolution.

Remember our old friend, the H-R diagram? Let’s take a look at an H-R diagram with only the stars from one star cluster plotted.

This is the H-R diagram for M67, an open cluster. Notice that there are no stars plotted on the upper-main-sequence—all of these stars have aged and moved up and to the right on the H-R diagram. This means that they have expanded and cooled.

The lower-main-sequence, however, is rich with stars—tons of stars. These stars have barely begun to evolve off the main sequence (the majority of a star’s lifespan).

What does this tell us about stars?

It’s key to remember that this H-R diagram contains stars that are all plotted at the same moment in time. So right now, in this one star cluster, we are observing that upper-main-sequence stars have evolved into giants and supergiants before lower-main-sequence stars.

In other words, lower-main-sequence stars—that is, lower-mass stars—live longer than massive stars.

Now, let’s take a look at the H-R diagram for a globular cluster—M13!

This H-R diagram is very different, but also very similar. We still see main-sequence stars and stars that have evolved. But now, we see a very clear horizontal branch—this is where stars are beginning to fuse helium in their cores. We also see that fewer stars are still on the main sequence.

What does this tell us?

Well, if fewer stars are still on the main sequence, that means more stars have evolved off of it—that is, more stars have had time to do so. So…we’re literally looking at an older star cluster.

There’s a common feature you’ll notice on most H-R diagrams of specific star clusters: the turnoff point. As you might have noticed on the diagram above, that’s the point where the stars in the cluster “turn off” of the main sequence.

The turnoff point is different for every star cluster. See all the little colored curves where these open clusters “turn off” of the main sequence?

Notice that if the turnoff point is high (higher up the main sequence), only some stars have evolved, but most are still on the main sequence. But if the turnoff point is low, most of the stars have evolved. So, we can tell the age of a star cluster by where the turnoff point is.

It turns out that most globular clusters are very old—around 11 billion years. Open clusters, on the other hand, tend to be younger. The oldest open clusters we know of are probably about 10 billion years old.

I think the coolest thing about H-R diagrams of star clusters is that they tell a story of the stars within them.

It can be easy to forget sometimes that our universe is a dynamic place, always changing. These star clusters are changing and evolving as we speak—albeit slower than molasses—and each one represents a snapshot of the life cycles of different types of stars.

Here’s a slideshow to make it easier to visualize how the H-R diagram changes with time for any one star cluster:

As with most concepts in astronomy, theory comes before observation, and we look for observations to check the answers in our theories rather than think of theories to explain our observations.

So here’s the question: can we actually find star clusters that look like the various snapshots of time on an H-R diagram?

The answer is yes!

Meet NGC 2264, a very young cluster that still resides within the nebula of its birth.

As you can see, the H-R diagram for NGC 2264 resembles the first step in the slideshow.

And for another snapshot of stellar evolution, we need look no further than another of my favorites: the Pleiades.

The Pleiades are another relatively young star cluster. The least massive stars in the cluster are still contracting (forming), and have not quite reached the main sequence.

The most massive stars, on the other hand, have already begun evolving off the main sequence.

Last but not least, let’s take another look at M67. (This time I’ve included the raw data.)

Notice that the H-R diagram for M67 more or less resembles the last two steps of the H-R diagram slideshow?

So…now that we’ve got a good idea of what H-R diagrams look like for star clusters of different ages, and we’ve gone over the life cycles of stars in other posts, let’s put the whole thing together.

Stars begin their lives deep within giant molecular clouds. One large cloud fragments into many smaller cores, which contract to form main-sequence stars.

As we’ve seen, low-mass stars—that is, the lower half of the main sequence—take longer to contract and are often still protostars while the more massive stars are beginning to evolve off the main sequence.

Keep in mind that the H-R diagram does not actually indicate movement through space. It is a graph of temperature vs. brightness, so when stars contract onto the main sequence, they are actually getting hotter and fainter. As they leave the main sequence, they are getting cooler and brighter.

But…wait a second. Why would they be getting hotter and fainter, and cooler and brighter? Shouldn’t it be the other way around?

It turns out that a star’s luminosity—the technical term for brightness—is actually directly related to its surface area.

So a star whose data point moves up and to the right on the H-R Diagram is actually expanding, and a star whose data point moves in the opposite direction is actually contracting.

Sound familiar from my post on how stars evolve beyond helium fusion?

Well, now you’ve seen how stars evolve, and you’ve seen the evidence we have—in the form of some of the night sky’s most gorgeous objects. Next up, we’ll take a look at another form of evidence we have that stars evolve: variable stars.

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