Cosmic Vocab: Black Hole Edition

Article March 21, 2026
An artist’s illustration of what a black hole might look like viewed from up close. Credit: Mark Garlick/Science Photo Library/Getty Images
An artist’s illustration of what a black hole might look like viewed from up close. Credit: Mark Garlick/Science Photo Library/Getty Images

It’s been a while since I’ve done one of my space vocab posts, and so far the ones I’ve done have been very solar system focused. So let’s cast a bit of a wider net this week and look at some of the most mysterious things we see out in space: black holes

Now there are black holes, black holes, and then there are black holes. They all have one basic thing in common, which is that their mass (whatever that is) is so densely packed that for some region around it its gravity is so powerful that not even light can escape it. But there are many variations on that basic theme, most of which have their own names.

Let’s pull out the cosmic dictionary and get to know some black holes!

 

The Anatomy

There are a few black-hole-related words that we should probably get out of the way first. For instance, if you’re a space nerd or a fan of ‘90s sci-fi films you’ve probably heard the term “event horizon” (great movie, by the way. Weird and creepy but a good time). This is directly related to that one thing all black holes have in common, a region beyond which nothing, not even light, can escape its gravity.

The SMBH at the heart of M87 is emitting a jet as seen in this Hubble image. Credit: ESA/NASA
This image of the black hole at the center of the massive galaxy M87 actually shows the shadow of the black hole’s event horizon against background light. Credit: Event Horizon Telescope Collaboration

If you’ve seen the black hole images produced by the Event Horizon Telescope, looking like a dark void surrounded by fuzzy light, what you’re actually seeing is the black hole’s event horizon. How far out the event horizon extends is another term, known as the Schwarzschild radius, and it depends on the black hole’s total mass. But if you want the tl;dr on what an event horizon actually is, just think of it as the point of no return. If you hit the event horizon, there is no force in the universe that is bringing you back. 

You may also hear black holes referred to as a “singularities”. This term generally refers to the actual mass of the black hole, which cannot be seen and is surrounded by the event horizon. It’s based on the idea that the black hole’s mass is congregated into a single point of no volume and infinite density. That arrangement is often compared to asking the universe to divide by zero (which, if you don’t recall back to middle school math, is an impossibility) and there are many theories claiming that whatever sits at the center of an event horizon, it can’t actually be a singularity.

 

The Mass

One way we distinguish one type of black hole from another is by looking at their masses. At the most basic level we divvy them up into three classes: stellar mass black holes, intermediate-mass black holes, and supermassive black holes. While being defined purely by their mass, they also tend to have different origin stories.

Stellar mass black holes are what you might call “ordinary” black holes. These are not the hulking monsters found in galactic centers, these are things living out in ordinary space with the rest of us. They may even be part of multi-star systems and have companions orbiting them. These black holes used to be stars, big stars that had more than, say, 20 times the Sun’s mass.

The supernova remnant W498, seen here in multiple wavelengths as viewed by multiple observatories, is what’s left of a massive star that exploded. It likely houses a stellar mass black hole at its center. Credit: NASA/CXC/MIT/L. Lopez et al/Palomar/NSF/NRAO/VLA
The supernova remnant W498, seen here in multiple wavelengths as viewed by multiple observatories, is what’s left of a massive star that exploded. It likely houses a stellar mass black hole at its center. Credit: NASA/CXC/MIT/L. Lopez et al/Palomar/NSF/NRAO/VLA

When these stars ran out of fuel at the end of their lives, their cores collapsed and became black holes. That either happened during a supernova explosion or (as we recently confirmed can happen) by skipping the explosion entirely and going directly to the black hole stage. It’s even possible that these black holes started out as multiple stars that died and formed white dwarfs or neutron stars that later collided and merged, and the resulting merged object collapsed to a black hole. The point is that, one way or another, these things start as stars.

These black holes can run up to about 15 times the mass of the Sun, though mostly they’ll be between five and ten times the Sun’s mass. Since they’re a byproduct of stars existing and there have been a lot of stars since the universe began, there may be hundreds of millions of these things cruising through the Milky Way.

The monsters you find at the centers of galaxies are the supermassive black holes (SMBHs). The smallest such critter we’ve ever found in a galaxy’s center weighed in at 50,000 solar masses. The one at the center of the Milky Way, with the pithy name of Sagittarius A*, comes in at just over 4 million solar masses. We don’t know for sure how large they can get, but the one most often cited as the most massive known SMBH is TON 618, containing something over 40 billion solar masses. They’re very, very large is the point.

As for where they come from, they may have multiple possible origins. We used to assume they had to grow by accretion, pulling in more and more mass and getting bigger and bigger, with occasional huge jumps in mass when galaxies collide and their SMBHs merge. We still think that plays a role, but we also know thanks to the Webb Telescope that there were huge SMBHs very early in the universe’s history, so a slow build-up can’t be how those formed. They didn’t have time. This may be another case where direct collapses happen, with huge, ridiculously massive gas clouds in the early universe going directly to giant black hole stage. 

This artist’s illustration imagines an active supermassive black hole surrounded by an accretion disk and emitting a jet. Credit: NASA/JPL-Caltech
This artist’s illustration imagines an active supermassive black hole surrounded by an accretion disk and emitting a jet. Credit: NASA/JPL-Caltech

Then there are the intermediate-mass black holes (IMBHs). These are a bit of a mystery. Theoretically they would have a mass somewhere between, say, a hundred solar masses and 50,000 solar masses. If nothing else 13+ billion years of stellar mass black holes hitting each other should have resulted in at least some black holes in this mass range.

But where are they?? We’ve found a few potential candidate IMBHs in the range of a thousand or so solar masses, but nothing definite. For something that should be the missing link between the stellar mass black holes and SMBHs, they’re proving remarkably elusive.

(I should probably also mention primordial black holes. These are purely theoretical objects that could be ridiculously, incredibly small. But we don’t know that they exist and even if they did once it’s possible they don’t anymore due to weird quantum mechanical stuff which gives me a headache if I think about it too much.)

 

Getting Active

Many of the black holes, of whatever sizes, that we see out there are contentedly not doing much. The Milky Way’s SMBH, for instance, is more or less just sitting there (these days anyway). But when it comes to those big ‘uns sitting in galactic centers, sometimes they can be a bit more on the active side. And then there’s a whole host of names that might get used to describe them.

Any actively feeding central SMBH can be referred to as an active galactic nucleus (AGN). These will usually be surrounded by an accretion disk, a flattened disk of material slowly making its way into the black hole. They will often also have jets, ridiculously long streams of material being shot out perpendicular to the accretion disk at nearly the speed of light. This is material from the disk that, due to not fully understood reasons, gets ejected instead of consumed.

The SMBH at the heart of M87 is emitting a jet as seen in this Hubble image. Credit: ESA/NASA
The SMBH at the heart of M87 is emitting a jet as seen in this Hubble image. Credit: ESA/NASA

If an AGN is exceptionally bright, it might qualify to be referred to as a quasar. This is actually a shortening of the term “quasi-stellar object” and refers to the fact that quasars are emitting so much brightness from such a small amount of space that from the great distances between galaxies they look like point sources the way stars do.

Some quasars also qualify as “blazars” (all blazars are quasars, not all quasars are blazars. Say that five times fast). If a quasar has one of those jets, and that jet happens to be pointed more or less right at Earth, so we’re looking down the barrel so to speak, then it’s a blazar. This word is a combination of the word “quasar” with the first letters of BL Lacertae, the first blazar to be confirmed. So, like, take the complete made-up-ness of “quasar” and double it.

If you have an AGN that is loud in radio waves, like a quasar, but is not nearly so compact in size, it might get termed a radio galaxy. Despite having “galaxy” in the name, this is usually a reference to the galaxy’s center and may only refer to the extent of the AGN’s jet system, rather than the entire galaxy. You will note this doesn’t necessarily make much sense, but whatcha gonna do? 

At points in its history when our Milky Way’s central SMBH was actively feeding, it may have resembled a Seyfert galaxy like NGC 5793. Credit: ESA/NASA
At points in its history when our Milky Way’s central SMBH was actively feeding, it may have resembled a Seyfert galaxy like NGC 5793. Credit: ESA/NASA

If you have an AGN that isn’t as bright as a quasar or as radio-loud as a radio galaxy, it will probably get classified as a Seyfert galaxy (in this case they are actually referring to the entire galaxy, not just the center, because who needs consistency?). Since they’re not so bright as quasars, the active center doesn’t drown out all the light from the rest of the galaxy. Our Milky Way has probably had some phases where Sagittarius A* was active and it likely would have been more of a Seyfert situation than a quasar.

Can you imagine our Milky Way with a center as much as ten thousand times brighter than it is today? I’ll take our currently chill SMBH any day.

 

Looking into the Black

Of course all of these designations have sub-designations because scientists are nothing if not fond of branching systems of categorization, but those are the basics when it comes to trying to classify your average black hole. They’re wild and mysterious objects and there is so much going on with them that we don’t yet understand.

This artist’s illustration imagines seeing a quasar up close. Credit: ESO/M. Kommesser
This artist’s illustration imagines seeing a quasar up close. Credit: ESO/M. Kommesser

One thing I should point out, since we’re talking about them, is that they’re not space vacuums. The really big ones have a lot of gravitational oomph, it’s true, and some of them do have stuff falling into them (that’s the AGNs, after all), but so many are just hanging out being all gravitational without actually consuming anything, which wouldn’t be the case if they were all giant space vacuums. 

I would also be remiss, in a post about black hole vocab, if I did not bring up my favorite word associated with black holes, possibly my favorite science vocab word of all time: spaghettification. This is when something—gas cloud, planet, star, what have you—gets too close to a black hole and experiences such extreme variation in gravity even on different parts of itself that it gets stretched farther and farther until it’s nothing but a spaghetti stream of atoms. And it’s a for real actual scientific term that real scientists actually use. And that, my friends, is awesome.