Why Globular Clusters are the Smallest

“From the intrinsic evidence of his creation, the Great Architect of the Universe now begins to appear as a pure mathematician.” –Sir James Jeans

Last time we met, I posed a mystery to you: why are globular clusters the smallest in the Universe?

And what’s more than that, we never find them in isolation! We always find them bound to galaxies (or, if not bound to a galaxy, then within a cluster of galaxies; thanks Steinn), but never just off in deep space, floating on their own like some lone Death Star.

Like all structures in the Universe, everything we’re talking about here — stars, globular clusters, galaxies, and clusters — formed from gravitational collapse. But all things don’t collapse equally! Something resists gravitational collapse, and regardless of what causes it, it’s always some type of pressure.

The Sun, for instance, is less dense than the Earth is, despite being 300,000 times as massive! What’s going on here? The tremendous pressure generated from the Sun burning its nuclear fuel actually pushes the outer layers of the Sun away from the center, balancing the force of gravity and preventing it from collapsing any farther.

In that case, you get a super-dense, collapsed star, like a white dwarf star. (To the left of the Sun-like star, above.) Without the pressure from nuclear fusion, it’s a lot denser, and its collapses even farther. Except it can’t collapse forever, because there’s a new source of pressure: the atoms themselves!

What does this have to do with globular clusters? Let’s go all the way back to when the Universe was filled with photons, nuclei and electrons, but before we formed neutral atoms!

Gravity is working as hard as it can to collapse everything down to smaller, denser structures. And it’s most successful where there’s the most amount of matter. But when the Universe is this young, the pressure from the photons flying around is simply too much! Whenever gravity gets the upper hand and a cloud of matter starts to collapse, the photons push back even harder, and the cloud gets washed out!

So all the tiny little structures that try to get formed — things that would be the size of isolated planets, stars, and open star clusters — get washed out. Until, that is, you form neutral atoms for the first time! That’s at the same time that the Cosmic Microwave Background (CMB) gets emitted. The CMB is this famous guy, for those of you who like pictures.

But now, with neutral atoms, they interact with photons way less frequently than they did when all the matter was ionized. What does this mean? It means the pressure drops tremendously, and all of these structures can now collapse!

But the small structures — the ones that would be small structures, anyway — have all been washed out! If you want to know what the smallest structure on its own you can form is, you have to look to the work of James Jeans, and hence the quote at the top. If you plug in the numbers for the density and temperature of the Universe at this time into the Jeans’ Length equation to find the mass of the smallest structure that you get, you know what you find?

The smallest structures you form are between 100,000 and 1,000,000 times the Mass of the Sun! So that’s why you never get a cluster, on its own, that’s smaller than a hundred thousand stars!

And why are they never on their own?

Quite simply, because gravity works like a web, and pulls small structures together into larger ones. By this point, nearly 14 billion years after these seeds for structure formation were sown, gravity has had enough time to pull all of these clusters into larger structures! (Err… probably. We have to remember that absence of evidence does not mean evidence of absence.)

And that’s why Globular Clusters are the smallest! Happy Monday!