What are globular clusters?

Globular clusters are some of the densest regions of stars in the sky. Living on the outskirts of their home galaxies, globular clusters can be home to tens of millions of stars, some of which are the oldest in their home galaxies.  

Globular clusters can be massive, but they pale in comparison to their home galaxies. While a large globular cluster might house tens of millions of stars, galaxies are often home to billions of stars.  

Messier 4, or M4, is the closest known globular cluster to Earth. The cluster sits just 5,500 light-years from Earth.  

Globular clusters do tend to be composed of old stars, but not entirely. Some of them (e.g., Omega Centauri) have a mix of stars of different ages, implying that there are episodes of star formation within the cluster over billions of years. But yes, the oldest generation of stars can be around 13 billion years old (e.g., M30), close to the age of the universe, at roughly 13.7 billion years. In contrast, some are only half that age (e.g., Palomar 1). 

Having multiple populations of stars in a single globular cluster or any set of globular clusters makes them more interesting and diverse objects to study, since each is a little like a tiny galaxy within our own (or within a nearby galaxy). There is evidence for things like black holes in the centers of globular clusters (e.g., M4), just like in the centers of galaxies. Also, some globular clusters may be the remnants of dwarf galaxies being absorbed by the Milky Way, such as Laevens 1 (which is out in the halo of our galaxy, at something like 20% of the distance to the Andromeda galaxy). And some may have formed as cast-offs from the formation of the Milky Way itself (e.g., Terzan 5).

It's pretty likely that Omega Centauri is the largest globular cluster in the Milky Way. At 10 million stars, it's much bigger than most, which have more like a million stars. Also, it has multiple populations in it and possibly a central black hole. It may be that Omega Centauri is a dwarf galaxy that is in the early stages of being absorbed by the Milky Way. Some of its stars may be getting stripped away, like Kapteyn's star. We've even named the stars being pulled out — "the Fimbulthul stream."

While Omega Centauri is probably the largest globular cluster in the Milky Way, that doesn't mean it's the largest one we know of. For instance, our nearby neighbor the Andromeda galaxy has a globular cluster, Mayall II, orbiting it — and this cluster is twice the size of Omega Centauri! 

The Nancy Grace Roman Space Telescope is designed to have revolutionary capabilities to enable astronomers to study the universe more efficiently than ever before. It has an enormous, 300-megapixel infrared camera with multiple infrared filters to allow it to take multicolor pictures of the universe with a field of view a hundred times greater than the Hubble Space Telescope. It has the same sharp images and same sensitivity as Hubble but can move to take new pictures far faster, so that surveying the sky with Roman is up to a thousand times faster than surveying with Hubble! 

If you look at the full moon, it's about half a degree across. That's roughly the size picture you would have to take to capture the full extent of a typical globular cluster like M55. (Omega Centauri and 47 Tucanae are up to twice this size.) The camera on Hubble has a field of view around 1/62 of the size of the full moon, so it would take dozens of individual pictures to capture the entirety of a globular cluster. 

The camera on Roman has a field of view 43% larger than the full moon, so you can image a whole cluster in one picture! This wide field of view allows Roman to study the fainter, typically lower-mass stars that inhabit the outer reaches of globular clusters and which are therefore poorly studied by smaller-field images. Plus, Roman's wavelength coverage extends further into the infrared than Hubble's, so [it] can see through more of the dust in the Milky Way and gain more information on the fainter, redder stars that Hubble can't see so easily. 

One other power of Roman is that it can measure the positions of stars extremely precisely. If we observe a globular cluster now, and then in a few years, and then again a few years later, we can easily see the stars moving inside the cluster. This tells us about the internal structure and how the cluster is evolving. It can also show us how the cluster as a whole is moving in the Milky Way and let us place how it is merging with our galaxy (or perhaps has long ago merged with our galaxy).

Oh, definitely. Globular clusters are still being discovered in the Milky Way and its halo. Some are behind the clouds of gas and dust in the plane of our galaxy and so can only be found using infrared light, which penetrates dust much more effectively than visible light. The cluster Mercer 3 is a good example of this; it was discovered only in 2008 and has a visual "extinction" (how much fainter the visible light is due to absorption by dust) of more than a billion times! 

Roman's infrared wavelengths, coupled with its exquisite sensitivity and angular resolution, make it very well suited to finding many new globular clusters hidden in our galaxy. In fact, we recently added a [Galactic Roman Infrared] Plane Survey as one of the major surveys the telescope will undertake, and it should certainly discover new globular clusters. 

Other clusters may have escaped detection because they are far outside the galaxy and small, and so they're very faint (e.g., William 1). Roman will be good at finding these, since it can cover a wide area of the sky and pick up small, low-brightness clumps of stars like this. It's hard to predict how many Roman will find, since we don't really know how many are out there!

One of the greatest stories yet to be fully told is how galaxies assemble. We see around us a host of galaxies with billions of stars. But how did they get here, and how did they combine to form the shapes we so often see (for instance, spiral galaxies)? Astronomers believe that galaxies grow in part by gathering gas into clumps, which form some stars, and which then combine into bigger clumps, which then merge and so on … until you have a giant galaxy like our own.  

In that process, there are globular clusters which form in one galaxy and then are separated and incorporated into the merged galaxy. We know of several, such as NGC 2298 (which formed in a dwarf galaxy) and M54 (which is probably still in its dwarf galaxy, currently being consumed by the Milky Way). Since this process happens so slowly, we can't watch it progress; instead, we have to look at different globular clusters at different points in their absorption into the Milky Way or nearby galaxies. 

The Nancy Grace Roman Space Telescope will be able to survey perhaps a hundred nearby galaxies to find the dwarf galaxies and globular clusters orbiting them, so the total number of globular clusters we know [of] may increase many times over. This wealth of new information should prove immensely valuable to astronomers who want to understand how galaxies like our own came to be.