Why does light escape stars?

  • Thread starter Shirley
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First, let me thank you in advance for your patience. I understand there is a lot of foundation I need to build before I can truly learn much of what I need to know, and I thank you for your assistance.

We know that gravity is strongest closest to large objects. Gravity seems to effect light. (My question may be predicated on a misunderstanding of the nature of light?) So, if light is effected by gravity, and gravity is strongest closest to very large objects (such as stars), why does light escape?

Thanks,

John
 

Hurkyl

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Well, first I'll explain the Newtonian picture.

In Newtonian gravitation, when considering the gravitation of spherically symmetric objects (stars are close enough), there are two regions to consdier:

Inside the star, there is zero gravity near the middle (you're being pulled every way equally). There are still high pressures, though. If you're outside the star, then the force of gravity dies off inversely proportionally to the square of your distance from the center.

Stars are big objects, so if you're on the surface of a star, you're still quite far from its center!

Basically, an object either has to be fantastically huge or fantastically dense in order to trap light and become a black hole.


Of course, Newtonian gravitation isn't adequate to talk about the interaction between gravity and light, but it's close enough to give an idea of what's happening.
 
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Thanks for the reply.

I do understand that Newtonian theory believes light to be particles, and under-predicts the pull of gravity on light.

So, according to current knowledge, a black hole is the only entity with gravitational attraction strong enough to capture light? Are "black holes" just another way of saying there's something that we cannot "eyeball" because no light escapes for us to see? Or has it been "proved" that black holes are collapsed giant stars?

John
 

Hurkyl

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Well, it wasn't Newton's theory that held light to be particles.

Anyways, yes, a black hole is simply something that traps light, it doesn't have to be a star. However, the prevailing theory is that the universe is filled with hydrogen gas, and hydrogen gas would ignite fusion (and thus become a star) long before it could become packed together enough to form a black hole.
 

jcsd

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Fiurstly your correct, gravity does affect light and inded the gravity of stars affects light noticeably.

The only object light can't escape from is a black hole, which is essentially a collapsed star, now black holes though they weigh several times more than our own sun are only a few kilometres across. This means that black holes are extrodinarily dense and it is the density of an object not it's total mass that gives it these properties.
Further to what Hurkyl said above:"Inside the star, there is zero gravity near the middle (you're being pulled every way equally). There are still high pressures, though. If you're outside the star, then the force of gravity dies off inversely proportionally to the square of your distance from the center." At any point inside the star you are only affected by the gravity of the material of the sphere with radius d, where d is the distance inbetween you and the centre of the star (assuming the star is perfectly spherical).
 
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Uh, you mean, if the object was so massive, the gravitational pressure would be so high, the temperature would reach 108 K, and the outward radiation would keep pressures from rising further?
 

jcsd

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That's more stellar evolution, but yes if star of high enopugh mass can no longer produce the pressure forces needed to stave off the gravitational forces then it will collapse into a black hole.
 

russ_watters

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Here's a hypothetical - is it possible to form a "cold" black hole? Ie, if the gas cloud collapses slowly enough, could it cool fast enough to avoid fusion and bypass that whole stellar evolution thing, going straight to a black hole?
 

jcsd

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No, becasue the gravitational pull creates the pressure,but I'm sure are at least theortical ways of creating a black hole without going through stellar evolution.

I'm sure you've heard of hypothetical primordial black holes which could of possibly of been formed when the universe was very dense due to density flucatuations, well obviously these are formed without steallar collapse, though in amuch earlier era.
 
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Perhaps we're straying, but I can't say that I understand the whole "event horizon" flowing outward thing. At all.

John
 

russ_watters

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Originally posted by Shirley
Perhaps we're straying, but I can't say that I understand the whole "event horizon" flowing outward thing. At all.
"Flowing outward"? Where do you see that? The event horizon is just the distance from an object (center of the black hole) at which the escape velocity is C.
 
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Originally posted by russ_watters
Here's a hypothetical - is it possible to form a "cold" black hole? Ie, if the gas cloud collapses slowly enough, could it cool fast enough to avoid fusion and bypass that whole stellar evolution thing, going straight to a black hole?
Assuming you had enough time and resources, you should be able to create a black hole via a Bose-Einstein Condensate.

Originally posted By John Cramer in Alternate Views. The BEC [Bose-Einstein Condensate] is so compact and dense that, with sufficient atoms added, a mini-black hole of atomic size should form.
http://www.npl.washington.edu/AV/altvw77.html

http://www.npl.washington.edu/AV/altvw108.html
 

Phobos

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Welcome to Physics Forums, Shirley! :smile:
 

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