The Beast’s environment: How a supermassive black hole shapes the fate of the Milky Way

We know that Sagittarius A* (Sag A*), the black hole at the center of our Milky Way galaxy, is supermassive and rotates rapidly, dragging space-time along with it. And this small detail—its rotational speed—which at first glance might seem trivial, conceals clues about its formation process.

One of the key aspects of understanding these beasts is how they got to be the size they are. Do they form as a result of the merger of smaller black holes? Or do they reach these enormous masses as they swallow (accrete, in scientific terms) the surrounding gas? The speed at which Sag A* spins would indicate that a significant portion of the black hole’s mass comes from the accretion of material from its surroundings. And it is precisely its surroundings that we are going to discuss today.

The cores of most galaxies, the so-called spiral and elliptical galaxies, contain a supermassive black hole at their center. The study of the environments of these objects is revealing that they have an enormous influence on the overall evolution of their host galaxy. Although a black hole does not emit light, when it is active, the matter falling onto it, in addition to helping it grow, releases huge amounts of energy in the form of observable radiation that it injects into its surroundings.

A supermassive black hole is roughly the size of our solar system, while its host galaxy is a billion times larger. However, the radiation and gas outflow resulting from the violent conditions near the black hole determine how the entire galaxy evolves over time. For example, the black hole can heat the gas and slow star formation, or in some cases activate it, and can also disperse chemically enriched material over large distances.

The regions near the centers of galaxies are extreme from many perspectives. In ours in particular, the density of stars is the highest in the Milky Way. Let’s look at the numbers: in the neighborhood of the Sun, the density of stars is approximately 0.1 stars per 3.3 cubic light-years. The galactic center, in the same volume, hosts 10 million stars, some of which are moving at extremely high speeds due to the presence of the central black hole. In approximately the same volume, where there are no stars in our environment—only the Sun and the nearest star, Proxima Centauri b, 4.24 light-years away—the center of our Galaxy hosts more than tens of millions of stars.

The galactic center is undoubtedly the place in the Milky Way with the highest stellar density, but there are other environments that, although not approaching those numbers, also host high star densities: globular clusters. A globular cluster is an association of stars that at its center can host up to 10,000 stars per cubic parsec (3.3 cubic light-years). Even with so many stars in such a small space, collisions are rare, and we know this because globular clusters, which we can see in more detail than the center of the galaxy, have stars at their centers, the so-called blue stragglers, which are young, massive stars formed by the collision of two older, lower-mass stars. Fewer than one in every 10,000 stars in globular clusters are blue stragglers, suggesting how rare stellar collisions are even in these extreme environments.

The largest store of dense gas

Let's return to the environment around Sag A* and another of its extraordinary properties, as the largest reservoir of dense gas in the galaxy, the material from which stars are built. Perhaps it's no surprise, then, that stars are forming there at the fastest rate in the entire Milky Way. And that brings us to one of the most basic questions in astrophysics: do stars always form the same way, or do the extreme conditions of certain environments modify the way they are built? In science, we like universal laws, which is why we find it so fascinating to understand what happens in the extreme environment of the center of the Galaxy, both to prove that the rules are broken and that they are followed.

In the absence of better information, we assume that the way stars are built—the recipe that tells us how many stars form with a given mass—is universal. That is, it stays the same at all times and in all galaxies. And yet, astrophysics has been searching for variations of this global law in all possible environments. And so far, except for the lowest masses (below the mass of the Sun) and the highest (above 10 times the mass of the Sun and back in time), it appears to be fairly invariable. To prove this, we must observe that it holds true wherever stars are forming, and in our Galaxy, the most extreme environment, which we still don’t fully understand, is around Sag A*.

The heart of the Milky Way is located just 26,000 light-years away, but the high density of stars and the vast, thick interstellar clouds of gas and dust obscure our view of the galactic center. This makes it difficult not only to study the validity of this universal law of star formation, but also to understand the effect of the black hole’s presence on gas feedback processes. So, even though we are gradually revealing their defining details, these fascinating objects hold valuable clues for science in their immediate surroundings, affecting the entire galaxy of which they are a part. And that is precisely what we are trying to do: relate the history of star formation to the history of the galaxy itself, which in turn is our own history.

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