Globular Clusters Are a Galaxy’s Fossils. But Where Do They Come From?

Globular clusters, almost as old as the universe itself, reside in the halo of each galaxy. Despite their long-standing popularity, astronomers are still disputed about their origin. Let’s explore these odd collections of stars! 

Omega Centauri; Source: Wikipedia

16’000 light years from Earth lies the dense globular cluster Omega Centauri, the largest of its kind with a diameter of 150 light years. What ancient astronomers thought to be one star turned out to be extremely wrong: Omega Centauri comprises 10 million stars. 

Most of the stars in Omega Centauri are metal-poor and old, about 12 billion years. That’s just two billion years younger than the universe itself!

Many of the properties of Omega Centauri make it a classical globular cluster. It…

• orbits in the Milky Way’s halo, distinct from the spiral structure where stars form in the lanes of gas and dust.
• is very dense, with the stars at the core thought to be separated by not more than 0.1 light years. In comparison, the Sun’s closest neighbor is four light years away. 
• has a spheroidal shape with mostly dim red dwarfs or dead remains, like white dwarfs and neutron stars.

However, what makes Omega Centauri different from other globular clusters is its theory of origin, as it has been speculated that Omega Centauri is the core of a dwarf galaxy “cannibalized” by the Milky Way. Its orbit around the galactic center, as well as the different ranges in stellar ages, are clear signs of that. 

But while there are formation hypotheses for the 150 globular clusters orbiting the Milky Way, as well as the millions of globular clusters orbiting other galaxies, they remain a big mystery. Not only are they odd in composition, their very nature is controversial. Nobody knows for sure where they come from. 

Globular cluster VVV CL001; Source: Wikipedia

As for stars, there are two main types of clusters: open and globular. 

Open clusters differ from globular clusters fundamentally. They are always found in a galaxy’s disk and contain young, blue, bright stars with short lifetimes. The Pleiades is the best-known open cluster, but what many don’t know is that our Sun was also once part of an open cluster when it was still in its stellar nursery. 

Globular clusters are oddballs like Omega Centauri but also include other popular objects like Messier 22 and Messier 5. When astronomers first looked at them with telescopes, they weren’t really interesting to look at. But as telescopes improved with better resolutions, the globular clusters turned out not to be single stars, but many of them, packed into a—astronomically speaking—tight space. 

Globular clusters gained traction and eventually helped astronomers find that the Milky Way is a spiral galaxy and that we’re not at the center of it. 

A majority of galaxies have globular clusters. Our next-largest neighbor, the Andromeda Galaxy, holds the record of containing the brightest globular cluster in the Local Group, Mayall II. It’s twice as massive as Omega Centauri and even has an intermediate-mass black hole at its center, 20’000 Suns heavy. 

Mayall II; Source: Universe Today

Just like Omega Centauri, it’s theorized that Mayall II is the core of a former dwarf galaxy, perhaps disrupted in the making of a large spiral galaxy like the Milky Way. 

If you’re looking for a haven of globular clusters, check out giant elliptical galaxies. Ellipticals are some of the biggest galaxies in the universe, though they aren’t as interesting to look at as spiral galaxies, they often sit at the center of massive galaxy clusters. 

These giant ellipticals also have a higher amount of globular clusters than the Milky Way – in fact, there are so many astronomers wonder how it’s possible. Messier 87, famous for having its central black hole photographed, has more than 15’000 globular clusters. The 150-or-so globular clusters of the Milky Way are nothing in comparison. 

M87, at the center of the Virgo Cluster, therefore paints the following picture: that globular clusters are part of galaxy formation in dense areas. Just the existence of M87 and other ellipticals at the core of clusters proves this. Giant ellipticals are the descendants of spiral galaxies after they’ve exhausted all their star-forming gases and merged with other galaxies to get bigger. 

Messier 87; Source: Wikipedia

It’s clear that the centers of galaxy clusters were very chaotic at the beginning of galaxy formation, so of course there would be more globular clusters. However, this doesn’t explain the origin of globular clusters. 

The basic theory is that globular clusters formed from giant molecular clouds that were abundant in the young universe. Molecular clouds are the nurseries of stars, and because giant molecular clouds aren’t as common anymore as they used to be, globular clusters are so old and can’t form these days. This theory proposes that all stars within globular clusters were born around the same time. 

But evidence shows something else: the stars within globular clusters may all be ancient, but they still vary in age over a few billion years. Additionally, they have different abundances of heavier elements. In other words, if globular cluster stars were all born at the same time, they would also have the same properties. And that’s not true. 

When astronomers looked at a globular cluster from the young universe, GN-z11, they found that the cluster is actively forming new stars and has a relatively high abundance of nitrogen. Now, this might be a clue that the nitrogen comes from young supermassive stars, or population III stars. 

These are yet theoretical types of stars, formed from pure hydrogen and helium, with masses ranging up to 10’000 times the Sun’s mass! Because of these immense masses, supermassive stars would have only been able to survive for a few hundred thousand years. In comparison, modern-day big stars live for a few million years, while the smallest stars, red dwarfs, can live for trillions of years. 

When such supermassive stars die, they would have spread metals (anything heavier than helium) into space, allowing for the stars in globular clusters to form. 

This would mean that the formation of globular clusters is linked to central supermassive stars, similar to how supermassive black holes define the centers of big galaxies. 

Globular clusters could therefore be tiny galaxies in their own sense – containing millions of stars and originating back to the young universe, when the sight of the heavens would have revealed a glorious world of colors.