Supermassive Black Holes are Galaxy Monsters

computer simulation of a supermassive black hole
This computer-simulated image shows a supermassive black hole at the core of a galaxy. The black region in the center represents the black hole's event horizon, where no light can escape the massive object's gravitational grip. The black hole's powerful gravity distorts space around it like a funhouse mirror. Light from background stars is stretched and smeared as the stars skim by the black hole. NASA, ESA, and D. Coe, J. Anderson, and R. van der Marel (Space Telescope Science Institute), Science Credit: NASA, ESA, C.-P. Ma (University of California, Berkeley), and J. Thomas (Max Planck Institute for Extraterrestrial Physics, Garching, Germany).

There's a supermassive black hole at the center of our galaxy. It can't be seen directly through telescopes or with our eyes, but astronomers know it's there.  In fact, there are supermassive black holes at the hearts of many galaxies.  How do astronomers know these monsters lurk in the galactic cores? They use a variety of methods to study light as it passes by a black hole and they also study the region around a black hole to understand how it affects nearby clouds of gas, dust, and even stars. 

Black holes are a favorite in science fiction stories and media. Sometimes they're used as a plot device to enable some kind of interstellar travel trick. Or, they get featured in time travel or some other important element of a story. As fascinating as such tales are, the reality behind these weird behemoths is more intriguing than writers can imagine. What are the facts surrounding supermassive black holes? Is there any science behind the science fiction depictions of supermassive black holes? Let's find out.

What Are Supermassive Black Holes?

Generally, supermassive black holes are just what their name says: really, really massive black holes. They measure in the hundreds of thousands of solar masses (one solar mass equals the mass of the Sun) up to billions of solar masses. They possess immense power and wield incredible influence over their galaxies.

Most supermassive black holes exist in the cores of galaxies. That central location allows them to (at least partially) help hold galaxies together. Their gravity is so immense, because of their incredible mass, that even stars hundreds of thousands of light years away are bound in orbit around them and the galaxy cores they inhabit.

Black Holes and Their Incredible Densities

Whenever astronomers talk about black holes, the main property they use that sets black holes apart other "normal" objects in the universe is density. This is the amount of "stuff" packed into the volume of a black hole. The density at the cores of black holes is so high that it essentially becomes infinite. Specifically, the volume (the amount of space a black hole and its hidden mass takes up) approaches zero. That means it's little more than a tiny pinpoint in space, but that tiny dot, called a singularity, contains an incredible amount of mass. That makes it incredibly dense. That density is spread out throughout the entire region of the black hole, from the singularity to the event horizon (which is the point where the gravity of the black hole is too strong for anything to resist. 

That sounds as if the interior of the black hole (beyond the event horizon) could be incredibly crushed, with no room. Interestingly, there's a thought experiment that says the average density of supermassive black holes can actually be less than the very air humans breathe. In fact, the greater the mass, the less dense the supermassive black hole is, if one considers the whole volume of the area from the singularity to the event horizon. The mass would be distributed through that region, with more mass at the singularity than in the "outskirts." 

If that's true, then it would not only be possible to approach a supermassive black hole, one could theoretically fall into a supermassive black hole and survive for quite some time until getting close to the singularity. However, there's one big problem: the gravity. It's so strong that anything swooping past the event horizon would be torn apart by the extreme gravitational pull. So much for wormhole travel! 

How Do Supermassive Black Holes Form?

The formation of supermassive black holes is still one of the mysteries of astrophysics. Normal black holes are the core remnants left behind from the supernova explosion of a massive star. The more massive the star, the more massive the black hole left behind.

One could, therefore, assume that supermassive black holes are created from the collapse of a supermassive star. The problem is that few such stars have been detected. Moreover, physics tells us that they shouldn't even exist in the first place. However, they do. The most massive stars ever detected contain nearly 300 solar masses each. Still, even these monster stars are a far cry from the types of masses that would be needed to create a supermassive black hole. To put it bluntly: a LOT more mass is needed to make a supermassive black hole than is contained in even the most supermassive stars. 

So, if these objects aren't created in the traditional fashion of other black holes, where do the monster black holes come from? The leading idea is that they formed as much-smaller black holes to build big ones. Eventually the build-up of mass would lead to the creation of a supermassive black hole. That is a hierarchical theory of building a supermassive black hole. There are some problems with that theory because it requires the study of "intermediate mass" supermassive black holes. They would be the "in b between step" from smaller black holes to the supermassive monsters. Astronomers are starting to detect more of these and study their particular characteristics to fill in the gaps in the hierarchical theory. 

Black Holes, the Big Bang, and Mergers

Another leading theory about the creation of supermassive black holes is that they formed in the first moments following the Big Bang. Of course, not everything is completely understood about the conditions during that time in order to figure out how black holes played a role and what spurred their formation. 

Observations of the known supermassive and intermediate-mass black holes suggest that the merger theory is likely the simplest explanation. Examination of the oldest, most distant and massive supermassive black holes, quasars specifically, shows there is evidence that the merger of many galaxies played a role. When galaxies merge, it appears their black holes do, too.  Mergers play a role in shaping the galaxies we see today, and so it makes sense that their central black holes may come along for the ride and grow along with the galaxies.

If this is the case then it would also seem to supply a partial solution to the intermediate black hole problem. In either case, the answer is not clear, yet. Much more work needs to be done to observe and characterize galaxies and their black holes.

Science in the Science Fiction

Getting back to science fiction and black holes, there are properties that completely bend the mind that writers have used. Stories of faster than light travel, interstellar travel and time travel pervade science fiction novels. There are even theories that black holes are gateways to alternative universes.

So is any evidence to support any of these ideas? Actually, yes, although only under very extreme circumstances. The idea of using black holes as wormholes that somehow connect us with the other side of the universe has been around for decades. The possibilities have even been calculated using serious physics and general relativity. So, theoretically, these things could happen, as was shown in the 2014 movie Interstellar. The physicist who worked with the filmmakers came up with some theoretical ideas that supported the film and worked scientifically. However, the technology required is still not available and a variety of special conditions need to be satisfied.  But who knows — much of the technology that humans use for flight today was also once thought impossible. 

Edited and updated by Carolyn Collins Petersen.