Name: How Big Can Black Holes Grow?
Uploaded: 2020-03-30T03:03:42.000Z
Duration: 5 min 10 s
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Nearly every large galaxy hosts a supermassive black hole at its center.

for instance, is four million times the mass of our Sun.

here are black holes out there over a thousand times bigger.

Black holes grow when stuff falls into them.

Despite what you may have heard, they actually don't suck things up like they're vacuum

They're more like a big mouth at the bottom of a steep valley --

that's why we say that matter falls into them.

Most of the stars and debris tumbling toward a black hole don't take a direct path—

And along the way, stuff collides, exchanges energy,

and most of it actually gets flung beyond the black hole's grasp.

Still, as they gobble up stars, gas, and dust,

it might seem like they could gorge themselves forever.

the point of no return for anything that crosses it, including light.

called the innermost stable circular orbit, or ISCO.

That's the inner edge of the accretion disk—

the disk of matter swirling around a black hole.

At this point, only light is fast enough to maintain a stable orbit around the black hole.

Anything else will start spiraling downward,

It hasn't technically passed the point of no return,

because if it got a good bump from another object, it could still avoid falling in.

it's doomed to eventually pass the event horizon,

because it can no longer move fast enough to escape.

As a black hole eats and gets more massive,

So stuff orbiting at a distance that used to be "safe" now becomes unstable.

It too will start spiraling toward the black hole—

As the ISCO expands, the black hole has access to more matter and can continue to eat.

At a certain distance from the event horizon,

matter is more gravitationally attracted to itself than to the black hole.

That's why giant gas clouds are able to collapse into stars and live in peace,

and our entire Galaxy isn't just one big accretion disk spiraling into its central

So, once a black hole reaches a certain mass,

everything orbiting the black hole is so far away

it'll just clump up into stars rather than fall in.

You can actually figure out what mass the black hole needs to reach for those to be

They calculated that black holes can theoretically grow

to between 50 and 270 billion solar masses.

That seems like a weirdly large range but it accounts for the fact that some black holes

which pulls the ISCO closer and lets black holes grow more massive before they max out.

Once it reaches that limit, whatever its mass is,

the black hole can only keep growing if something falls directly into it.

But that's such a precise, unlikely trajectory, it almost never happens.

Effectively, at that point, it stops growing.

But that's not quite the end of the story.

Bigger black holes do exist, because black holes sometimes collide and merge into one.

At that point, they pool their masses into one monstrous black hole.

But as huge as they are, black holes like this can't consume new matter.

The matter around them will forever be out of their reach.

getting a grasp on the science of black holes takes careful thinking.

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How Big Can Black Holes Grow?

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