Main content

## Cosmology and astronomy

### Course: Cosmology and astronomy > Unit 2

Lesson 1: Life and death of stars- Birth of stars
- Accreting mass due to gravity simulation
- Challenge: Modeling Accretion Disks
- Becoming a red giant
- White and black dwarfs
- Star field and nebula images
- Lifecycle of massive stars
- Supernova (supernovae)
- Supernova clarification
- Black holes
- Supermassive black holes

© 2023 Khan AcademyTerms of usePrivacy PolicyCookie Notice

# Black holes

Black Holes. Created by Sal Khan.

## Want to join the conversation?

- Could a blackhole ever be destroyed?(85 votes)
- When black holes meet, they do indeed form a bigger black hole.And yes black holes can get destroyed but not by another black hole.

Black holes are not entirely giant 'sucking machines', they also give off energy. I'm not talking about quasars and spitting out of gamma ray bursts, I mean Hawking radiation. Because of this, black holes actually have a temperature.

The temperature of a black hole is actually slightly above absolute zero, and so right before heat death, black holes will actually be giving off more energy than taking in, hence the decay of the black hole. It is predicted it would take 10^100 years for a super massive black hole to decay right before heat death (which is so far away I forgot how many zeros it had).(25 votes)

- So if light can't escape a black hole at a certain point, then how have people discovered them?(61 votes)
- because their gravity still effects nearby stars. sometimes, it actually rips the stars apart, which creates a big, glowing disk around it. also, they lens light that gets near them(56 votes)

- Have any planets ever gotten sucked into a black hole that we know about?????(10 votes)
- The "that we know about" parameter will make the answer an uneventful no. The problem with it is that the blackholes that we "know" about have either settled down and are just floating about waiting to hit something (or evaporate - hawking radiation) or are consuming stars or gas. The problem with viewing a planet being sucked into a black hole is that most planets will form near a star and when the blackhole starts sucking up the star the gasses will collide and the resulting friction will cause the event to become brighter and this effectively blinds us (this is like trying to look at a person standing between two headlights on a car at night and try to watch their lips move). The universe has been around long enough for just about anything to have happened, but I understand how cool it would be to witness it. :-)(15 votes)

- do blackholes ever like run out of steam and disappear? and how do objects bigger than the black hole fit in it and how does it grow?(7 votes)
- Yes, black holes emit a little amount of radiation called Hawking Radiation (named after the discoverer). This radiation comes directly from the black holes mass and the black hole slowly gets smaller overtime. This takes an extremely long time though. The average stellar black hole will take an incredible 10^66 years to decay (10 with 66 zeroes). A supermassive black hole will take an unfathomable 10^92 years to decay completely. For reference, the universe is 1.37 x 10^10 years old. When the event horizon shrinks to size zero, everything that remains inside the black hole (much less than the original mass) bursts out in a titanic explosion.(13 votes)

- could planets orbit around a black hole with enough speed?(14 votes)
- Yes,Our whole galaxy is orbiting around a super-massive black hole.(5 votes)

- Say an object with the mass of the moon were to meet the event horizon of an average sized black hole, approximately how long would it take for the object to go from the event horizon to the singularity? Also, approximately how fast does a black hole travel?(10 votes)
- Since the force here involves the gravitational field strength, I would say that the rate of acceleration that an object gets sucked in is very close to speed of light. Since its mass is infinite, its gravitational strength would be infinite as well.

However, it cannot go beyond speed of light due to Einstein's theory of relativity.

Thus the actual time of an object sucked in would be indeed very fast. It is hard for us to observe the of black holes from the first place...

I don't know if this has helped, but I thought that it wouldn't be bad to share my mind.(1 vote)

- I've seen A few computer-generated videos of black holes "eating" stuff, and I noticed that it always spirals around instead of going straight. Why does it do this instead of going straight into the black hole?(4 votes)
- Because the black hole is rotating, this can drag particles into spiral paths.(5 votes)

- If the photon has a mass of 0 e.v, why is it under influence of gravity? In other terms, why the photon is not able to escape beyond event horizon?(8 votes)
- Well, I don't think anything could be more infinite than the other. It's not like infinity could go on for infinity. That just doesn't make sense. Regular or supermassive black hole, neither is more infinite than the other. The only thing we can actually make a sense of with black holes is density. A lot of us know that supermassives are really dense. Maybe even all of us. I mean it's not like we can see them or anything, it's just that...well...we know stuff.(1 vote)

- I always thought that a black hole is a 4-d dimension, is that true?(3 votes)
- no,it is not so.....a black hole is a 3d object.....but it might be a fact or fiction that it takes us to higher dimensions of the universe..who knows??(5 votes)

- What can possibly happen if we just keep adding mass to the black hole ? Will its infinite density will just keep increasing or will it stop at some point(6 votes)
- We don't really know. We
*think*that black holes have an upper limit, although we don't have enough proof -- and apology that this is such a vague answer. In either case, we should not be worried :-)(2 votes)

## Video transcript

In the last video, we
saw that if we started with a massive star
about nine to 20 times the mass of the Sun,
and when it finally matures the remnant of
the star is roughly, or that remnant core of the star
is roughly 1 and 1/2 to 3 times the solar mass or
the mass of the Sun, then this remnant right here--
and let me just be clear, this nine to 20 times is
the mass of that star when it's in its main sequence. This 1 and 1/2 to
three times is the mass once it's shed off
a lot of the, I guess, outer
material of the star. And this is really the
mass of the remnant of the star, kind of
the core of the star. But that remnant,
once it stops fusing, once it stops having
outward pressure, and once it has enough density,
this, we saw in the last video, will cause a supernova. It will cause a shock
wave to move out through the rest of the
material and essentially cause it to blow up. And this will condense
into a neutron star. Now, in this video what
I want to talk about is what if we're starting with
a star that has a mass more than-- and this
is give or take-- but we don't know the
actual firm boundaries here. But what if we have a star
that is more than 20 times the mass of the Sun? And this is kind of
the original mass before the star
burns itself out. Or when that star is kind
of reached this old age, once it has an iron core,
it has more than-- so I could say the remnant--
the dense remnant has more than three to four
times the mass of the Sun. And remember, it's going
to have three to four times the mass of the Sun. But it's going to be far denser. It's just going to be a core. It's going to be in iron-nickel
core that's no longer fusing. So what happens to these stars? So it turns out that these
are so massive that even the neutron degeneracy
pressure will not be enough to keep the
mass from imploding. And these stars, all of
the mass in these stars will just keep imploding. So we imagine, in the first,
in kind of Sun-like stars, things would collapse
into white dwarfs. So maybe I should
draw that in white. So they would collapse
into white dwarfs. Now, that's not white either. There you go. They would collapse into
a white dwarfs eventually. So this is a white dwarf. And here, the pressure
that's keeping this from collapsing further is
electron degeneracy pressure. The atoms are squeezed so
much that the electrons are essentially keeping them
from squeezing anymore. But if the pressure
gets large enough, then you have the neutron star. So you have even more
mass and even a smaller-- and I'm not drawing this to
scale-- neutron stars are tiny. White dwarf stars
are on the scale of an Earth-like like planet. Neutron stars, we learned
in the last video, are on the scale of a city. So these are
superdense, super tiny. And this has more mass
than this over here. In fact, maybe I should
just draw it as a dot just so you have a sense
of how dense it is. It's really just like
one big atomic nucleus or, well, it's still small. But it's size of a city. It's like a nucleus
the size of the city. But this right here
is a neutron star. And what's unintuitive
about what I'm drawing is each of these smaller
things have more mass. This overcame the electron
degeneracy pressure to collapse even further. But if the mass is
large enough, and this is what we're talking
about in this video, even the neutron
degeneracy pressure will not be able to keep
that mass from collapsing. And there's even
theoretical quark stars where the quark
degeneracy pressure-- but if you get even
beyond that, then it all collapses
into a single point-- and I'm simplifying
here-- but it collapses into a single
point of infinite density, infinite mass density. And this is really the
mass of a black hole. And I'm calling it the mass of
a black hole, because there's different ways how you could
view where a black hole starts and ends or what exactly
is the black hole. So this is all the
mass of the black hole or we could say of
the original star. So when we're talking about
that remnant being times three to four solar masses,
all of that mass is now being contained. Well, not all of it. Some of it is released as
energy during the supernova. And that was also true
of the neutron star. But most of that
mass is now being contained in this
infinitely small point. And you'll hear physicists
and mathematicians talk about singularities. And singularities
are really points, even in mathematics, where
everything breaks down, where nothing starts
to make sense anymore, where the mathematical
equations don't give you a defined answer. And this is a
singularity because you have a ton of mass in an
infinitely small space. You essentially have an
infinite density right here. And this is hard to visualize. But you have kind of an infinite
curvature in space/time right here. And I can't visualize that. So maybe we'll think
about that in more videos. But the reason why I said
that there's different ways to think about where
a black hole is, or where it starts and ends, is
that this is where the mass is. And if there was any other
mass that was over here, it would obviously be
attracted to this mass and then become part
of that singularity. It would add to that mass,
that already huge mass, that's in an infinitely
small point in space. But the reason why the
boundary is hard to define is because there's
some point in space around that singularity
at which no matter what that thing is, no matter
how much energy that thing has, it will not be able to escape
the gravitational influence of the black hole, of
that ultradense mass. So even if it was
electromagnetic radiation, even if it was light,
and even if it's a light that's shone
away from the mass, it will eventually
have to go back. It will not be able to escape
the gravitational influence. And so the boundary where if
you're within that boundary-- that's really a sphere-- so that
boundary around the singularity where if you're
within the boundary, no matter what you do,
no matter if you're electromagnetic
radiation, you're never going to be able
to escape the black hole. If you are beyond
that boundary, you might be able to
escape the black hole. So this guy could escape. This guy over here, no
matter what he does, is going to have to go
back into the black hole. This boundary right here is
called the event horizon. This right here is
the event horizon, another word used in a lot
of science fiction movies. And for good reason,
because it's fascinating. And we'll actually learn in
future videos-- hopefully, about Hawking radiation-- we'll
see that that is not radiation from the black hole itself. It's the byproduct
of quantum effects that are occurring
at the event horizon. But the event horizon, it's
this kind of point in space, or this sphere in space,
or this boundary in space. Anything closer or
within the event horizon has to eventually end up in
the singularity, contributing to that mass. Anything on the outside
has a chance of escaping. So what would a
black hole look like? Well, not even light
can escape from it. So it will be black. It will be black in
the purest sense. It will not emit any
type of radiation from the black hole
itself, from that mass. And so here are some depictions
I got from NASA of black holes. And so just to be clear
what's happening here. What you're seeing
here is black. You can view that
as the black hole. When people talk
about the black hole, that's often what
they're talking about. But there's a point
of infinite density at the center of this
black sphere right here. And what you see as
that black sphere, that really is the boundary
of the event horizon. So this right here is the
boundary of the event horizon. And what we're seeing right
here is the accretion disk around the black hole. As all of this matter gets
closer and closer to it, it's being squeezed
more and more. It's moving faster and faster
and getting hotter and hotter. And that's why the way
this artist depicted it, it looks like the
stuff over here is redder and hotter than
the stuff further out. It's just accelerating as it
approaches that event horizon. Once it's in the event horizon,
we cannot even see the light it is emitting, even though it
would be starting to become unbelievably energetic. Here's some other pictures. This is a picture
of a star being ripped apart-- not a picture. This is actually an
artist's depictions. All of these are
artist depictions. We never were able to
get such a good pictures of actual action occurring
near black holes. These are artist depictions. But this is a star being
ripped apart by a black hole. So this star is getting pretty
close to this black hole. Already out here,
where the star is, it's very strong
gravitational attraction. So any mass that's being emitted
from the star in that direction is slowly being pulled
into the black hole. So the star is kind of being
ripped apart by the black hole. This is maybe a better
depiction of it. This is the star at first. And once it becomes under the
influence of the black hole's gravitation, it starts
to kind of elongate and gets ripped apart. And its matter starts
spiraling in closer and closer to that black hole. And then once it's
in the event horizon, we won't even see it anymore. Because even the light
from that matter, that intensely hot matter that's
entering into the black hole, cannot even escape
the black hole itself. Anyway, hopefully you
found that interesting. And I want to be
clear, we still don't understand a lot
about black holes. In fact, this whole
notion of a singularity, the fact that all the
math and all the theory breaks down at the singularity,
is a pretty good sign that our theory isn't complete. Because if our
theory is complete, we would maybe get something a
little bit more sensical than just all of our equations
not making sense at that infinitely dense point. Anyway, hopefully you
found that interesting.