Placeholder Image

Subtitles section Play video

  • As weve seen over the past few episodes, a lot of really epic stuff happens when a

  • star dies. If the star’s core is less than 1.4 times the mass of the Sun, it becomes

  • a white dwarf—a very hot ball of super-compressed matter about the size of the Earth.

  • If the core is heftier, between 1.4 and 2.8 times the Sun’s mass, it collapses even

  • further, becoming a neutron star that’s only 20 km across. The neutron soup inside

  • of it resists the collapse, and prevents the core from shrinking any more.

  • But what if the mass is MORE than 2.8 times the Sun’s? If that happens, the gravity

  • of the core can actually overcome the tremendous resistance of the neutrons and continue its collapse.

  • What force can possibly stop it now?

  • It turns out, none. None more force. There is literally nothing in the Universe that

  • can stop the collapse. The core of the star is about to go bye bye.

  • Way back in Episode 7 I talked about escape velocity, and it’s about to become a major

  • player in the unfolding events of the collapsing core of a high mass star. In brief, it’s

  • the velocity at which you need to fling something off the surface of an object to get it to escape.

  • For the Earth, the escape velocity is about 11 km/sec. Get something moving that quickly,

  • and it’s gone; itll never fall back. The Sun, which has much stronger gravity than

  • Earth, has an escape velocity of over 600 km/sec.

  • A neutron star, with its immense gravity, can have an escape velocity of 150,000 km/sec

  • that’s half the speed of light!

  • Keep that in mind, and let’s go back to the collapsing core of the star. As it shrinks,

  • its gravity gets stronger and stronger. That means its escape velocity gets higher and

  • higher. When it’s neutron star-sized the escape velocity is half the speed of light,

  • but if it’s more than 2.8 times the mass of the Sun, the core will keep collapsing.

  • When its size drops just a little bit more, down to roughly 18 km, an amazing thing happens:

  • The escape velocity at its surface is equal to the speed of light.

  • And, well, that’s a problem, because in our Universe, nothing can travel faster than

  • the speed of light. Not a rock, not a rocket, not even light itself. Once the core of the

  • star shrinks down smaller than that magic size, nothing can escape.

  • No matter can come out, so it’s like an infinitely deep HOLE, and no light can come

  • out, so it’s BLACK.

  • We should come up with a snappy name for such an object.

  • A black hole is the ultimate end state for the core of a high mass star. Whatever happens

  • in a black hole STAYS in a black hole. That region of space, that surface around the black

  • hole where the escape velocity is the speed of light, is called the EVENT HORIZON for

  • that reason. Any event that happens inside can’t be known. It’s beyond the horizon for us.

  • Black holes mess with our concepts of space and time. The math and physics of black holes

  • is incredibly complex, so much so that even after several decades of study, physicists

  • still argue over a lot of their properties.

  • This has led to a lot of misconceptions about them, too.

  • All right, let’s get this out of the way right now: The Sun cannot become a black hole.

  • It takes a stellar core at least about three times the mass of the Sun to overcome neutron

  • degeneracy pressure. That means the original star must have something like 20 times the

  • Sun’s mass or more. So were safe from THAT particular scifi scenario.

  • Here’s another misconception: A lot of people think of black holes as cosmic vacuum cleaners,

  • sucking in everything near them.

  • But that’s not really true. They have powerful gravity, yeah, but only when youre very

  • close to one. The power of a black hole comes from its mass, certainly, but just as important

  • is its SIZE. Or, really, its LACK of size.

  • If you could turn the Sun into a black hole, which you can’t, but let’s pretend you

  • could, then the Earth would orbit it pretty much exactly as it does now. From 150 million

  • kilometers away, the Earth doesn’t care if the Sun is big or tiny. Were so far

  • away that it doesn’t matter.

  • It gets to be a big deal when you get close. Remember, from episode 7 about gravity, the

  • strength of gravity you feel from an object depends on how massive it is and your distance

  • from its center. The closest you can get to the Sun is by touching it, being on its surface,

  • about 700,000 km from its center. If you get any closer to its center, youre INSIDE

  • it. The material OUTSIDE of your position is no longer pulling you down and so the gravity

  • you feel will actually decrease.

  • But if the Sun were crushed down to about 6 km across it would be a black hole. You

  • could get much closer than 700,000 km to it, and as you did you’d feel a stronger and

  • stronger pull as you approached it.

  • So from far away, a black hole with, say, ten times the Sun’s mass would pull on you

  • just as hard as a normal star with that same mass.

  • You can orbit a black hole, too, as long as you keep a safe distance between you and it.

  • Orbiting a ten-solar-mass black hole would be just like orbiting a ten-solar-mass star

  • except not so hot and bright.

  • Black holes are weird enough without the misconceptions.

  • Black holes also come in different sizes. The kind I’ve been talking about has a minimum

  • mass of about 3 times the Sun’s, and might get as high as a dozen or more times the Sun’s

  • mass, if the parent star was big enough. We call these stellar-mass black holes. If it

  • happens to gobble down more matter, it gets more massive, and the event horizon grows

  • as well. The black hole gets bigger.

  • The idea that huge black holes could form in the centers of galaxies was first proposed

  • in the 1970s, and it wasn’t much later that the first one was found, in the center of

  • our own Milky Way galaxy. Weve measured its mass at a whopping 4.3 million times the

  • Sun’s mass! And now we think every major galaxy has one at its heart, too, and in fact

  • may be crucial in the formation of galaxies themselves. I’ll discuss those more in a future episode.

  • Here’s a fun thought: What would happen if you fell into one? Say, a stellar black

  • hole with ten times the Sun’s mass?

  • You’d die. But what happens in the few milliseconds before you left the known Universe forever

  • is actually pretty interesting.

  • As weve seen many times in our own solar system, tides are important. They arise because

  • gravity weakens with distance, so a big object like a moon gets stretched by its planet’s

  • gravity; the far side of the moon is pulled less than the near side.

  • A black hole has incredibly intense gravity, so the tides it can inflict are serious indeed.

  • Theyre so strong that if you fell into a stellar mass black hole feet first, the

  • force of gravity on your feet can be MILLIONS OF TIMES STRONGER than the force on your head.

  • Remember, even the meager tides of a planet can rip moons apart. When you multiply that

  • force by a million, youre in trouble.

  • As you fall in, your feet are pulled so much harder than your head that you stretch, pulled

  • like taffy. You’d become a long, thin, noodle, kilometers in length, but narrower than a hair wide.

  • Astronomers call thisand no, I’m not kiddingspaghettification.

  • This would happen pretty close to the black hole, just a few dozen kilometers out. If

  • you fell in from a long distance, you’d be moving pretty near the speed of light by

  • that point, and you’d only have a millisecond or so before it killed you anyway, so yay?

  • Note that this is only for stellar mass black holes. Supermassive black holes are far bigger,

  • millions or billions of kilometers across. Compared to that size, the distance between

  • your head and feet is small, so the tides across you aren’t nearly as severe. You’d

  • fall in pretty much intact -- if that makes you feel any better.

  • But compared to either flavor of black hole, a star still has substantial size, and one

  • that gets too close to any black hole can be disrupted via tides. In March 2011, astronomers

  • witnessed just such an event. In a distant galaxy, a star apparently got too close to

  • a black hole, and was torn apart by the ferocious tides. As the star was disrupted, it flared

  • in brightness, momentarily blasting out a trillion times the Sun’s energy! That’s

  • how we were able to see it even though it was several billion light years away.

  • But I’ve saved the weirdest thing for last. One of Albert Einstein’s biggest ideas is

  • that space isn’t just emptiness, it’s an actual thing, like a fabric in which all

  • matter and energy is embedded. What we perceive as gravity is really just a warping of this

  • space, like the way a bowling ball on top of a bed warps the shape of the mattress.

  • The more massive an object, the more it warps space.

  • Not only that, but space and time are basically two parts of the same thing, what we now call

  • space-time. You can’t affect one without affecting the other. Einstein calculated that

  • when a massive object warps space, it also warps time; someone deep inside the gravitational

  • influence of an object perceives time as ticking more slowly than someone far away from that

  • object. I know, it’s bizarre; we think of time as justflowing, and everyone should

  • see it move at the same rate. But the Universe is under no obligation to obey our preconceptions.

  • Einstein was right (he was right a lot).

  • This slowing of time is stronger the stronger the gravity of the object is. So your clock

  • ticks a bit slower than someone far away from Earth, for example. The effect is tiny, but

  • real, and weve actually measured it on Earth with extremely precise clocks!

  • However, if you get near a black hole, the effect gets a lot stronger. In fact, black

  • holes warp space-time so much that, at the event horizon, time essentially stops! You’d

  • see your clock running normally, and you’d just fall inbloop, gone. But someone

  • far away would see your clock ticking more slowly as you fell in. And this isn’t a

  • mechanical or perception effect; it’s actually woven into the fabric of space. To someone

  • outside looking down on you, your fall would literally take forever.

  • But then, they wouldn’t be able to actually see you. The light you emit would have to

  • fight the intense gravity of the black hole to get out, and to do that it would lose energy.

  • This is very similar to the Doppler redshift I’ve talked about in earlier episodes, and

  • is called a gravitational redshift. When youre right at the event horizon, just when an outside

  • observer would see your clock stop, they’d also see the light coming from you infinitely

  • redshift! Your light would lose ALL its energy trying to leave the vicinity of the black

  • hole, and you’d be invisible.

  • And from your viewpoint?

  • Buckle up, because this is...WOW.

  • You’d see the universe speed up, and just as you hit the event horizon, all of time would passall of it. And all

  • that light coming at you from the Universe would be blue-shifted, becoming such high

  • energy that you’d be fried. But since youre about to fall into a black hole, you probably wouldn’t care.

  • See? Like I saidWOW.

  • Black holes are so strange, with such fiercely complicated math and physics to explain them,

  • that scientists are still trying to figure out even basic things about them. For example,

  • some scientists argue that the event horizon as we understand it may not actually exist,

  • and that when you apply quantum mechanics to black hole physics, you find particles

  • can slowly leak out. Were still new at this, and struggling to understand what may

  • be the most complex objects in the cosmos.

  • Black holes, as bizarre and counterintuitive as they are, keep popping up from here on

  • out as we poke our noses into more and bigger astronomical objects. While they may seem

  • scary and weirdand let’s be honest: they arethey have literally shaped most

  • of the objects we see in the Universe.

  • Today you learned that stellar mass black holes form when a very massive star dies,

  • and its core collapses. The core has to be more than about 2.8 times the Sun’s mass

  • to form a black hole. Black holes come in different sizes, but for all of them, the

  • escape velocity is greater than the speed of light, so nothing can escape, not matter

  • or light. They don’t wander the Universe gobbling everything down around them; their

  • gravity is only really intense very close to them. Tides near a stellar mass black hole

  • will spaghettify you, and time slows down when you get near a black holenot that

  • this helps much if youre falling in.

  • Crash Course Astronomy