There are tens of millions of black holes in the Milky Way alone, and trillions more in the rest of the observable universe. All of these black holes have varying sizes, from the tiniest hypothetical black holes to the largest, terrifying ones. So, let’s dive in!
Primordial Black Holes
These types of black holes are… hypothetical. But if they do exist, they’re probably the oldest objects in the universe. In the early universe, when everything was hot and cramped (sort of like a sleeping bag), these primordial black holes would form when an area of space was just a teensy bit denser than the surrounding area. The smallest primordial black hole that could still be around today would be a trillion kilograms or so, roughly the mass of a big mountain. But these black holes would be smaller than a proton! But… let’s move on to the kinds of black holes that we know exist.
Stellar Black Holes
To create one of these black holes, all you have to do is compress enough matter into a single point. These black holes are formed through some of the most violent events in the universe; when neutron stars crash into each other, and when stars die. The smallest black hole in the universe (called GW170817) is about 2.7 times the mass of the sun, while only being about 16 km wide!
Another “lightweight” black hole is the orbital companion to the star V723 Mon. V723 Mon is 23 times larger than the sun (about 30 million km in diameter) but is being bullied by a tiny black hole just three times the mass of the sun, and about 17.2 km wide! (Ironically, this black hole is named “Unicorn”.)
One of the largest stellar-mass black holes we know is called M33-X-7. It spends its time in the Triangulum galaxy, eating a 70 solar mass blue giant star! As the material goes toward the black hole (sort of like when you pull the drain plug in a bathtub) friction heats it up to stupidly hot temperatures, making the gas around the black hole (ironically) very bright. And yet, M33-X-7 is only 15 times the mass of the sun and has a diameter of 92 km. To grow bigger, black holes need to devour stars, or better, merge with each other.
The technology to detect black hole mergers is very new, so we’re detecting lots of interesting things! Like two massive black holes merging in a galaxy 17 billion light-years away. As they spun around each other, they released more energy than all the light from the Milky Way in 4400 years combined. The new black hole that was formed is about the size of Germany and is about 142 solar masses.
And here, we find a strange gap. There are hundreds of black holes up to 150 solar masses, and then here is… nothing for a really long time. Then we suddenly hit black holes with millions of solar masses! This is pretty odd because you would expect that black holes just getting bigger and bigger. But for the biggest black holes, this process isn’t fast enough to produce these monstrosities. If you look at how long it would have taken for these black holes to grow that big, it would be longer than the age of the universe, which doesn’t make any sense! To explain the largest black holes, we might need the largest stars that ever existed, called Quasi-stars.
These Quasi-stars would be thousands of times larger than the sun, and even bigger than the biggest stars we have today. We don’t know if Quasi-stars existed, but if they did, they would have formed in the very early universe, where the matter was so dense. These Quasi-stars would form really quickly, and the pressure on the core of the star would be so great that the core would collapse into a black hole. The black hole could feed for millions of years in the middle of the star, and get much bigger than black holes today. These black holes would be several thousand times the mass of the sun and wider than the entire Earth! These Quasi-black holes might have eaten and merged with each other to become supermassive black holes.
Supermassive Black Holes
And here, we find the largest bodies in the universe. These behemoths lie at the centres of galaxies, and they are massive. The one at the centre of the Milky Way (called Sagittarius A-Star) is very calm and is not actively eating anything at the moment. We know it’s there because we can see a number of stars orbiting a seemingly empty spot. And despite its mass of 4 million solar masses, its radius is still only 17 times the radius of the sun. Smaller than most blue hypergiants, but millions of times more massive.
Since supermassive black holes are, well… massive, and since they lie at the centre of galaxies, many people imagine that they’re sort of like the sun at the centre of the solar system, keeping everything in orbit. But there is a big difference. While the sun makes up 99.87% of the mass of the solar system, supermassive black holes usually only have 0.001% of the mass of the galaxy. The millions of stars in the galaxy are not gravitationally bound to supermassive black holes. Instead, it’s the dark matter that is lurking in all galaxies.
Many supermassive black holes aren’t friendly to their surrounding area. Instead, they attempt to eat everything around them. As gas flows into black holes that are feeding (such as the one in the BL Lacertae galaxy), most of the material doesn’t actually make it into the black hole. Instead, it gets converted into plasma that gets ejected out of the poles at nearly the speed of light! If Earth was orbiting this black hole, it would seem 115 times larger than the sun in the sky.
Now, black holes get so big that stars look like grains of sand next to them! The black hole at the centre of the galaxy Cygnus A has a black hole with 2.5 billion solar masses! 2.5 BILLION! And it’s 14.7 billion km wide! Meaning that if you put it into our solar system, it would swallow up Neptune!
Another pretty large black hole sits in the centre of the galaxy Messier 87. This black hole has 6.5 billion solar masses and is one of the few black holes that we have taken a picture of. Actually, it’s not possible to take a photo of a black hole, so instead, the picture is actually of the glowy gas disk around a menacing shadow. This “shadow” is so large that a whole solar system would fit in it. However, there are even bigger black holes, so big that you can’t even classify them as “super” massive.
Ultramassive Black Holes
Ultramassive black holes are the giants of giants. Take, for example, OJ 287. This black hole has a mass of 18 billion solar masses and is three times as large as the Messier 87 black hole. It’s so big that it actually has a supermassive black hole (over 150 million solar masses) orbiting it! It can comfortably fit three solar systems side by side inside its event horizon.
Let’s now look at the largest black hole ever. TON 618, a black hole consuming galaxies’ worth of matter, making so much light that it’s visible from over 18 billion light-years away. It has an incredible 66 billion solar masses and is so large that it takes light a week to reach the singularity after crossing the event horizon. Eleven solar systems could fit inside it, side by side. It also eats the equivalent of the sun in a day. It is very likely that it’s the largest body in the universe.
In any case, black holes are huge, strange, and scary. Let’s learn more about them!
Sources:
This Absolute Monster of a Black Hole Eats The Equivalent of a Sun a Day
The Largest Black Hole in the Universe – Size Comparison
Black Hole Size Comparison 2019
Black Holes Explained – From Birth to Death
How Big Is the Largest Black Hole in the Universe?
Black Holes | Science Mission Directorate.
Black hole dubbed ‘the Unicorn’ may be the smallest one discovered yet
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