Globular clusters are, well, globe-shaped collections of stars. Millions of ‘em, usually, packed into a ball just a few light years across. They’re much smaller than galaxies, and in fact galaxies like the Milky Way have hundreds of them orbiting it, like bees swarming around a hive.
The stars in globulars are usually very old, indicating that studying these beasties can tell us a lot about how galaxies form. So scientists pointed Hubble at a cluster of galaxies – a gravitationally-bound collections of many, sometimes thousands, of galaxies – to see what globulars they could see. What they found was pretty cool.
The cluster they chose was the Virgo Cluster, the nearest such swarm of galaxies. It’s only 55 or so million light years away, which is practically in our face. That makes the globulars easier to see, and in their surveys the astronomers say they were able to distinguish something like 90% of the total numbers of globulars (from the ground, they’d look like stars) – they bagged over 11,000 globulars in the Virgo cluster, an incredible number (there are about 2000 galaxies in the cluster).
Of particular interest are the dwarf galaxies (like IC 3506, pictured above from the Hubble survey; nearly all the “stars” in that picture are actually globular clusters), dinky galaxies that have fewer stars than massive ones like the Milky Way. Since they’re smaller, they tend to have fewer globulars orbiting them, and are more sensitive to forces around them.
Astronomers found two interesting effects: dwarf galaxies within about 3 million light years of the center of the Virgo cluster had more globular clusters orbiting them than dwarf galaxies farther out, but dwarf galaxies closer in, less than 130,000 light years from Virgo’s center, tended to have few or no globulars at all!
This is being interpreted as meaning that globular clusters tend to form more easily in slightly denser environments such as those near (but not at) the center of galaxy clusters. However, if they venture too close to the core, they have their globulars stripped away.
M87 is a massive elliptical galaxy smack in the heart of Virgo. It probably has something like a trillion stars, five times the number the Milky Way has. Over billions of years it’s built up its mass, eating the stars from other galaxies as they fly by (it also eats entire galaxies). Apparently it’s done that with globulars from dwarf galaxies too, stripping off their globular clusters and keeping them for its own. That explains why no dwarf galaxies too close to M87 at the center of Virgo have globulars (and a chemical analysis of the globulars orbiting M87 indicate they are low in heavy elements like iron, suggesting they come from farther out in the cluster).
The image there is of M87. Take a close look; just about every single point of light you see there is a globular cluster! The beam coming from the center is a blast of particles and radiation being emitted from M87’s central supermassive black hole (which is a whole ‘nuther story unto itself).
Although the Milky Way is not really a part of any cluster (it is moving toward the Virgo Cluster but is not really a part of it) our environment is different, and the way we formed is probably different as well from galaxies in Virgo. But just like comparative anthropology, comparative galaxology (OK, I made that up, but you know what I mean) can tell us more about how we got here, and what’s happened since the Milky Way formed 10 billion years ago. It’s a big puzzle, but we have big pictures to help us figure it out.