September 30, 2020

The enigmatic origin of globular clusters

Stars do not usually appear isolated in the universe. On the contrary, they tend to focus on those huge clouds we call galaxies, and even within galaxies they often form “flocks,” more or less large groups we call clusters. There is a good reason for that, and that is that stars are usually born in a group. Many of these groups remain united for a few million years, but as they move through the interior of the galaxy they become frayed, due to the gravity of other nearby stars or encounters with a cloud of gas. We call these ephemeral gangs of cumulus stars open. But in the outskirts of the galaxies, far from those frenetic regions in which the nebulae fight for a span of more space, we find quite remarkable groups of stars: globular clusters, which we are going to talk about today.

Globular clusters are conglomerates of stars that we find in almost all galaxies, except in the smallest ones. As the name implies, they are approximately spherical, and usually contain hundreds of thousands or millions of stars. They are quite small for the amount of stars they contain, so their “population density” is relatively high, and can be very high in the nucleus. To get an idea, between the Sun and the nearest star there are 4.2 light years; if we were in the nucleus of a dense globular cluster, up to 900 stars would be made in that same volume.

These busy stellar neighborhoods are also very old. Their stars seem to be of the strict second generation, made with the gas that the first stars of the universe expelled, and most of the clusters seem as old as the galaxies in which they are found. Their natural habitat, then, has to be found outside the body of the galaxy, because if they had to move through the interstellar medium, full of gas and other stars, they would have been undone as happens to the open clusters and would not have reached our days. The globular clusters are inhabitants of the halo, a kind of spherical shell that surrounds the galaxies and that is a remnant of the first days of these.

Uncertain origins

Thus, we have balls of stars that are considerably dense, very old and that turn the galaxies into very wide orbits, sometimes quite far from the body of the galaxy. Where did these peculiar formations come from? We do not know for sure. An idea that was handled for a while was that they were “failed galaxies”, too small gas clouds that ended up being “adopted” by larger galaxies.

That idea had an interesting point: modern cosmology tells us that galaxies were formed from swarms of gas clouds in which there were larger and smaller clouds. The larger ones tended to phagocytize the small ones, and thus, naturally, systems formed with large galaxies in the center surrounded by a retinue of smaller ones, the satellite galaxies. That is exactly what we see today. In this scheme, globular clusters would be especially small satellite galaxies, and with some somewhat peculiar properties that would have to be explained in some way. For example, why they are so compact and other satellite galaxies are not.

But this idea has a defect in its same waterline: the gas clouds that gave rise to the galaxies accumulated inside dark matter clouds. Dark matter is five times more abundant than gas, and therefore its gravity is five times more intense, so the gas goes where dark matter tells it to go. Actually, the dynamics we have just told, from big clouds engulfing small ones, is a dynamic of dark matter clouds. The gas we now see in the galaxies only followed what dark matter did. And here comes the serious problem: in globular clusters there is no trace of dark matter.

The absence of dark matter in globular clusters is already noticeable. Since in them we have the order of one million stars concentrated in a small space, one could innocently think that perhaps a certain amount of dark matter would have grazed the gas to that space and, once there, the stars would have formed. If that was what happened, after that something else happened that stole all the dark matter from the cluster. And that didn’t happen with one or two, but with the overwhelming majority of globular clusters. Clearly, there is a different mechanism operating here than the formation of satellite galaxies, which in their majority do conserve significant amounts of dark matter.

Clusters from collisions

That is why an article published a few days ago in the Astrophysical Journal, in which the authors propose a new idea about how globular clusters were formed: that they were the result of crashes between those clouds that were giving rise to galaxies.

The idea is very ingenious: we know the case, at present, of groups of galaxies that collide with each other. A group of galaxies is made up of three elements: the galaxies themselves, the diffuse gas between them and the dark matter, which constitutes the bulk of the mass. When the two groups collide the galaxies do not know anything: they are very far from each other and the probability that two galaxies collide is very small. Dark matter does not find out too much either: it costs a lot to exchange energy with other forms of matter, so it basically passes by. Since dark matter is what generates most of the gravity, galaxies follow dark matter. What happens to diffuse gas? Something very different: gas atoms can exchange energy with other atoms, so they stop when the two clouds collide. Dark matter and galaxies pass by, but the gas gets stuck in the collision zone. We have seen this in several collisions of groups of galaxies, the most famous of which is the Bullet Cluster.

The idea of ​​this article to explain the formation of globular clusters is that these shocks should have been much more common in the early stages of galaxy development, when there were many clouds of gas and dark matter moving in all directions because the galaxies have not yet They had phagocytized them. Those clouds surely already contained stars, and those stars had enriched the gas with new chemical elements, such as carbon. If at that point there was a clash between two of those clouds, what would happen? Well, as in the case of galaxies, that dark matter and stars would pass by, but the inner gas of the clouds would slow down, as is the case in the Bullet Cluster. If the gas slows down a lot it could happen that the dark matter and the stars will continue their way and leave there, orphaned, to a cloud of hot gas and compressed by the energy of the collision.

The proposal of these authors is that globular clusters were formed from these orphan clouds. That is why they have no dark matter, and that is why they are located on the periphery of the galaxies, which is where those clouds moved and could crash from time to time. The whole muscle of the article is to show that in a typical galaxy those shocks were reasonably probable, that the number of crashes was similar to the number of globular clusters we see today, and that after the crash the gas was in a state capable of form stars In addition to this they get some very suggestive results, such as calculating the composition that the gas must have so that stars form after the crashes, or calculate the minimum mass that a galaxy must have so that these crashes can occur nearby. Both are things that we can compare with the properties of real globular clusters and the galaxies that host them.

This may be the correct mechanism to understand the formation of globular clusters or it may not, that will only be said by the time and scrutiny of the scientific community. But it is still fascinating that some objects that we have known for centuries, and that at first glance seem simple, require so much and so beautiful physics to come into the world.


  • In this article we describe an idea that has just been presented in public. It seems promising, but like all new ideas we cannot take it as established knowledge. In the coming months or years it will be examined, criticized and defended, and only after that process can we say anything firm about it.
  • The word cumulus It is used in astrophysics with the general meaning of “group.” So there are star clusters, such as globular clusters and open clusters, but there are also clusters of galaxies. In this article we have preferred to use the word only with groups of stars, to avoid confusion.
  • Although what we see of galaxies is their gas and their stars, most of their mass is deposited in dark matter, and it has been dark matter that has guided the evolution of galaxies, especially in their initial moments .



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