Dying Star

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Dying Star

Postby quixata » Mon Aug 01, 2005 4:13 am

I do not know how relevant or even meaningful this question is.
But I do have some confusion here.

If the mass of a burnt up star is equal to the Chandrasekhar Limit (around 1.4 times the solar mass) then nothing stops the Dwarf to collapse infinitely i.e. the radius becomes 0.

Also, in the case of a very massive star (say, mass of the burnt up star is as high as 10 times the solar mass) nothing stops the core to collapse to a black hole.

Then how are the above two invisible balls different?
May be I have some wrong conception here.
Any clarification?

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Postby wendy » Mon Aug 01, 2005 5:36 am

A dwarf star does not collapse to zero, but to some small size, which is still bigger than the schwazwald-radius (ie GM/2c2).

For something like the sun, the mass measure GM/c² is 4845 feet. This means that a sphere of this diameter and weighing as much as the sun would be a black hole.

However, gravity is not able to overcome the atomic forces as it gets down to this size, and the sun as dwalf star might be, say 10 miles across. This means that it is not a black hole, and that matter can escape from it. A sufficiently fast bolloid might break the thing into two bits, and the bits escape from each other.

On the other hand, the mass of say 6784 feet would make a sphere where the escape velocity is greater than the speed of light, and nothing would escape from it.
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Postby quixata » Mon Aug 01, 2005 7:49 am

Thanks for the reply.
but I am still unclear.

Where exactly the Chandrasekhar Limit (around 1.4 times the solar mass) stands in this respect?

Most of the Dwarfs, when they are just burnt out have mass less than this limit and hence they have finite radii.

Chandrasekhar (in the '30s) found the upper limit for the mass of these dwarfs which is around 1.4 times the solar mass.
(Ref. http://en.wikipedia.org/wiki/Chandrasekhar_limit and http://scienceworld.wolfram.com/physics ... Limit.html)

Chandrasekhar also showed that if the mass equals exactly the limiting value then the resulting body will have 0 radius.

If the mass is higher than the limit then the resulting core will take other forms.

But my confusion lies here. Has it been proved that no star can have mass equal to the Chandrasekhar Limit?

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Postby jinydu » Mon Aug 01, 2005 8:55 am

I thought that beyond the Chandrasekhar limit, the star would collapse into a black hole. I don't see how it's possible for a star to have a mass exactly equal to the Chandrasekhar limit, since the loss or gain of a single electron would ruin that balance.

Those two links should give you the information you need. If it's still not enough, you could try a Google search.
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Postby quixata » Mon Aug 01, 2005 12:28 pm

Thanks. I have a slightly clearer conception now.
Perhaps I just wanted to hear that it is not possible for a star to have a mass exactly equal to the Chandrasekhar Limit.

jinydu wrote:I thought that beyond the Chandrasekhar limit, the star would collapse into a black hole.


Stars with mass bigger than the limit will not necessarily collapse to form black holes. only stars with much bigger mass will. For stars with mass moderately biggger than the limit will collapse into neutron stars where the energy due to the degeneracy of the neutrons will balance against the gravitational force. These stars will be much denser compared to the dwarfs.

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Postby jinydu » Mon Aug 01, 2005 2:00 pm

That sounds reasonable. I know for sure that there are three possibilities for "dead" stars. In order of increasing mass, they are:

White Dwarf, Neutron Star, Black Hole

Of course, if there are three "states", there are two boundaries; one between the white dwarf and the neutron star, and the other between the neutron star and the black hole.

One of those boundaries is called the Chandrasekhar limit, but I don't remember which one.

In fact, in the decades before black holes were finally vindicated by experimental evidence, the idea was attacked by Albert Einstein, Arthur Eddington and other scientists who considered the idea of an "infinitely small object" distasteful.

Note: More recently, some scientists have proposed that there could be an intermediate state between a neutron star and a black hole: a "quark star". In the last few years, telescopes detected two objects which could be quark stars, but that is still debated.
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