There are few things in the universe quite as exciting as black holes.

They’re in all the movies–I even wrote a post a few years ago on what the movies get wrong about them! Though some movies, like Interstellar, do a pretty good job of capturing the relativistic effects.

There are also plenty of popular misconceptions floating about–probably thanks to those fictional portrayals!

Most of the time, when we geek out over black holes, we’re talking about stellar-mass black holes. That is, the remains of massive stars.

Wait a second…most of the time? What other kind of black hole could we be talking about?

I guess I kinda gave it away in the post title, huh?

I touched on the idea of a supermassive black hole during our exploration of the Milky Way Galaxy, when we took a dive into our galaxy’s nucleus.

Okay, I guess we didn’t really dive inside the actual nucleus…

I sure tried my best, though, given that humans can’t leave our solar system yet!

When we explored our galaxy’s nucleus, this is what we saw:

In that post, I took us through a flyby of all the labeled regions in this image, before heading straight for the bright radio source in the center: Sgr A, or Sagittarius A.

Sagittarius A, the strongest source of radio-wavelength radiation in the galactic core, is a sphere of low-density gas. Deep within that sphere lies a much smaller but extremely compact phenomenon: Sgr A* itself.

Sgr A*, read as “Sadge-A-star,” isn’t just a black hole. It’s a supermassive black hole.

And it’s the gravitational center of our Milky Way Galaxy.

But here’s the big question.

Is the Milky Way alone? Or do other galaxies sport supermassive black holes in their cores?

As a matter of fact…they do.

Alright, let’s back up for just a second. In order to better understand supermassive black holes, we need to ask ourselves…what is a normal black hole?

The simplest explanation is that a black hole is a singularity: an object of zero radius, but infinite density.

Okay…did that break anyone else’s brain, or is it just me? You too? Okay, good, moving on.

It’s not easy to actually picture a point in space that has absolutely zero size, but is filled with so much stuff that it’s infinitely dense. Instead, let’s explore how an object gets to be a singularity in the first place.

Here are the two main evolutionary tracks that stars take. See the track that ends with a black hole–on the bottom right? Yeah, that’s the track we’re going to follow.

This is what happens to a massive star whose core alone is 3 times as massive as the sun (3 M_☉).

The definition of a star that I generally work with on this blog is an object that is undergoing nuclear fusion in its core. Stars constantly wage an internal war between the outward pressure of their nuclear power and the inward force of their own gravity.

But stars only have as much nuclear fuel as they formed with. Eventually, they run out.

At that point…

Boom!

Well, there’s a few other steps–which I covered here. But, ultimately, their cores collapse.

All stars become compact objects at the end of the line. The type of compact object–white dwarf, neutron star, or black hole–depends on the mass of the core.

Low-mass stars (like our neighbor red dwarf, Proxima Centauri) and medium-mass stars (like our sun) collapse into white dwarfs, compact balls of carbon roughly the size of the Earth.

Massive stars, on the other hand, collapse into either neutron stars or black holes–depending, again, on the mass of the core.

White dwarfs and neutron stars result when the collapsing core is too massive for “conventional” physics to support them. They must rely on the material strength of degenerate matter.

If the collapsing core contains more than 3 M_☉ (solar masses) of star stuff, though…no force in the universe can stop it. There is no material strength that can support an object 3 times the mass of our sun that is not engaging in nuclear fusion.

That explains why the core collapses into a singularity, an object with zero size! Such an object can’t have any size greater than zero because the stuff it’s made of would have more room to collapse.

It still has 3 M_☉ of stuff, though, and that much stuff in a space of zero radius means infinite density.

Now, we have a black hole.

But…there’s just one problem.

It’s a stellar-mass black hole.

How the heck do we get a supermassive one?

Honestly…good question!

A supermassive black hole can’t be the remains of a collapsed star. All stellar black holes with measured masses are smaller than 20 M_☉. But with supermassive black holes, we’re talking about more like millions of solar masses. Our Milky Way’s black hole, Sgr A*, is 4 million M_☉.

There just…aren’t really stars massive enough to produce black holes like that. It doesn’t work according to our current understanding of star formation and stellar physics. (And such a massive star would almost certainly be among the brightest–and, as a result, easy to detect!)

So…what the heck is a supermassive black hole?

The good news: it is a singularity. It operates according to the same whacky physics as a stellar-mass black hole–which means we at least have some understanding of it.

Also keep in mind that, while 4 million M_☉ sounds like a lot of mass, galaxies with black holes are actually a whole lot bigger than that–on the order of 10¹¹ to 10¹³ M_☉! (That’s 100 billion to 10 trillion solar masses!)

Sgr A* is only 0.00001% the mass of the Milky Way. And, in general, it seems supermassive black holes do tend to be similarly small compared to their host galaxies.

But we’re not quite sure yet how they actually form.

One thing we do know: their masses correlate to the mass of their host galaxies’ central bulges.

Wait a second…what about that thing at the top? What the heck is a quasar?

Just a bit of foreshadowing on my part! I’ve been super excited to cover quasars–and they’re coming up just a little further down the line, when we explore active galaxies.

But for now, how about we take a look at the actual photographic object showcased here?

Messier 87 (or M87 for short) is a giant elliptical galaxy, found in the constellation Virgo. And it’s still getting bigger.

Here it is, for your viewing pleasure:

So, what does it mean that the mass of a supermassive black hole correlates to the mass of a galaxy’s central bulge?

For one thing, it gives us a clue to how these supermassive black holes formed. We may not know the actual process, but it seems clear that they and the central bulges of their host galaxies formed together.

It’s also a clue that the supermassive black holes haven’t grown a whole lot in the interim. Otherwise, the correlation would have been thrown off over time.

We do have one idea that might begin to explain supermassive black holes–and its name is dark matter!

I know. I’ve foreshadowed dark matter a lot by now, haven’t I?

Well, never fear–we will finally be taking a closer look at it in my next post! (Though, for now, it will still be a sneak-peek. We’ll look even closer when we dive into cosmology, after our exploration of active galaxies.)