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Nearly every large galaxy hosts a supermassive
black hole at its center.
The one at the heart of the Milky Way,
for instance, is four million times the mass
of our Sun.
Which is…big.
But despite the name “supermassive,”
here are black holes out there over a thousand
times bigger.
So, how big can black holes get?
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
cleaners.
They’re more like a big mouth at the bottom
of a steep valley --
that’s why we say that matter falls into
them.
But black holes are messy eaters.
Most of the stars and debris tumbling toward
a black hole don’t take a direct path—
they swing around in some kind of orbit.
And along the way, stuff collides, exchanges
energy,
and most of it actually gets flung beyond
the black hole’s grasp.
So black holes grow slowly.
Still, as they gobble up stars, gas, and dust,
it might seem like they could gorge themselves
forever.
But there is actually a limit.
When it comes to black holes,
we talk a lot about the event horizon—
the point of no return for anything that crosses
it, including light.
But there’s another important threshold,
that you probably haven’t heard of,
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,
like water toward a drain.
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.
But if it remains uninterrupted,
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,
its gravitational pull gets stronger.
So stuff orbiting at a distance that used
to be "safe" now becomes unstable.
It too will start spiraling toward the black
hole—
which means the ISCO moves outward.
As the ISCO expands, the black hole has access
to more matter and can continue to eat.
Which makes the ISCO expand even more,
which gives the black hole more to eat.
And so on.
But only for so long.
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
black hole.
So, once a black hole reaches a certain mass,
the ISCO lines up with that distance.
In other words, at that point,
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
the same distance.
And in a paper published back in 2015,
researchers crunched the numbers.
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
are spinning,
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.
They can only grow by colliding.
The matter around them will forever be out
of their reach.
Since we can’t just see these things,
getting a grasp on the science of black holes
takes careful thinking.
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