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Author Topic: Would a huge star be invisible?  (Read 2911 times)

Offline Maniax101

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Would a huge star be invisible?
« on: 22/10/2009 15:08:24 »
When an object increases in mass, the grav. field grows. How big would a star have to be for its escape velocity to exeed the speed of light? If these huge stars (I presume) exists, then we wouldn't be able to see them... no?

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« Last Edit: 24/10/2009 21:13:51 by chris »


 

Offline lightarrow

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Re: Would a huge star be invisible?
« Reply #1 on: 22/10/2009 15:44:24 »
When an object increases in mass, the grav. field grows. How big would a star have to be for its escape velocity to exeed the speed of light? If these huge stars (I presume) exists, then we wouldn't be able to see them... no?
They call them black holes...
It doesn't depend on mass only, but on dimensions too. What counts is the gravitationale field at the object's surface.
 

Offline Maniax101

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Re: Would a huge star be invisible?
« Reply #2 on: 23/10/2009 13:37:58 »
Ah, no. A black hole comes from when the internal pressure of a star due to nuclear reactions looses to gravity, mostly due to that the star is "dying".

What I had in mind was the really really huge stars, when they reach a certain mass, would the wink out of existense and become black holes, with no shrinkage before, even though they are "healthy" burning stars?
 

Offline Mr. Scientist

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Re: Would a huge star be invisible?
« Reply #3 on: 23/10/2009 13:50:30 »
Ah, no. A black hole comes from when the internal pressure of a star due to nuclear reactions looses to gravity, mostly due to that the star is "dying".

What I had in mind was the really really huge stars, when they reach a certain mass, would the wink out of existense and become black holes, with no shrinkage before, even though they are "healthy" burning stars?

It's not about mass. It's density.

This is where you are going wrong.
 

Offline Soul Surfer

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Re: Would a huge star be invisible?
« Reply #4 on: 24/10/2009 00:18:48 »
It is suggested that in the early days of the universe some of the earliest large mass concentrations could have collapsed into black hoes with a relatively small amount of radiation altohiugh the Quasars are probably large black holes feeding on a large amount of material early in the universe
 

Offline Mr. Scientist

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Re: Would a huge star be invisible?
« Reply #5 on: 24/10/2009 04:08:49 »
It is suggested that in the early days of the universe some of the earliest large mass concentrations could have collapsed into black hoes with a relatively small amount of radiation altohiugh the Quasars are probably large black holes feeding on a large amount of material early in the universe

These are primordial black holes - and they have [all] dissipated and thermal radiated now.
 

Offline Soul Surfer

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Re: Would a huge star be invisible?
« Reply #6 on: 24/10/2009 09:10:51 »
That is untrue Mr Scientist.  The effective life of even quite a one solar mass black hole is vastly longer than the current age of the universe  and these large thousan solar mass and above black holes ave absolutely vast lifetimes.  the short lived primodial mass black holes that are talked about in the text extremely small with only the maas of an asteroid not even the earth.
 

Offline Mr. Scientist

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Would a huge star be invisible?
« Reply #7 on: 25/10/2009 07:07:02 »
True - a bit ahead of myself. The smaller primordial black holes no longer exist. Some of the more supermassive primordial black holes will begin their evaporation quite soon.
 

Offline Homely Physicist

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Would a huge star be invisible?
« Reply #8 on: 01/11/2009 01:57:33 »
Interesting question! The mass and size proportions of each star are different. Whether it is rotating, or has an excess of +ve or -ve charge, affects the point at which it can be described by black hole metrics. The average speed of each particle inside the star is also very important.

When an object increases in mass, the grav. field grows. How big would a star have to be for its escape velocity to exeed the speed of light? If these huge stars (I presume) exists, then we wouldn't be able to see them... no?
They call them black holes...

To cut a long story short, escape velocity does not come into whether an object is classified as a black hole or not: it's whenever a local concentration of mass shrinks into a sphere with a radius smaller than it's Chandrasekhar limit (assuming non-spinning, charge neutral case). Theoretically, it might be possible to have an object that's not a black hole yet not visible directly either. However, our laws of condensed matter physics come in to play and no-one has found such a form of matter that could arise inside stars.

True - a bit ahead of myself. The smaller primordial black holes no longer exist. Some of the more supermassive primordial black holes will begin their evaporation quite soon.

Interesting... I wasn't aware of any definite observational evidence for primordial black holes yet.
« Last Edit: 01/11/2009 03:43:25 by Homely Physicist »
 

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Would a huge star be invisible?
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