# Thread: Why does a star become a black hole???

1. Join Date
Sep 2010
Posts
2

## Why does a star become a black hole???

Ok so I know that stars become black holes (well some of them) and HOW they become black holes but what I don't understand is how the star is a black hole when it get crunched down to a tiny fraction of it's size but isn't a black hole when it's full size.

I guess what I'm really asking is what changes in a stars mass/gravitational pull from when it's a big star to the point where it's matter is crunched down into a much smaller object?

I hope the question makes sense....

2. Order of Kilopi
Join Date
Jan 2010
Posts
5,065
It doesn't have to do with the absolute amount of mass, black holes can go from microscopic to supermassive.
The relevant parameter is the density, the mass per unit of volume.

Think in terms of escape velocity. The escape velocity on the surface of the earth is somewhere around 10-15km/s. Suppose we shrink the earth to half its current size, the escape velocity will go up (with rē), we're getting closer to the center of gravity. Now suppose we keep shrinking the earth, and the escape velocity keeps increasing. Then at some moment we'll have the escape velocity equal the speed of light, and you get a black hole.

3. Welcom to Baut, DeepSpaceThinker! It's a great question.

Balck holes happen when a lot of mass is crammed into a tiny space.

Think about a star at the end of its life cycle. It has burned up almost all of its usable fuel, but is still very massive.The mass of the star is held up by the extreme temperature and pressure in its core. When the star runs out of fuel, and the pressure pushing out is no longer more powerful than the gravity pulling in, gravity wins out and the star begins to collapse.

Now, think about how gravity works: gravitational attraction falls off like 1/r^2, so the farther away an object is, the easier it is to move away from the star. So accordingly, the closer it is, the harder it is to move away, or the faster must a particle move to escape the star's pull. As the star collapses further and further, it is not losing any mass, so the 'escape velocity' at the surface, the velocity a particle must have to escape the star's gravity without further acceleration away from the star, increases. When the star reaches a certain density, and enough mass is packed into a small enough radius, the escape velocity at the surface becomes faster than light can travel. When this happens, any object, even light, that passes through this radius can not escape the gravity of the star, and we call it a 'black hole'.

Hope this helps.

4. Established Member
Join Date
Sep 2010
Posts
127
Wow, that was a very simple and easy to understand explanation. Great question OP, and great response Rooby!

5. Newbie
Join Date
Aug 2010
Posts
3
I had the same question and i decided to search for previous posts about it. Great answer in any case, thanks

6. Established Member
Join Date
Jul 2006
Posts
221

## Blackholes, Expansion, and Photons

Originally Posted by Roobydo
...When the star reaches a certain density, and enough mass is packed into a small enough radius, the escape velocity at the surface becomes faster than light can travel. When this happens, any object, even light, that passes through this radius can not escape the gravity of the star, and we call it a 'black hole'. ...
I was just reading about escape velocity and it seems like the light could still go to a nearly infinite distance before it succumbed to the pull of gravity. The light can never escape but it also does not just stop inside that radius.

This leads me to another thought that since light is just energy and if the gravitational field redshifts it as it moves away and since redshifting is a loss of energy then at an infinite distance the energy of the photon will have been completely absorbed by the blackhole.

I used to think that the light would just get turned around (sort of reflected) back towards the blackhole.

If all my above musings are true then wouldn't the spacetime of the blackhole grow just like during the inflationary period of the big bang? Because the inflation/expansion of the universe is driven by the energy of photons that are being redshifted - I think I read that somewhere.

7. Banned
Join Date
Oct 2005
Posts
27,229
Originally Posted by xylophobe
Because the inflation/expansion of the universe is driven by the energy of photons that are being redshifted - I think I read that somewhere.
The current expansion isn't being driving by that-- the current dynamics of the expansion (and has been true for a very very long time) are almost entirely ruled by rest mass and "dark energy", neither of which has anything to do with light. The cosmological redshift of the background radiation is more what would be termed a "diagnostic"-- a way for us to learn about the history of the universe, not something that is affecting it in any kind of important way. However, the inflationary epoch, very very early in many Big Bang-type models, would have been an epoch when most of the energy in the universe was in the form of light, so the gravity would have been ruled by light also. "Inflation" is thought to be a period when gravity changed sign and became repulsive-- so all that light energy would indeed have been responsible for the inflation. I don't know if its energy can be associated with "expansion energy" though-- some play that kind of trick and balance the rising gravitational potential with falling energy density, I just don't know how rigorous that is.

8. Established Member
Join Date
Jul 2006
Posts
221

## Blackhole Conversion

Originally Posted by Ken G
... I don't know if its energy can be associated with "expansion energy" though-- some play that kind of trick and balance the rising gravitational potential with falling energy density, I just don't know how rigorous that is.
It seems like if a blackhole can redshift light into oblivion then the blackhole converts outward momentum/energy into inward momentum/energy because the opposite is true for the photon that is directed towards the blackhole - it gets blueshifted. So the blackhole steals energy from any outward-bound photon and adds the energy to the inward-bound photon.

I don't know if it would be the "momentum" of the photon, or what, but the overall energy of outward photons gets transfered to inward photons - using "momentum" in this view implies that the balance of momentum for the blackhole does not equal zero. Whether this makes sense or not I do not know but I do know that momentum must be preserved for collisions in supercolliders.

9. Banned
Join Date
Oct 2005
Posts
27,229
Originally Posted by xylophobe
So the blackhole steals energy from any outward-bound photon and adds the energy to the inward-bound photon.
General relativity is quite a bit tricky in regard to energy, because energy is not an "invariant" quantity-- it is not a quantity that is the same for all observers (only rest energy is, and a photon has no rest energy). With quantities, like photon energy, that are not invariant, the story you tell about "what is happening to the energy" can be quite different from different reference frames (but all the stories must agree that there is an observed redshift). One story, for example, is that when a redshifted photon is observed, the redshift was always there-- it was redshifted "because" time itself is going slower near the black hole, so the photon was emitted red. Another story is that it redshifted during transit, by the effect you are describing. Or, an even more complicated, yet useful, story is that it was redshifted by a combination of time going extremely slowly at the point of emission, coupled with a net blueshift between the outward motions of the source and the observer, both zooming outward through a "space itself that is falling" into the black hole, where the further out observer is moving more slowly outward than is the source (hence a net blueshift added to the extreme redshift due to the gravitational time dilation at the source). Any of these stories is simply connected to the coordinate system being used, i.e., the language being used.
I don't know if it would be the "momentum" of the photon, or what, but the overall energy of outward photons gets transfered to inward photons - using "momentum" in this view implies that the balance of momentum for the blackhole does not equal zero. Whether this makes sense or not I do not know but I do know that momentum must be preserved for collisions in supercolliders.
True, but supercolliders aren't using general relativity-- there aren't strong gravitational fields. Even if you classically conserve energy in a gravitational field, it's not a transfer from outgoing to infalling matter, it's a transfer from the kinetic energy of the matter to the "gravitational potential energy," which in effect goes right into the rest mass of the outward particle-- so the outward particle keeps its own energy. That's what doesn't happen for photons, since they have no rest energy-- and there it is sometimes said that energy simply isn't conserved because you need to query different observers over a range of reference frames to add up all the energies, and energy just doesn't work that way.

10. Order of Kilopi
Join Date
Jan 2010
Posts
5,065
Originally Posted by xylophobe
I was just reading about escape velocity and it seems like the light could still go to a nearly infinite distance before it succumbed to the pull of gravity. The light can never escape but it also does not just stop inside that radius.
Actually light will "stop" (or better, never reach beyond) that radius. The reason for that is that light has a constant speed c relative to its 'patch' of spacetime.
An easy way to consider is this, think of having a photon right outside the EH moving 'outwards'. It will redshift along the way, but it will get away. So even if a photon came from beyond the EH, and just a little bit outside it will get away eventually. Light doesn't "slow down" and then fall back in, it either gets away completely or it doesn't get away even the tiniest bit, and the boundary between those two is a clear EH.

11. Originally Posted by DeepSpaceThinker
Ok so I know that stars become black holes (well some of them) and HOW they become black holes but what I don't understand is how the star is a black hole when it get crunched down to a tiny fraction of it's size but isn't a black hole when it's full size.

I guess what I'm really asking is what changes in a stars mass/gravitational pull from when it's a big star to the point where it's matter is crunched down into a much smaller object?

I hope the question makes sense....
What changes? It's size. The same reason a 150kg weight lifter is different from a 150kg couch potato. They both have the same mass but their size is very different. Thus the 150kg weight lifter can do many things the 150kg couch potato can not.

Now the real underlying question is probably why a star stays big when the black hole does not. This is because of the pressure from the release of energy from the nuclear process. This causes an outward pressure much like if you blew into a plastic bag with holes in it. Stop blowing and the plastic bag will collapse but blow into it and it will inflate (get bigger). When a star runs out of fuel their isn't that outward pressure to keep the star big and gravity pulls everything towards the centre, it actually always did this but the outward pressure balanced this effect. As the star's material collapses under its own gravity it heats up. If it is big enough then it might get hot enough, as things are compressed they get hotter, to start burning heavier elements for fuel. Eventually this will run out for even the most massive stars when the material left is lead. Lead produce an excess of energy so has no net outward pressure. Thus such a star might, if it has enough mass, will collapse under its own weight.

I'm sure someone has already given a much better description but there you go ... that is my layman explanation.

12. Originally Posted by xylophobe
I was just reading about escape velocity and it seems like the light could still go to a nearly infinite distance before it succumbed to the pull of gravity. The light can never escape but it also does not just stop inside that radius.

This leads me to another thought that since light is just energy and if the gravitational field redshifts it as it moves away and since redshifting is a loss of energy then at an infinite distance the energy of the photon will have been completely absorbed by the blackhole.

I used to think that the light would just get turned around (sort of reflected) back towards the blackhole.

If all my above musings are true then wouldn't the spacetime of the blackhole grow just like during the inflationary period of the big bang? Because the inflation/expansion of the universe is driven by the energy of photons that are being redshifted - I think I read that somewhere.
In physics, especially BH physics, it's crucial to remember (and expose) from what viewpoint you are describing a situation.

To an external observer light traveling radially (directly from the singularity) of the BH will not slow then turn, but will be 'red-shifted'. Suffice it to say that the photon will not be slower, but 'weaker'. It is helpful to remember that the change in the photon to an outside observer is not due to gravitation pull on the photon, but that it is traveling from a location where time is passing slowly to a location where time is passing more quickly. Thus, the photon is not moving slower through space, but it is oscillating slower through time. However, the trajectory of a photon traveling on a non-radial path will be bent- it is possible that the photon could even orbit the black hole inf it's path is tangential to the horizon.

13. Established Member
Join Date
Jul 2006
Posts
221

## The Photon Source

Originally Posted by Roobydo
In physics, especially BH physics, it's crucial to remember (and expose) from what viewpoint you are describing a situation.

To an external observer light traveling radially (directly from the singularity) of the BH will not slow then turn, but will be 'red-shifted'. Suffice it to say that the photon will not be slower, but 'weaker'. It is helpful to remember that the change in the photon to an outside observer is not due to gravitation pull on the photon, but that it is traveling from a location where time is passing slowly to a location where time is passing more quickly. Thus, the photon is not moving slower through space, but it is oscillating slower through time. However, the trajectory of a photon traveling on a non-radial path will be bent- it is possible that the photon could even orbit the black hole inf it's path is tangential to the horizon.
After thinking about it for a while and considering your statements the "stealing" of energy from the photon only depends upon if the photon is generated within or near to the blackhole. If the photon was from an external source then it would get blueshifted on the way in and then redshifted on the way out so there would be no net energy gain by the blackhole.

14. Order of Kilopi
Join Date
Jan 2010
Posts
5,065
Originally Posted by Roobydo
it is possible that the photon could even orbit the black hole inf it's path is tangential to the horizon.
Indeed, see photon sphere.
Note that this happens quite a bit above the EH.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•