In contrast to things like planets or  stars they have no physical size limit,  

and can literally grow endlessly. Although in  reality specific things need to happen to create  

different kinds of black holes, from really tiny  ones to the largest single things in the universe.  

So how do black holes grow and how  large is the largest of them all?

This video will not discuss how black holes work  or how they form since we've looked at that in  

detail in our black hole and neutron star series,  you can check them out afterwards. For now we are  

interested in finding the largest thing in the  universe. Let us start really, really small.

Primordial Black holes

The smallest kind of black holes may or may not  exist. If they do, they are probably the oldest  

objects in the universe, older even than atoms.  They would have formed just after the big bang,  

when the universe was so dense with  violent energy, that any tiny pocket  

that was just slightly more dense than  its neighbors could produce a black hole.

The smallest Primordial Black Hole that  could still be around would be a trillion  

kilograms or so, the mass of a big mountain.  And yet they would be no bigger than a proton.  

A Primordial Black hole with the mass of  earth would barely be larger than a coin.  

This makes them very hard to find, so we  haven't actually observed any yet – if  

they exist they may even be the mysterious  dark matter that holds galaxies together.

Let's move on to the kinds of black holes  that we know for sure ARE out there.

Stellar Black Holes

To make a black hole we need to compress enough  matter so that it collapses into itself. After  

that, the more mass we throw at it, the  larger it becomes. In today's universe,  

only the most violent cosmic events can create  the necessary conditions, such as the merger of  

neutron stars or when the core of a very massive  star collapses in a supernova. To have a unit to  

work with here, we'll use the mass of our sun,  about 2 million trillion trillion kilograms.

The smallest known black hole  has 2.7 times the mass of the sun  

which works out as a sphere around 16 km  in diameter, large enough to cover Paris.

Another lightweight black hole is the  companion to the V723 Mon red giant star.  

This star is 24 times larger than our sun,  30 million kilometer in diameter. And yet,  

it is thrown around by a tiny black hole  just 17.2 km wide. This tiny thing bullying  

the star is so much smaller that we can  barely even show them in comparison.

One of the largest known stellar black holes is  M33 X-7. It currently spends its time eating a 70  

solar mass blue giant, bit by bit. As all that  stolen matter circles towards the black hole,  

like water going down a drain, friction heats  it up to temperatures high enough to shine  

500,000 times brighter than our Sun! And  yet, X-7 is 'only' 15.65 solar masses  

and 92 km wide, just big enough  to cast a shadow on Corsica.

To grow much larger, black holes have to either  devour a lot of stars or better, merge with one  

another. The instruments that make it possible  to detect these mergers are very new so we are  

currently discovering a lot of exciting things.  Like two massive black holes that we detected in  

a galaxy 17 billion lightyears away. As they spun  around each other violently, they released more  

energy in the form of gravitational waves than the  combined light from all the stars in the milky way  

in 4400 years. The new black hole they formed is  about the size of Germany and is 142 solar masses.

And here we hit a curious gap in scale.

There are lots of black holes up to 150 solar  masses. And then there is nothing for a long  

time. Until we suddenly hit black holes,  that are millions of times more massive.

Which is a bit confusing, because we had this  idea that black holes are consistently growing  

and growing. But for the most massive black holes  this process is not fast enough to explain their  

existence today. The universe is simply not  old enough for these supermassive black holes  

to have formed by eating stars and merging with  each other. Something else must have happened.

To explain how we got the largest black  holes in the universe, we might need the  

largest stars that ever existed: Quasi Stars.  To get a sense of scale, we can compare them  

to the largest stars that exist today. Our  Sun is like a grain of sand next to them.

We don't know if Quasi Stars actually existed but  they are an interesting concept when it comes to  

supercharging black hole development. The idea is  that the matter in the early universe was so dense  

that quasi stars could grow to thousands of  times the mass of our sun. The cores of these  

stars might have been crushed by their own weight  so much to actually collapse into black holes  

while the star was still forming. In contrast to  stars today that would destroy themselves in the  

process, inside quasi stars, a deadly balance  could emerge. Gravity pressed the supermassive  

star together, feeding the black hole and heating  the material falling in to such a degree that the  

radiation pressure kept the star stable. And so  these quickly growing black holes might have been  

able to consume the quasi star for millions of  years and grow far bigger than any modern stellar  

black hole. Black holes several thousand times the  mass of the Sun and wider than the entire earth.

These black holes might have become  the seeds for supermassive black holes.

Supermassive black holes

So now, we arrive at the kings of our  universe, the largest single bodies in  

existence. The centers of most galaxies contain a  super massive black hole, and they are monstrous.

In the Milky Way we have Sagittarius A  Star, a super massive black hole with  

about 4 million solar masses that is calm and  collected and just does its thing. We know  

it sits there because we can see a number of stars  being thrown around by a seemingly empty spot.  

And despite its incredible  mass, it's radius is still  

only 17 times our sun. Smaller than most giant  stars, but millions of times more massive.

Because Supermassive black holes are so  massive and located at the center of galaxies,  

many people imagine them as being a bit like  the Sun in the solar system. An anchor that  

glues everything else together and forces it into  an orbit. But this is a misconception. While the  

sun makes up 99.86% of all the mass in the solar  system, SuperMassive Black Holes usually only have  

0.001% of the mass of their galaxy. The billions  of stars in galaxies are not gravitationally  

bound to them, instead it is the gravitational  effect of dark matter which holds them together.

Many supermassive black holes aren't gentle  giants, especially when they are feeding on  

the clouds of mass in their galaxy. The one at the  center of the BL Lacertae galaxy is devouring so  

much material that it produces jets of plasma  accelerated to nearly the speed of light.  

If Earth were orbiting this huge body, it would  seem 115 times larger than our Sun in the sky…  

and we'd be burnt to a crisp in seconds  by its glowing hot accretion disk.

At this point black holes become so large that  stars seem ridiculously tiny compared to them.

The galaxy Cygnus A has a super massive black hole  with 2.5 billion solar masses and 14.7 billion km  

wide, which would mean that if it took the place  of our Sun, it would swallow all the planets and  

stretch halfway to the edge of our Solar System.  It is devouring so much mass and material that it  

churns its disk into a kind of magnetic funnel,  spewing gas out making tremendous radio lobes,  

towering over the galaxy, half a million light  years in diameter. That is 2.5 Milkyways wide.

Another pretty large Super Massive Black  hole sits in the galaxy Messier 87. It  

has 6.5 billion solar masses and was the  first black hole we got an actual photo of.  

Or rather of the glowing gas around  the edge of a menacing shadow.  

This sphere of darkness is so large  that it covers our entire Solar System.

And yet, there is a scale even  above these kinds of objects...

Ultramassive black holes

Now we reach the most massive black holes, perhaps  the largest single bodies that will ever exist.  

These black holes have eaten so much that they've  grown to tens of billions of solar masses, their  

gravity the engine for a 'quasar'- an accretion  disk shining brighter than thousands of galaxies  

full of stars. So massive that they deserve a  title of their own - Ultramassive Black Holes.

The Ultra Massive Black Hole at the center of  galaxy OJ 287 is 18 billion solar masses. It  

is so big that it has a Super Massive black hole,  nearly forty times larger than sagittarius A star,  

orbiting it! This thing defies imagination  and is really hard to compare to anything.  

It can comfortably fit three Solar  Systems side by side inside of it.

Let us end this insane competition  and get to the king of kings.

TON 618, a black hole that we can observe  consuming galaxies worth of matter is  

shining with the brightness of a hundred trillion  stars, visible from 18 billion light years away.  

It has an incredible 66 billion solar masses.  A black hole so large that it would take light  

a week to reach the singularity after  crossing the event horizon. About 11  

Solar Systems could sit inside of it side  by side. It may very well be the largest  

single body in the universe. But in  reality, it is probably even larger.  

Since TON 618 is so far away, we only see  what it looked like 10 billion years ago.

In any case, black holes are scary and mysterious  

and gigantic. They will be here after everything  else dies, and growing larger and larger.

So now let us do the trip again. From  the smallest possible black hole,  

all the way up to the largest.

Let's try something new today,  we can call it: “Behind the Lies”  

a short behind the scenes bit about the  necessary inaccuracies in this video  

because it's really not actually possible  to rank black holes like trading cards.

How so? Well, while we have catalogued  millions of stars, we really only have  

good data on a couple of dozen black holes.  That's because black hole gazing wasn't really  

a thing until 50 years ago – and technically  still isn't, because we can't see black holes.

We can only derive their properties from studying  their gravitational effects on the matter around  

them, like the orbit of stars that come close  to them. This effect depends on the mass of the  

black hole, which we can approximate at  the most basic level with Kepler's Laws.  

But this comes with huge  uncertainties and error bars.  

Then we have to convert mass to size  next, which brings new uncertainties.  

For example, we calculated the radius from the  mass using the Schwarzschild equation which for  

the sake of simplicity assumes black holes are  perfectly round and don't spin: a kind of black  

hole that doesn't really exist. The reality is  that physics on these scales is a bit fuzzy.

So some of the black holes we talked about  here may be way smaller or way bigger. We  

just don't know for sure. We shimmied around  this problem by comparing different sources  

with different kinds of values and using  different mass calculations to arrive  

at a standardized list that allowed us  to be as accurate as humanly possible.  

You can look at all of this in our source doc. As  a result this script was written with the tears of  

experts we drove crazy with our obsession  for the best values they could live with.

In this process, tons of stuff  got cut and didn't make it into  

the final video – but luckily we  found a way to not waste all of it:

We created a lot of black hole merch, spanning the  whole range from somewhat bonkers to more serious.  

This way we get to explore a topic  from different angles – and you  

get to continue having fun with  black holes after this video ends.