Supermassive Black Holes: Galactic Behemoths

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).

Black holes, particularly those of the supermassive variety, are often the subjects of science fiction novels and interesting movie plots. They invariable are part of some interstellar travel trick, or get featured in time travel or some other important plot 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 these incredible objects?

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 posses immense power and wield incredible influence over their galaxies. Yet, as influential as they are, we can't really see them. We have to deduce their existence from the effect they have on their surroundings.

For example, supermassive black holes exist mainly 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 black holes are discussed, one property that sets them apart from other "normal" objects in the universe is their density. This is the amount of "stuff" packed into the volume of a black hole. The density at the cores of normal 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 but it still contains an incredible amount of mass.Another way to think of this is that a black hole is actually a very small area (some say a pinpoint) containing huge amounts of mass. That makes it incredibly dense.

Unbelievably, one can calculate that the average density of supermassive black holes can actually be less than the very air we breathe. In fact, the greater the mass, the less dense the supermassive black hole is. So, it would not only be possible to approach a supermassive black hole, one could even fall into a supermassive black hole and survive for quite some time until getting close to the core. Of course, that's theoretically, because the extreme gravitational pull of all the mass in the black hole would tear anything apart long before it hits the singularity at the core.

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. One shouldn't be stable enough to persist. However, they do exist; the most massive stars ever detected were found within the past decade. They are nearly 300 solar masses. 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: you need a LOT more mass to make a supermassive black hole than is contained in even the largest 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?

Perhaps the most common theory 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 and while we see black holes accreting mass all the time, there is still a gaping hole in the theory. Namely, we have rarely observed a black hole in the "intermediate" stage. If these objects are created through accretion, then we should also see black holes in between these two masses, in the midst of formation. Astronomers are on the hunt for those intermediate-mass monsters and they are starting to find them. Understanding the process they go through to become supermassive is going to take some more work. 

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, we need to understand more about the conditions during that time in order to figure out how black holes played a role and what spurred their formation. 

Observational evidence tends to 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. 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 supple 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

As with any black hole object, there are properties that completely bend the mind. 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 there a shred of evidence to support any of these claims? 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 have been around for years. The possibilities have even been calculated using serious physics and general relativity.

The problem is in the "special conditions". These seem to eliminate any real possibility of using black holes for such purposes, mostly because it seems unlikely that these special conditions will ever exist. But who knows — much of the technology that we have today was also once thought impossible. So, don't give up yet.

Edited and updated by Carolyn Collins Petersen.

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Millis, John P., Ph.D. "Supermassive Black Holes: Galactic Behemoths." ThoughtCo, Apr. 27, 2017, thoughtco.com/supermassive-black-holes-3072389. Millis, John P., Ph.D. (2017, April 27). Supermassive Black Holes: Galactic Behemoths. Retrieved from https://www.thoughtco.com/supermassive-black-holes-3072389 Millis, John P., Ph.D. "Supermassive Black Holes: Galactic Behemoths." ThoughtCo. https://www.thoughtco.com/supermassive-black-holes-3072389 (accessed January 18, 2018).