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Author Topic: What's the mass threshold between becoming a star and then a black hole ?  (Read 7461 times)

Offline neilep

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Hullo,

This is an artists impression of a black hole.



...and this isn't...This is in fact a whole bunch of stars called M24



Nice isn't it ?..there's a lot of stars there..I lost count after three !

In the film 2010 lots of these monoliths


 were being chucked into Jupiter. I assume to give it enough mass to turn it into a Sun. Which it did do...which was nice !

My questions are :

How close is Jupiter then to becoming a star ?..how much more mass would it take for it to give out starry luff ?... ten times it's mass ?..a hundred ?..a hundred thousand ?

..and then, how much mass would it need to be to become black hole ?

Effectively...what is the minimum mass for star quality and blackhole joy ?





 

Offline Soul Surfer

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Jupiter is approximately one thousandth the mass of the sun.  small stars that we see are about one tenth the mass of the sun and the smallest stars that glow are probably about one hundredth the mass of the sun so Jupiter needs about ten times as much material to start to glow from mass alone.  However in the story of 2010 the monoliths didn't just add mass to Jupiter they added energy by all flying together to meet at the centre of the planet so if you arranged for this to happen you could probably get away with a lot less mass.

The Chandrasekhar limit for the collapse of stars to neutron stars is about 1.4 solar masses and the limit for the collapse of neutron stars to black holes is about 2-3 solar masses but this does not mean that all stars more than three times the mass of the sun are in imminent danger of collapse into a black hole because this is in fact very difficult for several reasons.  notably the gravitational and nuclear fusion energy and residual angular momentum so you need to start with a star more than ten times the mass of the sun before it has a good chance of collapsing int a black hole.
 

Offline syhprum

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It has been recognised recently that there is a continuum between planets and stars with a class of dwarf star that heats up due to gravitational commpression but does not become sufficiently hot in its centre for nuclear fusion to take place.
Over many millions of years these cool down and become planet like objects
 

Offline neilep

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Jupiter is approximately one thousandth the mass of the sun.  small stars that we see are about one tenth the mass of the sun and the smallest stars that glow are probably about one hundredth the mass of the sun so Jupiter needs about ten times as much material to start to glow from mass alone.  However in the story of 2010 the monoliths didn't just add mass to Jupiter they added energy by all flying together to meet at the centre of the planet so if you arranged for this to happen you could probably get away with a lot less mass.

The Chandrasekhar limit for the collapse of stars to neutron stars is about 1.4 solar masses and the limit for the collapse of neutron stars to black holes is about 2-3 solar masses but this does not mean that all stars more than three times the mass of the sun are in imminent danger of collapse into a black hole because this is in fact very difficult for several reasons.  notably the gravitational and nuclear fusion energy and residual angular momentum so you need to start with a star more than ten times the mass of the sun before it has a good chance of collapsing int a black hole.

THANK YOU VERY MUCH IAN.

May I ask....is it just gas giants that become stars ?....Would a rocky planet like this one be able to become star if it accumulated enough mass ?......rocky mass that is !!
 

Offline neilep

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It has been recognised recently that there is a continuum between planets and stars with a class of dwarf star that heats up due to gravitational commpression but does not become sufficiently hot in its centre for nuclear fusion to take place.
Over many millions of years these cool down and become planet like objects

THANK YOU SYHPRUM,

Is that what they call a black dwarf ?.....I did hear or read somewhere that stars like ours when they eventually go through their dying process that the final eventual outcome is something called a black dwarf !

 !!...could what you describe and what I've mentioned be something similar ?...I also recall that the process takes so long that the Universe hasn't been around long enough for any to be formed yet !..that is... formed by the dying star process !
 

Offline Soul Surfer

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Yes it would in theory be possible for a rocky body to become a star if it was big enough. It would need to be several times the mass of the sun, become a star in a very late phase of its life, evolve very quickly and explode violently to become a neutron star or a black hole but it is not possible for such a body to form.  The reason for this is that most of the mass of any cloud condensing into stars is hydrogen or helium.  As eventually rocky planets are forming they initially contain quite a lot of hydrogen and helium but it is blown away by the central star as it has a very bright phase prior to settling down in its position on the main sequence.
 

Offline Soul Surfer

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Small stars when they burn out become "white dwarfs".  These have very few internal sources of energy and are essentially just cooling down,  but as you suggest they will take many times the current age of the universe before they stop glowing.

The theoretical final state of these bodies is a bit of a mystery because they are mostly made out of degenerate plasma in which the electrons are crushed much closer to the nucleii than they would be in normal atoms so as they cool down the energise will eventually reach a point where they will tend to try to form normal atoms.  This could make the surface of the cooling star "fluff up" because the outer atmosphere then occupies very much more volume.  This may even, I think, cause them to evaporate slowly and they may end up after many millions of times the age of our universe as nothing.
 

lyner

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Yes it would in theory be possible for a rocky body to become a star if it was big enough.
The problem is one of probability, I think.
There is so little material  around with the density of rocky planets that the probability of getting enough into one place is very low.
I know that they reckon that gas giants have dense elements at their centres but how much? Is it the equivalent to  several Earths inside Jupiter?
 

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