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  • Go to Brilliant.org/SciShow to learn more.

  • [ intro ]

  • 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 isbig.

  • But despite the namesupermassive,”

  • 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.

  • And If you want to hone your scientific thinking,

  • Brilliant is offering a new course on Scientific Thinking where you can get started.

  • It uses puzzles to help you ground yourself in the foundations of physics

  • and then guides you through exploring the physics in your everyday life.

  • If you want to keep going, Brilliant also offers over 50 other courses in science, engineering,

  • computer science, and math.

  • All of them are available offline through the Brilliant app,

  • so you can learn on the go, wherever you are.

  • If you're one of the first 200 people to sign up at Brilliant.org/SciShow,

  • you'll get 20% off the annual Premium subscription.

  • So check it out and see if it's right for you -- and let us know if you learn anything

  • cool!

  • [intro]

Thanks to Brilliant for supporting this whole week of SciShow!

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B1 中級

ブラックホールはどのくらいの大きさに成長することができますか? (How Big Can Black Holes Grow?)

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    林宜悉 に公開 2021 年 01 月 14 日
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