Black Holes: From Teeny Tiny to the Biggest Things in the Universe#

Did you know the biggest things in the universe are black holes? Unlike planets or stars, they don’t really have a physical size limit; they can just keep growing, potentially forever. Now, to make the different kinds of black holes, from the really small ones to the absolute giants, specific things need to happen. So, how exactly do they grow, and just how big is the biggest one out there?

(Just a heads-up: This won’t get into how black holes work or how they form – we covered that already in our black hole and neutron star series, which you can totally check out later if you’re curious. For now, we’re laser-focused on finding the biggest thing around.)

Let’s start our journey by going really, really small.

Primordial Black Holes: The Potential Oldest Objects#

These are the smallest kind of black holes, and they might or might not even exist. If they do, they’re probably the oldest things in the universe – older than even atoms!

• How they might have formed: They would have popped into existence right after the Big Bang, back when the universe was incredibly dense and full of wild energy. Any tiny spot that was just a little bit denser than the spots next to it could have squeezed down to make a black hole.
• Smallest Possible Size (if they exist): The smallest Primordial Black Hole that could still be hanging around today would be about a trillion kilograms, roughly the mass of a big mountain. And get this: even with that much mass, it would be no bigger than a proton.
• Larger Primordial Black Hole Comparison: Imagine one with the mass of Earth – it would barely be bigger than a coin.

Because they’d be so tiny, they’re incredibly tough to find. We haven’t actually seen any yet. If they are out there, they might even be the mysterious dark matter that helps hold galaxies together.

Okay, let’s move on to the black holes we know for sure are out there.

Stellar Black Holes: Born from Stars#

To make a black hole, you first need to squish enough matter together so it collapses in on itself. Once you’ve got one, you just keep throwing more mass at it, and it gets bigger.

• How they form today: In the universe as it is now, only the most violent cosmic events can create the right conditions. Think things like:
  • Two neutron stars merging.
  • The core of a super massive star collapsing during a supernova explosion.
• Measuring Mass: To keep things easy, we’ll use the mass of our Sun as our unit. Our Sun is about 2 million trillion trillion kilograms.
• Smallest Known Stellar Black Hole: The smallest one we’ve found has 2.7 times the mass of the Sun. In terms of size, it would be a sphere about 16 km across – big enough to cover a city like Paris.
• Another Lightweight Example (V723 Mon): There’s another small black hole that’s buddies with a red giant star called V723 Mon. This red giant is huge, 24 times larger than our Sun, with a diameter of 30 million kilometers. And yet, this giant star is being whipped around by a tiny black hole just 17.2 km wide! The black hole is so much smaller than the star that it’s hard to even show them together in a picture. This tiny thing is practically bullying the star!
• One of the Largest Known Stellar Black Holes (M33 X-7): This one is busy snacking on a 70 solar mass blue giant star, eating it little by little. As all that stolen matter spirals towards the black hole (kind of like water going down a drain), the friction heats it up so much that it shines 500,000 times brighter than our Sun! Even with all that action, X-7 itself is ‘only’ 15.65 solar masses and just 92 km wide – only big enough to cast a shadow on a place like Corsica.

To get much bigger, stellar black holes need to either eat a ton of stars or, even better, crash into and merge with other black holes.

• Detecting Mergers: We’ve got new instruments now that let us spot these mergers, and we’re finding a lot of cool stuff! Like two massive black holes we found merging in a galaxy 17 billion light-years away. As they spiraled violently around each other, they let out more energy in the form of gravitational waves than all the stars in the Milky Way combined produce in 4400 years! The new black hole they made is about the size of Germany and weighs in at 142 solar masses.

The Curious Mass Gap#

Right about here, we hit a weird spot in the scale. We see lots of black holes up to about 150 solar masses. And then… nothing, for a long, long time. Suddenly, we jump to black holes that are millions of times more massive. This is a bit confusing because you’d think black holes would just keep growing steadily. But for the truly massive ones, just eating stars and merging isn’t fast enough to explain how big they are today. The universe simply hasn’t been around long enough for them to get that huge that way. Something else must have happened.

Quasi Stars: Supercharging Early Black Holes?#

To figure out how we got the biggest black holes, we might need to look at the biggest stars that ever existed: Quasi Stars.

• Scale Comparison: To give you an idea, our Sun is like a grain of sand next to these hypothetical stars.
• Concept: We don’t actually know if Quasi Stars really existed, but they’re an interesting idea for how black holes could get a huge head start.
• How they might have worked: The idea is that matter in the early universe was so packed together that stars could grow to be thousands of times the mass of our Sun. The centers of these monster stars might have been crushed by their own weight so much that they collapsed into black holes while the star was still forming.
• A Deadly Balance: Unlike stars today that would blow themselves up, inside a Quasi Star, a strange balance might have formed. The huge gravity pulled the star in, feeding the growing black hole. As material fell in, it got so hot that the radiation pushing outwards kept the star stable.
• Fast Growth: This setup might have allowed these quickly growing black holes to eat the Quasi Star for millions of years, getting far, far bigger than any modern stellar black hole. We’re talking black holes several thousand times the mass of the Sun and wider than the entire Earth. These could have been the starting points, the seeds, for the truly massive black holes.

Supermassive Black Holes: Kings of the Galaxy#

Now we arrive at the absolute rulers of our universe, the single largest bodies we know of. Almost every galaxy has a supermassive black hole right at its center, and they are monstrous!

• Our Milky Way’s SMBH: In our own galaxy, the Milky Way, we have Sagittarius A* (pronounced ‘A Star’). It’s a supermassive black hole with about 4 million solar masses. It’s pretty calm and just does its thing. We know it’s there because we see stars zipping around a seemingly empty spot in space.
• Mass vs. Size (Sag A):* Even though it’s incredibly massive (4 million times the Sun!), its radius is still only about 17 times our Sun. That’s smaller than most giant stars, even though it’s millions of times heavier!
• Galactic Anchor Misconception: Because they’re so massive and sit in the middle, lots of people think supermassive black holes are like the Sun in our solar system – the main anchor holding everything in orbit. But that’s not quite right. While the Sun is 99.86% of the mass in our solar system, Supermassive Black Holes usually only make up about 0.001% of their galaxy’s mass. The billions of stars in a galaxy aren’t really bound to the black hole; they’re held together mostly by the gravity of dark matter.
• Not Always Gentle Giants: Many supermassive black holes aren’t quiet like Sag A*. Especially when they’re eating big clouds of gas and dust! The one at the center of the BL Lacertae galaxy is chowing down so much that it blasts out jets of plasma moving almost at the speed of light. If Earth were orbiting this beast, it would look 115 times larger than our Sun in the sky… and we’d be instantly vaporized by its glowing hot accretion disk.
• Stars Become Tiny: At this scale, stars seem laughably small compared to the black hole.
• The Cygnus A Example: The galaxy Cygnus A has a supermassive black hole with 2.5 billion solar masses. It’s incredibly wide – 14.7 billion km. If you put it where our Sun is, it would swallow all the planets and stretch halfway to the edge of our Solar System! It’s eating so much stuff that it twists its accretion disk into a kind of magnetic funnel, shooting gas out to make giant radio lobes towering over the galaxy, spanning half a million light-years! That’s wider than 2.5 Milky Way galaxies.
• The Messier 87 Example: Another pretty big one is in the galaxy Messier 87. It has 6.5 billion solar masses and was the first black hole we ever managed to get a picture of! (Well, a picture of the glowing gas right around the edge of its menacing shadow). This sphere of darkness is so large it would cover our entire Solar System.

But believe it or not, there’s a scale even bigger than these…

Ultramassive Black Holes: The Record Holders#

Okay, now we’re talking about the biggest black holes – maybe the largest single bodies that could ever exist. These things have eaten so much they’ve grown to tens of billions of solar masses. Their gravity powers something called a quasar, which is an accretion disk so bright it outshines thousands of entire galaxies combined! They’re so massive they deserve their own special name: Ultramassive Black Holes.

• The OJ 287 Example: The Ultramassive Black Hole in the center of galaxy OJ 287 is 18 billion solar masses. It’s so ridiculously huge that it actually has a Supermassive black hole (one nearly forty times bigger than Sagittarius A*!) orbiting it! This thing is hard to even wrap your head around. It could comfortably fit three whole Solar Systems side-by-side inside of it.

Let’s finish this crazy comparison and get to the king of the hill.

• The TON 618 Example: TON 618 is a black hole we can see actively eating stuff equivalent to entire galaxies. It shines with the brightness of a hundred trillion stars, making it visible from 18 billion light-years away. Its mass is an unbelievable 66 billion solar masses! This black hole is so vast that if you were at its edge, it would take light a whole week to reach the singularity at the center. You could fit about 11 Solar Systems side-by-side inside it. It might just be the largest single body in the universe.

Here’s a cool/scary thought: Because TON 618 is so far away, we’re only seeing what it looked like 10 billion years ago. In reality, it’s probably even bigger now.

Black holes are definitely scary, mysterious, and unbelievably gigantic. They’ll likely be around, getting bigger and bigger, long after everything else is gone.

So, that was our trip from the smallest possible black hole all the way up to the largest one we know of!

Behind the Lies: Why Ranking Black Holes is Tricky#

Let’s do something a little different now, like a quick peek behind the curtain. We can call this: “Behind the Lies”. This is a short look at why the simple ranking we just did has some necessary inaccuracies. You see, it’s really not possible to rank black holes perfectly like collecting trading cards. Why’s that?

• Limited Data: We’ve cataloged millions of stars, but we only have really good information on maybe a couple dozen black holes.
• Black Hole Gazing is New: Studying black holes hasn’t really been a major thing for very long – maybe 50 years or so. And technically, we still can’t see black holes directly.
• How We Study Them: We can only figure out what black holes are like by studying how their gravity affects the stuff around them, like the paths of stars that get close.
• Approximating Mass: We can get a basic idea of a black hole’s mass (which affects its gravity) using things like Kepler’s Laws (like how planets orbit the Sun). But these calculations come with big unknowns and error bars.
• Mass to Size Conversion: Then, we have to turn that mass estimate into a size estimate, which adds more uncertainties. For example, the size (or radius) is often calculated using the Schwarzschild equation. But this equation, to keep things simple, assumes black holes are perfectly round and don’t spin. The problem is, a black hole like that doesn’t really exist in reality.
• Physics is Fuzzy: At these extreme scales, physics gets a bit… fuzzy. So, some of the black holes we talked about might actually be quite a bit smaller or bigger than we said. We just don’t know for sure.
• How We Handled It: We worked around this by looking at information from different sources that used different methods and calculations. We tried to create a standard list that got us as close to the truth as humanly possible. (You can actually see all the nitty-gritty details in our source document if you’re curious!).
• The Human Cost: Putting this together meant driving experts a little crazy with how much we pestered them for the best possible numbers they could agree on. Lots of information got cut along the way and didn’t make it into the main video script.
• Not Wasted Effort: To make sure all that effort wasn’t for nothing, we came up with a way to use some of that extra knowledge: we made some cool black hole merchandise! It ranges from kind of silly to more serious stuff. This way, we get to explore the topic from different angles, and you can keep having fun thinking about black holes even after the video ends.