I pretty much understand how stars of a certain size/mass become a blackhole ..BUT i'm puzzled.
Surely the light from the star as it colapses , will , to an observer( at some distance ) ,remain put , at least for millions of years. As, at the event horizon time comes to practical standstill. I'm thinking of those last remaining photons that just have enough legs to get away. I'm even thinking maybe they don't ever get away , so a black hole in effect should look like a small dim star?
your thoughts please
thx

See:

Usenet Physics FAQ (scroll down to the section on black holes)
Ted Bunn's Black Holes FAQ

__________________
"All your bias are belong to us." Ara Pacis
"A witty saying proves nothing." Voltaire

It would take only a few milliseconds before even light from an intense
gamma-ray source, falling through the event horizon, was so redshifted
that it could no longer be picked up by a radio antenna a mile long.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"The other planets?
Well, they just happen to be there, but the point of rockets is to explore them!"
-- Kai Yeves

It would not be the first time I was completely wrong about some thing... But for me a star collapsing into a Black Hole is just a moment of gravity exceeding light velocity. What we see changes as the light emission stops. However very little has actually changed. Its still a Star. And the orbiting material is what we see. Any matter outside the event horizon is still emitting light and thus visible.
Mark.

Very dim, very red-shifted--in fact, so dim that less than one photon of energy gets out at the end--and in Quantum mechanics, you can't have less than a photon, so it's zero.

This is one of these places where quantum mechanics and relativity start to scuffle.

__________________
-----
Todd (Bowie, MD, US, North America, Earth, Sol System, Vega region, Local Bubble, Orion arm, Milky Way Galaxy, Local Group, Virgo A Cluster, Virgo supercluster, the universe in which spock is clean shaven)

Quidquid latine dictum sit, altum sonatur.

personal page: http://blog.astrosketches.info

Not really. The original poster's notion that the light would continue
on for a very long time is the result of applying a simple mathematical
idea without using any actual numbers. Quantum mechanics and
general relativity don't come into conflict until much closer to the
singularity, at distances near the Planck length.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"The other planets?
Well, they just happen to be there, but the point of rockets is to explore them!"
-- Kai Yeves

I think it changes waaaaaay too much to call it a star anymore.
Pretty much the only thing that remains the same is the gravity
field at a distance. Even that can change greatly as the star
collapses, going supernova, throwing off most of its mass.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"The other planets?
Well, they just happen to be there, but the point of rockets is to explore them!"
-- Kai Yeves

this kind of answers my question?

so what happened to the frozen stars theory ?
In fact, more or less the same thing can be said about the material that formed the black hole in the first place. Suppose that the black hole formed from a collapsing star. As the material that is to form the black hole collapses, Penelope sees it get smaller and smaller, approaching but never quite reaching its Schwarzschild radius. This is why black holes were originally called frozen stars: because they seem to 'freeze' at a size just slightly bigger than the Schwarzschild radius.

That is what we see from a distance. That is not the final answer.

Try the same collapsing BH scenario with an observer that free falls into the BH.

None of the pictures painted in the previous scenarios are entirely correct or complete. Here's what I believe an observer at a safe distance would see:

As the star approached the event horizon, matter would begin to flow in an initially narrow stream from the star toward the black hole because of tidal forces. Distant observers would see a thin stream of very hot gas begin to spiral from the surface of the star toward the event horizon of the black hole, gradually building up an accretion disk just above the event horizon. He would see gas flowing from the accretion disk turning redder and dimmer and moving ever more slowly as it came nearer to the event horizon, disappearing altogether just before reaching the event horizon.

Here is what I believe someone falling into the same black hole would experience as he fell directly into the black hole from a direction that would not take him close to the star but well before the star came into the vicinity of the black hole:

He would see nothing when looking directly at the center of the black hole. The position of the black hole would be marked by a circular region containing a total absence of stars or any other light-emitting objects. Stars would be packed with increasing density just outside the disk of total darkness, the density decreasing with distance from the edge of the disk of total darkness. As he came closer to the black hole, the size of the totally black disk would appear to increase and fill his field of view. He'd see nothing unusual outside the black disk except that the stars outside the disk appeared brighter and brighter. In contrast to what observers outside saw time slowing for objects approaching the black hole and dimming, he would see time speeding up for objects more remote from the black hole. Aside from the apparent acceleration of time for him, he would notice nothing unusual if his own dimensions were small compared with the diameter of the black hole. In fact, he'd notice nothing different at all when he passed through the event horizon. On the other hand, if the diameter of the black hole was comparable with his own height, he would experience forces tending to compress him into a much smaller size.

But the question seems to relate to a star collapsing to become a black hole, not a star approaching a black hole.

I'm not sure what you mean by "comparable", here.
A solar-mass black hole, with a Schwarzschild radius a thousand times the height of an adult human, applies tidal forces on the order of a billion g along the length of a free-falling adult human crossing the event horizon. A 100,000 solar-mass black hole, with a Schwarzschild radius 100 million times the height of an adult human, produces manageable tidal forces on the order of a tenth of g under the same circumstances.

I'm not sure if the word "comparable" is appropriate when multiplication constants greater than a million are involved.

Grant Hutchison

grant hutchison: But the question seems to relate to a star collapsing to become a black hole, not a star approaching a black hole.

dcl: Thank you, grant hutchison, for waking me up. You are right. Sorry, I led myself astray from the question at hand. Following, hopefully, is the answer to the question that was asked:

The scenario most frequently cited as leading to production of a black hole is collapse of the core in a star with mass beyond the Chandrasekhar limit when it exhausts its supply of hydrogen. As the pressure from the core falls below the amount necessary to support the weight of the overlayers, the latter begin to fall into the core, and the process cascades until the core density becomes so great that the core can no longer support that weight and collapses into free fall along with the rest of the star. As the density increases, an event horizon suddenly comes into existence at a point at the very center of the core and rapidly expands. Matter overtaken by the event horizon falls into it and the added mass inside the expansion of the event horizon to accelerate, taking in more and more mass. The process is explosive, sweeping over the rest of the star in an instant. In the meantime, a shock wave from the initial explosion sweeps outward through the rest of the star, resulting in what rest of the world sees as a nova or supernova explosion, depending on the original mass of the star. The conversion from a star to a black hole is thus not instantaneous but is nevertheless extremely fast since it takes only a very brief instant for the shock wave to reach the surface of the star. The star does not disappear at the instant that the event horizon comes into existence. In fact, the star does not disappear even at that instant, for the shock wave throws extremely hot gases into the space surrounding the star, and it is only when those gases cool sufficiently that what's left of the star disappears. No radiation escapes when mass falls through the event horizon, but the escaping gases continue to radiate heat radiation after they cease to be visible to the human eye and to astronomical instruments.

For stars of large mass, genre 25+ solar masses, I would agree with you. For a smaller star, maybe 5 solar masses, the event horizon will evolve outward, growing as the star collapses or implodes. The EH will evolve outward for all events, but more dramatically for supernovas. Eventually, it becomes larger than the radiating surface and we have the birth of a true BH.

alainprice: I agree with you.

But you cannot have natural star collapse BH below cca 20-25 solar masses for the original star.

Regarding the posts so far, aren't the Schwarzschild solutions non-rotating? Haven't all the posts used the Schwarzschild solutions?

No, one real object is standing perfectly still. Everything else is rotating relative to it.
I'm not telling you which one it is, though.

__________________
[Foot mouth in put]
Si tacuisses, philosophus mansisses.

Schwarzchild = non-rotating and non-charged

Kerr is rotating but can't remember if it accepts charge.

I was misreading ' K e r r ' as ' K e n ' , which is pretty funny, see post title. Now I have to go look up Kerr black holes.

Thanks, John M.

Edit: Ok, from the Wiki article on 'Black Hole', "In 1963 Roy Kerr extended Finkelstein's analysis by presenting the Kerr metric (coordinates) and showing how this made it possible to predict the properties of rotating black holes. In addition to its theoretical interest, Kerr's work made black holes more believable for astronomers, since black holes are formed from stars and all known stars rotate."

Wiki link: http://en.wikipedia.org/wiki/Kerr_bl...ral_relativity

The article is good reading, with the usual Wiki reservations (be skeptical).

It's actually funny to read some of the work done on black holes (e.g. the accounts in Kip Thorne's "Black Holes and Time Warps: Einstein's Outrageous Legacy)--doing exact computations for the general case or even one specific realistic case is just too hard. So, physicists compute a lot of non-realistic cases--e.g. nonrotating, rotating with neutral charge, etc. to get a picture of what is going on--kind of like studying a forest by plucking a random leaf here and there.

I recall some of the more funny images presented by some studies--e.g. of course one computed what happens when a perfectly spherical star collapses into a black hole. But what about real stars with flattening due to rotation, prominences, stars distorted by nearby stars, etc. They are hard cases--so, instead I believe it was Kip himself who computed computed--this is the funny one--the result of a cubical star collapsing into a black hole. (the end result was the black hole would still be spherical, or an oblate spheroid due to rotation). Other computations included, star with a "mountain" on it, etc. Since they all gave the same end result, a plain-old spherical/oblate spheroidal black hole, the conclusion is that probably any real star with less-funny shapes does the same.

Now, I wondered--Robert Foreward, in an appendix to "Dragon's Egg", claimed a torroidal black hole would, among other things, rotate the 4-dimensional spacetime so that time is one of the spatial dimensions, and some other spatial dimension is now time, so you could travel through time by accelerating your spacecraft through it. Now--in light of the above, I figure a torroidal black hole is unstable and collapses very quickly to a sphere--but how would one arise? A super race maybe taking a very long string of black holes, moving them into a huge Kemplerer rosette (would this be "torroidal enough"?) and reducing the radius till they merge.

__________________
-----
Todd (Bowie, MD, US, North America, Earth, Sol System, Vega region, Local Bubble, Orion arm, Milky Way Galaxy, Local Group, Virgo A Cluster, Virgo supercluster, the universe in which spock is clean shaven)

Quidquid latine dictum sit, altum sonatur.

personal page: http://blog.astrosketches.info