In this series we are exploring the weird and wonderful world of astronomy jargon! You’ll feel a little old and red with today’s topic: globular clusters!
In globular clusters you will find a lot of older, redder stars packed closely together. In many ways, they’re the retirement homes for the galaxy.
Globular clusters are relatively small, with most no more than a couple dozen parsecs across. But each one contains hundreds of thousands, and sometimes millions, of stars. That puts the average distance between stars at about 1 light-year, but in their cores the stars pack together over a thousand times more tightly than in our own neighborhood.
The clusters are old. Indeed, they are some of the oldest objects in the universe. They have not seen new star formation for at least 8 billion years.
The Milky Way hosts about 150 known globular clusters, although astronomers suspect that the true number is closer to 200. Each galaxy has its own retinue of clusters, which larger galaxies hosting more and smaller galaxies hosting fewer. You can see some of them with the naked eye, like Omega Centauri, which is a massive cluster weighing 4 million solar masses sitting 17,000 light-years away from us.
Astronomers divide globular clusters into two rough categories. One population is relatively younger (around 8-10 billion years old), tend to have more heavier elements, and tend to be closer to the central core of the galaxy. The other population is older (10-12 billion years old), have more pristine elements, and are scattered around randomly.
While there is no universally accepted theory for the formation of globular clusters, astronomers suspect that the younger population formed alongside the galaxy, while the older clusters formed separated and were captured later.
Most interestingly, globular clusters lack any significant amounts of dark matter. Dark matter makes up most of the mass of normal galaxies, so this is a clue that clusters had a different history. Either clusters tried to grow into galaxies but failed, or they started out as smaller star clusters that got out of control.
Either way, astronomers are very interested in the nature of globular clusters, since they can tell us about the formation and evolution of galaxies.