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• 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.

• 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