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Learning objectives

By the end of this section, you will be able to:

  • Explain the event horizon surrounding a black hole    
  • Discuss why the popular notion of black holes as great sucking monsters that can ingest material at great distances from them is erroneous
  • Use the concept of warped spacetime near a black hole to track what happens to any object that might fall into a black hole
  • Recognize why the concept of a singularity—with its infinite density and zero volume—presents major challenges to our understanding of matter

Let’s now apply what we have learned about gravity and spacetime curvature to the issue we started with: the collapsing core in a very massive star. We saw that if the core’s mass is greater than about 3 M Sun , theory says that nothing can stop the core from collapsing forever. We will examine this situation from two perspectives: first from a pre-Einstein point of view, and then with the aid of general relativity.

Classical collapse

Let’s begin with a thought experiment. We want to know what speeds are required to escape from the gravitational pull of different objects. A rocket must be launched from the surface of Earth at a very high speed if it is to escape the pull of Earth’s gravity. In fact, any object—rocket, ball, astronomy book—that is thrown into the air with a velocity less than 11 kilometers per second will soon fall back to Earth’s surface. Only those objects launched with a speed greater than this escape velocity can get away from Earth.

The escape velocity from the surface of the Sun is higher yet—618 kilometers per second. Now imagine that we begin to compress the Sun, forcing it to shrink in diameter. Recall that the pull of gravity depends on both the mass that is pulling you and your distance from the center of gravity of that mass. If the Sun is compressed, its mass will remain the same, but the distance between a point on the Sun’s surface and the center will get smaller and smaller. Thus, as we compress the star, the pull of gravity for an object on the shrinking surface will get stronger and stronger ( [link] ).

Formation of a black hole.

Formation of a Black Hole. At left in this illustration an astronaut stands atop a bluish sphere. At center, the astronaut stands atop a smaller white sphere which is surrounded by arrows pointing inward toward the center of the white sphere. Finally, at right, a very thin and elongated astronaut hovers just above a small black dot. The text below the black dot reads: “No escape”.
At left, an imaginary astronaut floats near the surface of a massive star-core about to collapse. As the same mass falls into a smaller sphere, the gravity at its surface goes up, making it harder for anything to escape from the stellar surface. Eventually the mass collapses into so small a sphere that the escape velocity exceeds the speed of light and nothing can get away. Note that the size of the astronaut has been exaggerated. In the last picture, the astronaut is just outside the sphere we will call the event horizon and is stretched and squeezed by the strong gravity.

When the shrinking Sun reaches the diameter of a neutron star (about 20 kilometers), the velocity required to escape its gravitational pull will be about half the speed of light. Suppose we continue to compress the Sun to a smaller and smaller diameter. (We saw this can’t happen to a star like our Sun in the real world because of electron degeneracy, i.e., the mutual repulsion between tightly packed electrons; this is just a quick “thought experiment” to get our bearings).

Questions & Answers

what is the correct arrangement of the horoscope
edwin Reply
what causes voids in space like bootes void?
NARUTO Reply
I think that it has somehow managed to get a lot of dark energy which makes star formation hard
Tanmay
Or this area might have a lot of gas clouds which obstruct our view of its star. that means it has normal amount of galaxies but we can't see them
Tanmay
but why scientists are saying that it bounces galaxues away while containing some inside it 🤔🤔🤔
NARUTO
I never heard anyone say that
Tanmay
well a YouTube channel called the seeker just said that scientist doing observations on this phenomenon is causing galaxies to bounce of
NARUTO
hi
Bas_
we live in a place which only has 3 dimensions and our imaginations are only limited till there neither is the space infinity nor are our imaginations!
Samuel Reply
hi guys can we all discuss about a thought which I have?
Samuel
yes why not tell us
Tanmay
I do not know
Clint
it's up to u to do your homework
Clint
we live in many dimensions, x,y,z, time, gravity, light, ...... an n num of Dem,
Clint
Is this the question? we live in a place which only has 3 dimensions and our imaginations are only limited till there neither is the space infinity nor are our imaginations!
Tanmay
ppl thought we lived on a flat earth, 2 Dem, them 3, then 4 Einstein, ...... the list goes on
Clint
ur turn to work
Clint
work
Clint
young buzzards chew at the various of the old, find new food
Clint
work, there is no free ride in learning
Clint
read everything, process it later, keep reading
Clint
one day, you will make a contribution to science, but u have to read, u have to take ur bumps
Clint
I think we should read and understand what it wants to tell us what you learnt where can it be used and than go forward
Tanmay
Infinity is beyond our imaginations
Samuel Reply
maybe, maybe not.... it depends on the limit of ur thinking
Clint
have u stop thinking?
Clint
no?
Clint
then u do not know you own limits of ur imagination....
Clint
I think our imagination is beyond of infinity...
Suman
u may choose to stop imagining, but it's u that chooses
Clint
the depth of the mind has not been determined
Clint
if someone knows his imagination is limited then how does his imagination will exist, the imagination that is limited then those are just thoughts but unlimited imagination is creation of something new...
Suman
A person trapped in a well for a lifetime can never imagine the vastness of the sea so our imagination very well depends upon your physical limits
Tanmay
how would one describe the 3 dimensions, i mean what defines their boundaries . . . ?
Chalo
3 dimensions mean 3 mutually perpendicular coordinate in space
Gurjinder
how mass came to our galaxy when it was jast a point.
Nashita Reply
From energy
suvas
mass is energy, engery is mass, a very small amount of mass equals a huge amount of energy.... books tell that God is light, and if light is there, there is mass. Where it came from, why are we here, those are meta-physicial questions or religious ones
Clint
light bends toward planets, it's equilance can be computed to equal mass and vic-versi
Clint
a point? define a point?
Clint
a point has infinite points between it and any object
Clint
ur question isn't wrong, but mass, like light are fundamentals.... that means they are just there, we question them yes, dissect them yes, but why are they there? Answer that question and ur the smartest person on this rock
Clint
show me where we can create light without mass or mass without light
Clint
ur 8th grade right?..... keep thinking,... u have a future and will go far
Clint
infinite means where we humans are unable to count further, I think energy is expanding itself...
Suman
who are you Sir?
Suman
I can't argue with u there.... as long as light exists, matter will increase
Clint
good point
Clint
I think there is something opposite of light too... if there would be ...then anti- matter will also exist
Suman
actually anti photon and photon are the same thing. photons are there own anti particles
Tanmay
We theorised that all matter in our universe came from a single point which is called big bang but we don't know what exactly happened at t=0 sec
Tanmay
a point? big bang? hyper condensed matter, but even if true it was not a point, it had light, space, dimension....
Clint
that's just a theory.
Suman
what caused that infinitesimal point to get so concentrated and expand outwards , what made that happen 🤔 😳
NARUTO
ah, but u speak of God.... with God all things are possible
Clint
the matter we see in our universe came from a giant dust cloud which made most of the stars and protostar of current Sagittarius A* black hole
Tanmay
so ur point is what? .... u believe in God?
Clint
I don't believe in god
Tanmay
I would like to have a proof that God exists
Tanmay
theory of a pulsating universe is a theory, theory of big bang is a theory.,... it's up to each and every individual student to start/believe somewhere/something
Clint
Naruto we exactly don't know what happened at t=0
Tanmay
we need to believe in these theories and challenge then test them to their limits and prove enhance or disprove it
Tanmay
but a student must believe in his imagination first then others'. And try to prove his imagination. This is what we call progress.
Suman
all we can do is theorize we will never know
NARUTO
this is what happened to Newtonian mechanics we believed it tested it it's limits and then enhanced it
Tanmay
physics uses existing phenom to at best guess what in the heck it is, and what it can do... we name things with a limited vocabulary and put numbers where they fit in equations we determine to match as precisely as possible
Clint
in 10th century we never new we can ever see and work at atomic level but now we have hopes the will happen with these theories
Tanmay
those are called physical constants.... and until someone describes a vocab, and rules that's in Greek (forgive me Greece) that we try to work within..... but I'm all open to another language if u can at least match the one that we have
Clint
You should imagine a theory where other theory fails other wise it is of no good use your theory would not get recognition and would not help science to progress
Tanmay
Einstein did not develope a new language, he added a dialect
Clint
the roots of science is always asking questions and trying to solve it
NARUTO
then he showed that quantum can be reduced to classical to prove he was indeed talking the same language.... physics
Clint
This is turning to a philosophy slightly going out of topic
Murrino
points are hard to work with they are infinitesimal sphere or 1 dimensional dot? They can't have any rotation that's why string theory was born to work at that small point. These strings are easy to study and do research
Tanmay
Yes I was also thinking about it so let's converge all of the discussion
Tanmay
if string theory is true then that proves warm holes true and time traveling possible right?
NARUTO
that's right Naruto, we question first,.... but we don't always solve it... I bet if u had access to a crate, and toys u could kick
Clint
worm*
NARUTO
As per big bang theory all the matter came from a point at t=0 . Everything including time and space emerged from it. But we can't prove this theory with current technology and we also don't know what exactly happened at t=0
Tanmay
As per big bounce theory tells that universe was always there and time and again the universe goes into big bounce and the previous universe converges into a point and new universe emerge from it.
Tanmay
Can someone tell me what multi verse theory states?
Tanmay
doesn't it states that there's other universes exist and parallel universes to ours
NARUTO
that's right tam, but first work with what's there, and before u go off half cock, look at terminology, search 5x more.... when u know what they know, then u kick there butt and tell them what they are missing..... but be humble
Clint
other life forms.... maybe
Clint
we are not the only back water plant
Clint
say what do yall think cause those huge dark voids in the universe?
NARUTO
but I will tell each of u something, what grows here can grow there, what is here maybe there, but that single point (who or what ever it was) is a part of that
Clint
what do you mean by part of "that"?
Tanmay
Naruto do you mean the great void which lack a lot of stars than the other areas?
Tanmay
something's like that maybe, the ways to know them
Dennis Reply
oh ok you can actually reply to your own comment by tapping it once
Tanmay
you should do this
Tanmay
what are laws of the universe
Dennis Reply
The law of attraction
Lambent
Are you talking about fundamental forces Dennis?
Tanmay
you can go to google and type 4 fundamental forces of the universe
Tanmay
let me give you gist of it there are 4 fundamental forces namely gravitational, electromagnetic, strong nuclear and weak nuclear
Tanmay
the came into being just after the bid bang
Tanmay
I mean big bang
Tanmay
you should watch this video for better understanding ***youtu.be/X9otDixAtFw
Tanmay
wow some more than information
Rajeshwari Reply
since space gets bigger dose it affect distance between the sun and earth
Swazi Reply
yes the distance between earth and sun increase at a rate of 15 cm per year
Tanmay
ohh really I didn't knew that , you learn something new everyday
NARUTO
true
Tanmay
thanks so much my friend
Youssef
Dear tanmay and other friends I want to ask that if both sun and earth are moving away from their center than why their inter relative distance is increasing rather squeezing taking in consideration of masses I am student so I apologize further and wanna guidance if possible
haseeb
The Earth does not get close to or “touch” the Sun because it is in orbit around the Sun. A circular orbit means that the orbiting body always stays the same distance from the thing it orbits. The Earth's orbit is nearly circular and keeps us about 93 million miles from it.
Konica
Konica nor the orbit can prevent our earth from getting closer (if it was coming closer) and nor is the orbit of earth nearly circular
Tanmay
And Haseeb can you make your question a little simple or something I didn't get it. What do you mean by moving away from their center?
Tanmay
And then comes your increasing rather than squeezing taking mass into consideration I never heard of that and I didn't understand that
Tanmay
Which formula are you using for that squeezing part?
Tanmay
What's difference between Astronomy and Astrophysics?
Gurjinder Reply
Actually astrophysics is one of the many branches of astronomy. It usually deals with the mathematical calculations of predicting the motions of heavenly bodies and also it studies the evolution of our universe by using physics.
Debabrata
So is there any observational astrophysics?
Gurjinder
Yes there are 2 types of astrophysicsist one is theoretical and other one is observational. I don't know about astrophysics though
Tanmay
Thanks
Gurjinder
welcome
Tanmay
😊
Gurjinder
Is there a way for us to find out that we are in a simulation or not?
Tanmay Reply
I think mathematics is a key
Arghyabindu
If we are in a simulation then I don't think that the super intelligent species which is controlling this simulation will give us any hint about how to know it.
Debabrata
There has to be some kind of bugs even a carefully designed app has some bugs in our world
Tanmay
Yeah.... There might be some loophole in their programme code.... And who knows.... We could find the bugs in a super massive black hole
Debabrata
But here we are talking about a very very very massive simulation by a very advanced species...... So they might ve already planned on how to deal with all the bugs and glitches
Debabrata
There's a really interesting theory that states even if we are in a simulation, it would be coded in such a way that as soon as one of us (a sentient being inside the simulation) had the slightest hint/evidence that we were in a simulation, the simulation would be edited to remove that information
ben
(I keep commenting on posts and they just dont post sometimes, does anyone else have this problem or should I just delete the app?)
ben
same happened to me but I see yours))
NARUTO
(ah well it must just be a little problem with the app, cheers for letting me know 🙌)
ben
(Well never happened to me though it takes some time to post large messages)
Tanmay
I saw a comment where he told that the universe is a game and finding it's nature is the level as soon as we find its nature the universe collapse and make another universe with more complex nature and this continues
Tanmay
(ahh that may be what it is actually because it is only the longer messages, thanks for letting us know)
ben
That was really a unique way of seeing the universe
Tanmay
Impractical way are there such as leaving your mortal body and entering to the astral body....cons of this idea is you can't tell us what you have saw or feel in that endemic confrontation
Arghyabindu
what a friend told me now
Arghyabindu
We are just a component of the universe trying to understand the other components of the universe...... So I guess that sometimes it makes the matter more and more complicated
Debabrata
what a friend told me now
Arghyabindu
Wait if we are in a simulation then does that means our future is predicted and we can't change it like some kind of rpg game
Tanmay
Yes IF we are in a simulation then definitely the future is already programmed..... But it may be in early access stage..... And the future might not be written at the moment.
Debabrata
so we can do time travel upto the point where our future is written
Tanmay
Maybe or may not be..... Depending on the script or program written in the simulation..... The intelligent species should be controlling every step we take and every thought that come into our mind.
Debabrata
Have you seen sword art online anime?
Tanmay
well never mind it's just repeated stuff
Tanmay
Nah.... I don't watch anime
Debabrata
ok
Tanmay
hii guyz
Lambent
Hey
Debabrata
BLM ✊🏾✊🏾✊🏾
Laila
?
Kilten
thanks
Sahil
If we can simulate consciousness somehow, then there is a bright chance that we 'might' be living in a simulation all along.
Ganesh
which is the biggest star in the universe? if anyone know answer,please reply.
Ciffee Reply
which is the biggest star. ?
Ciffee
u y scuti
Sunidhi
sun
Konica
for our galaxy
Konica
Konica UY Scuti is the biggest Star ever observed and it's located in our galaxy.
Gurjinder
what are the branches of astronomy?
SCIENCE Reply
So there are literary many branches of astronomy like- Radio astronomy, Ultraviolet astronomy, X ray astronomy, etc. (just imagine the electromagnetic spectrum) There's also Astrophysics (My favorite), astro chemistry, astrobiology, astro geology, planetary science, astrometry
Debabrata
factors that contribute to the development of ancient astronomy to modern astronomy
Andrea Reply
When will be we found them?I am so waiting.
Everything Changes when Galileo Galilee observed Sky with a telescope. There's no doubt that ancient astronomers knows a lot of things about sky , but there observation was with naked eye. i wanna share some of the glimpse of Indian ancient astronomy:
Gurjinder
1.They know the difference between the Star and Planets at that time. 2. They know which star is multiple star system and their rotation with respect to each other. 3. They know the value of speed of light. And their are lots of things which they knows even without telescope.
Gurjinder
why is a supernova
pooja Reply
well there are different types of supernovas. there is a type 1a supernova and that happens when a white dwarf reaches the mass of 1.4 of the sun. A type 2 supernova is when a high mass star starts to fuse iron in the core and the star collapse then expands to create that supernova
PopularHellboy
Practice Key Terms 3

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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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