Black Holes: Crash Course Astronomy #33

By | December 7, 2019


As we’ve 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; it’ll 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 we’re 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 you’re 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. We’re 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, you’re 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. We’ve 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 we’ve 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. They’re 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, you’re 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 this – and no, I’m not
kidding – spaghettification. 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 just… flowing, 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 we’ve 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 in — bloop, 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 you’re
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 pass — all 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 you’re about to
fall into a black hole, you probably wouldn’t care. See? Like I said…WOW. 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. We’re 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 weird — and let’s be honest:
they are — they 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 hole — not that this helps much if you’re falling in. Crash Course Astronomy is produced in association
with PBS Digital Studios. Head over to their YouTube channel to get sucked into even more
awesome videos. This episode was written by me, Phil Plait. The script was edited by Blake
de Pastino, and our consultant is Dr. Michelle Thaller. It was directed by Nicholas Jenkins,
edited by Nicole Sweeney, the sound designer is Michael Aranda, and the graphics team is
Thought Café.

100 thoughts on “Black Holes: Crash Course Astronomy #33

  1. Lilianah Martinez Post author

    could anyone get a PDF of all this information? a little hard to write down everything he's saying when he's talking so fast 😀

    Reply
  2. nazhifah himatul ulya Post author

    black hole is one of most terrible nightmare. while watching this video i couldnt stop imagining how it would feel like being sucked into a black hole. what will i see inside? i wont see anything because i die, but what if i could still be alive for example, what will i see inside the hole? i really want to know whats inside the hole. will it be a total void of anything, mirrorring the universe with the hole as the main portal? ive heard about singularity but i want to see how singularity is inside there

    Reply
  3. Julia Deleon Post author

    My mind exploded, I felt that I'm listening to an alien language :'( mind blowing~

    Reply
  4. German Lopez Post author

    I love this dude, why there are no more videos with him? 10:12

    Reply
  5. Magnus Juul Post author

    I can't get over the way he pronounces kilometer. the o sound should be pronounced the same way as the o in Tom for example

    Reply
  6. ljupce trninkov Post author

    So a black hole is a core infinitescimally small which has pulling force faster than the speed of light and thus no light can escape it?

    Reply
  7. EulogizeMe43 Post author

    Thank you for taking an extremely complicated subject and making it easy for us to understand. Love these videos!

    Reply
  8. Guinness Post author

    "Spagettification" – the Astronomer's masturbation term

    Reply
  9. david ortega Post author

    it is much more science about black holes, there must be more videos :c

    Reply
  10. Blacktop Mafia Post author

    So I can throw my ex wife into a black hole and I won’t see her again? Great!

    Reply
  11. Whitney Graham Post author

    i'm sorry but i lost it when he said "you'd die"

    Reply
  12. Amit Garg Post author

    time never stops .. It may be different but ultimately never stops. it can't be stop..

    Suppose, Somehow you are sitting at the top of blackhole and survived. Your body also have motion. If motion is there , time is there … It may be completely different from earth's time but still time exist.

    may be one second on blackhole = 100 or 10000 years on earth but still time is there.

    Same applies to black hole itself , they have motion in them that's create gravity . So time is there! ..

    Reply
  13. The Science Man Post author

    Black Holes are so fascinating to learn about!

    Reply
  14. Carmen Mencar Post author

    "not perspective effect" so it is not because light is affected by gravity and would hit the observer slower as the clock gets closer to the black hole?

    Reply
  15. Ranziger Käsekuchen Post author

    if time stands still at the event horison how can anithing ever enter the black hole ?(how can it grow and where goes all this matter ?)

    Reply
  16. Moepowerplant Post author

    Any Love Live fans here who can relate to the video?

    Reply
  17. David Rozin Post author

    So, is it right that if we were able to hover in one place within the event horizon of a black hole, then we would see the future of the entire universe?

    Reply
  18. Kile B Post author

    But a time does aired I'm sure there was another theory on theif you're reading this then there's probably another theory on thescience special astronomy has been wrong many times and things change so keep your mind open like the Galaxy thank you for putting out this information though I love can't deny he looks like a younger version of Walter White

    Reply
  19. Coda Mission Post author

    Only 4 years after this video was released, a team was able to composite an image of Messier 87 and it's supermassive black hole. We are an incredible species

    Reply
  20. Coda Mission Post author

    I want to fall into a supermassive black hole and watch the world pass me by.

    Reply
  21. Alan Stephens Post author

    I can not believe that these "CrashCourse" postings have been going on for years and I did not know it. Since discovering them a few days ago, I've been watching a couple every day! This explanation regarding black hole gravity and "Escape velocity at the surface being equal to the speed of light" made a light go off in my head. Love this channel.

    Reply
  22. FlickCenterGaming Post author

    nothing can travel faster than the speed of light, except my new kitten on catnip.

    Reply
  23. Aaron Gibb Post author

    I want to take 100 tabs of acid and then fly into a black hole, I wouldn’t care if I died at that point

    Reply
  24. alkafrazin Post author

    Wouldn't it be half the speed of light to just under the speed of light, depending on the mass of the star?

    Reply
  25. diatonix2 Post author

    Very interesting, but please make a pause between sentences! I'm getting a headache listening to you.

    Reply
  26. physicself Post author

    Why do you keep saying 'velocity' when you mean 'speed'?? This conflates two very different concepts and obscures the meaning of both.

    Reply
  27. J.B 91 Post author

    In one video you say the mass must over 20x the SUN to be a Black hole. Here its only 2.8. I am confused.

    Reply
  28. Charodeiski Post author

    10:05 – 10:15 HOLD UP! Wouldn't this by definition mean, that from the perspective of the outside observer, the black hole actually NEVER forms in the first place?! I mean if person A touches the event horizon, his time dilation factor would have reached 1 by default, and as is being said, all of infinity would pass at the moment person A touches the event horizon from HIS perspective. If we were to see person A as the "representative" of the black hole (or more precisely the black hole's frame of reference) compared to us, wouldn't this by definition mean that a black hole actually can't form to begin with? Because the moment it theoretically gets formed, it's creation would take place in an infinite future in the reference frame of the outside observer???

    Reply
  29. Dyoto Orion Post author

    WE should come up with a Snappy Name name for such an Object! <3 Hahahahaa! 😀

    Reply
  30. Explosivefibers Post author

    So if you would fall into one, d*ck first, you would get the longest in the universe? Asking for a friend

    Reply
  31. langhard1 Post author

    if i enter a black hole while i match the surrounding entry speed , would the spagetty thing stil happen ?

    Reply
  32. Doug G Post author

    Black holes are the embodiment of rage and terror. Yikes

    Reply
  33. Tux Penguin Post author

    "If I kill you, you're taking the Universe with you."

    –Black hole

    Reply
  34. biohazard429 Post author

    I want a t-shirt that reads, Whatever happens in a blackhole stays in a blackhole. I know that's very geeky but I'm a major nerd haha

    Reply
  35. Daniel Lopez Post author

    they should of crashed into it harder, everybody in the company who made this video

    Reply
  36. S Dunny Post author

    At the risk of not sounding like an idiot … I still can’t grasp this concept … can someone explain this in an easier way to understand … I love astrology but I have trouble understanding some of this stuff

    Reply
  37. S Dunny Post author

    Are black holes like constantly eating up space ? I’m confused

    Reply
  38. Lizard King Post author

    so what does it actually LOOK like when stuff falls into a black hole, if time slows to an absolute stop. does that mean everything that falls in just sort of.. accumulates around the edge of the event horizon, like a giant cosmic katamari?

    Reply
  39. Ron Frank Post author

    If we were being slowly, and inevitably, pulled into the black hole at the center of our galaxy….wouldn't that make the rest of the universe appear to be expanding? At least from our point of view.

    Reply
  40. US Gaming Post author

    Falling into black hole increase our speed to light speed then if we increased it again then Time travel happens I am I correct.

    Reply
  41. theapocilip Post author

    Time travel is possible. Go visit a black hole, come back to earth and it’s the year 20 000

    Reply
  42. Baguio Byben Post author

    Can anyone explain why light can't escape a blackhole when nothing goes beyond the speed of light? Isn't this somehow logically incorrect? If light is faster than everything shouldn't the blackhole not affect it? I seriously don't understand how a blackhole can do this so can someone enlighten me.

    Reply
  43. Helder De Almeida Post author

    So basically black hole is a portal to the unknown universe.

    Reply
  44. Stas Post author

    Please, let Phil teach another course! I miss him so much 🙁

    Reply
  45. Matt Miller N8QZH Post author

    And when the neutrons are compressed to this point, magic things happen and the laws of physics evaporate..
    It opens a portal to……… ????

    Reply
  46. Muhammed Sillah Post author

    how does a neutron star's gravity increase when it shrinks(more density right?) if no matter is added to it? isn't gravity directly proportional to mass?

    Reply
  47. RONA DESPABILADERAS Post author

    Good day. I am a graduate student of Bicol University (Philippines) and I am currently writing my thesis on the use of flipped classroom in teaching. Your video is very informative and is suited to the level of my learners. Please allow me to use it in my study. Thank you very much.

    Reply
  48. A CB Post author

    if someone far away watches you fall into a black hole, it would be over within fraction of a second, you would appear to zip into the black at a very fast speed. Why? Because the observer is in a faster time frame, he is watching you in fast forward mode, this is partly why everything that orbits black holes, even at significantly large distance, seems to move at insanely fast velocity, This suggests that slower time does not mean literally that time is slower, but that the frame rate is slower, or that there are missing frames relative to the rest of the universe.

    Reply
  49. NABIL Iben sobih Post author

    I would be pleased if you add a video explaining a little bit better the degeneracy pressure. It is commented several times throughout the videos but the concept remains a bit vague. What are the physiscs behind?
    Thank you

    Reply
  50. 수미Sumi K Post author

    I want to ask a question. I want to study about cosmology. Are the basic of Cosmology and Astronomy same ? Please, somebody tell me.

    Reply
  51. langhard1 Post author

    but how about entering the hole at the speed that matches the tide ?

    Reply
  52. Margret Fortune Post author

    Yet black holes do decay by discarding massless particles, so not everything is trapped.

    Reply
  53. Crystal Poe Post author

    My 7 year old (who's been obsessed with learning all he can about space since he was 2) wants me to say "Black holes freak me out and I wouldn't like to die in one"

    Reply
  54. ZJZ Post author

    The term of escaping velocity equals light speed is totally confusing. First, how is it equivalent to nothing can escape from it? It only means even if you throw a thing at speed of light, it will still fall back. How about I keep pushing a thing with constant speed like 10km/hr? Second, the gravity at any point inside a black hole except its center is finite, that means the negative potential energy you need to overcome is finite. Any object moving at near light speed would have nearly infinite kinetic energy. How came it's not enough to overcome the finite amount of negative potential energy?

    Reply
  55. PepperShakerKetchupBottle Post author

    I think my new favorite word is Spaghettification.

    Reply
  56. TOFKAS01 Post author

    9:30 And from the inside you would see the universe aging in a very fast pace (theoreticaly) 10:07 ….Can it be perhaps be, that black holes are some kind of "time-capsule" for atoms? That they are perhaps the seed for the next universe and the next big bang?

    Reply
  57. Suthin Scientist Post author

    If you fell into a black hole, you'd be torn apart within a few kilometers of the singularity.

    Reply
  58. Michael Clement Post author

    As an object shrinks its angular momentum gets faster however in the case of a black hole the size of the object's matter is zero and therefor the angular momentum should be infinity, which of course is not possible as its faster than the speed of light. While understanding that we don't know what happens in regards to angular momentum are their any theories out there?

    Reply
  59. Paul Lilly Post author

    When I die, I want to die in a black hole so that I can watch all of time elapse in my last few moments.

    Reply
  60. Lemuel Acosta Post author

    So the only possible way of travelling to the future is to get near a black hole

    Reply
  61. BD Bailey Post author

    I have a question. You talked about spaghettification but assured us not to worry because "it's over in a millisecond." Yeah, but!! But later on you talk about time dilation, which I haven't finished the video yet and haven't heard you use that term so far, but it's what you're talking about. Right. See where I'm going? It may seem to us the poor schmuck who fell in couldn't have suffered because it was over in a flash. But just as an object becomes infinitely massive as one approachers light speed, so time will eventually stop as one approaches a black hole, no? So as he's stretched out, what's he feeling then? Is it taking eons and he's wanting to get on with it already??? Do his feet feel heavy???? Is he wishing he had a change of socks?????

    Reply
  62. Arianna Parker Post author

    That’s odd because I’ve seen neutron stars that have a bout a mass of 10 times the sun

    Reply
  63. alien hiabov Post author

    information still not understood…. gatta go look at a washing machine to see if i can get an idea. it sounds stupid!

    Reply
  64. Barry Healey Post author

    Dr. Plait, does a black hole still have the core matter from a neutron star at its center or has it collapsed into a microscopic singularity and the mass is destroyed? If so how is the gravity sustained?

    Reply
  65. DuncanAndFriends Pranks Post author

    There must be a link between black holes and dark matter

    Reply
  66. MrEnjoivolcom1 Post author

    I'd like further explanation on the entire "all of time passing before you as entering the hole". Like…a bit more please.

    Reply
  67. RA-75 Post author

    Einstein also took credit for other people's work – Lorentz, Poincare, Michelson-Morley, and the first person to talk about gravitational effects on sunlight occurred in the 1800's. In short Einstein was a plagiarist – funny that he gave all his 1905 Noble Prize money to his ex-wife. She was the real brains behind the photoelectric effect!

    Reply

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