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Chandrasekhar Limit

by shounakbhatta
Tags: chandrasekhar, limit
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shounakbhatta
#1
Jan24-13, 01:06 PM
P: 268
Hello,

Stars with mass higher than the Chandrasekhar limit ultimately collapse either to become neutron stars or black holes. Stars with a mass below this limit are prevented from collapsing by the degeneracy pressure of their electrons.

When does the star become neutron star and when does it become a black hole?

Thanks.
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94JZA80
#2
Jan24-13, 01:55 PM
P: 119
Quote Quote by shounakbhatta View Post
Hello,

Stars with mass higher than the Chandrasekhar limit ultimately collapse either to become neutron stars or black holes. Stars with a mass below this limit are prevented from collapsing by the degeneracy pressure of their electrons.

When does the star become neutron star and when does it become a black hole?

Thanks.
this isn't quite accurate. main sequence stars w/ a mass of ~8 solar masses or more collapse into neutron stars or black holes at the end of the "super-giant" phase of their lives. main sequence stars w/ less than ~8 solar masses become white dwarfs at the end of the "giant" phase of their lives, and eject their outer layers in the form of a planetary nebula. white dwarfs range from less than a solar mass up to ~1.4 solar masses - the Chandrasekhar Limit - beyond which electron degeneracy is not enough to stave off further gravitational collapse.
Chronos
#3
Jan24-13, 03:05 PM
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What happens in any star is highly dependent on its core. Stars between .4 and 8 solar masses will ultimately become white dwarfs after exhausting their fuel. Stars in excess of 8 solar masses can accumulate iron in their cores. Iron is the end of the stellar nucleosynthesis road for any star. When the mass of iron in a stellar core reaches the Chandresakhar limit, it will inevitably suffer a core collapse resulting is a supernova. The remnant will ordinarily be a neutron star, or possibly a black hole. The key here is the ability to fuse silicon into iron. Lower mass stars [less than 8 solar] cannot fuse silicon to iron [at least not in any significant quantities] hence cannot accumulate enough iron in their core to trigger a core collapse even [i.e., go supernova]. Such stars pass quietly through the red giant phase and retire as white dwarfs. The pressure and temperature of a core collapse event is necessary to produce remnants more compact than a white dwarf.

Chronos
#4
Jan24-13, 03:15 PM
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Chandrasekhar Limit

The other case where the Chandrasekhar limit come into play is in type 1a supernova. These are produced, not by a core collapse event, but, by overfeeding a white dwarf remnant to the point it exceeds the Chandrasekhar mass limit, which overwhelms electron degeneracy resistance causing it to go BOOM. The explosion is so violent it utterly destroys the white dwarf.
94JZA80
#5
Jan24-13, 03:39 PM
P: 119
i was on my way out of the office when i hastily typed my quick response, and was going to come back and elaborate on what i said, but it looks like you got that part covered Chronos
Quote Quote by Chronos View Post
The other case where the Chandrasekhar limit come into play is in type 1a supernova. These are produced, not by a core collapse event, but, by overfeeding a white dwarf remnant to the point it exceeds the Chandrasekhar mass limit, which overwhelms electron degeneracy resistance causing it to go BOOM. The explosion is so violent it utterly destroys the white dwarf.
based on what i've read, a Type 1a supernova is quite literally an explosion (as opposed to an implosion event) that, as you said, occurs when a white dwarf exceeds 1.4 solar masses (the Chandrasekhar Limit) as matter from a companion star is being dumped onto it. at that point, a runaway thermonuclear chain reaction takes over and destroys the white dwarf in a spectacular explosion that leaves no remnant (neutron star, black hole, etc.) behind. what i don't understand is why an explosion happens at the Chandrasekhar Limit in this instance, instead of further collapse to a neutron star or a black hole. does it have to do with the difference between the electron degenerate matter that composes a white dwarf and the iron that composes the core of a massive star? or am i missing something rather obvious?

*EDIT* - just read up a bit on white dwarfs and type 1a supernovae. it seems the current view is that white dwarfs never actually quite reach the Chandrasekhar Limit during the accretion process, and that the pressure caused by gravity as the white dwarf approaches within 1% of the Chandrasekhar Limit heats the core enough to start thermonuclear fusion, which starts the runaway chain reaction that ultimately destroys the white dwarf in an explosion.
Chronos
#6
Jan24-13, 08:35 PM
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There is nothing to moderate the explosive force of a 1a supernova. In a core collapse event the core is smothered by the stellar envelope.
shounakbhatta
#7
Jan26-13, 09:50 AM
P: 268
I believe the electron degeneracy principle comes into play when it prevents from internal gravitational collapse of the white dwarf star. I believe electron degeneracy principle is the stellar application of Pauli exclusion principle stating that no two electrons can be in the same state. Am I right?

Now, I have one question: Is there a scenario when the electron degeneracy pressure is no more applicable and eventually the star will collapse within its own gravitational pressure inevitable?

Thanks.
shounakbhatta
#8
Jan26-13, 10:14 AM
P: 268
Another thing:

The Chandrasekhar limit (CL) is for white dwarf star. Stars above CL becomes neutron star or black hole and Tolman–Oppenheimer–Volkoff limit (TOV) is the upper limit for neutron star right? Above which the neutron star becomes a black hole, right? Is this where the electron degeneracy pressure is no more applicable?

Thanks.
Chronos
#9
Jan26-13, 10:55 AM
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The electron degeneracy pressure is merely too weak to influence events once the Chandrasekhar limit is reached. Obviously once a star collapses to form a neutron star [or black hole, for that matter], electron degeneracy pressure is no longer in play. The TOV limit is the upper limit for neutron star mass. Beyond that, the neutron degeneracy pressure to too weak to prevent the star from further collapse - either a black hole or some intermediate state such as a quark star results. Given no quark stars have been positively identified to date, prevailing opinion is a black hole is the probable outcone of exceeding the TOV limit. We do not know the precise value of the TOV limit. The equation of state [EOS] for degenerate matter is poorly understood. When it was originally calculated, the TOV limit was predicted to be 0.7 solar masses - which is obviously too low. Based on the most massive known neutron star [1.97], current thinking is it is somewhere between 2 and 3 solar.
shounakbhatta
#10
Jan26-13, 12:57 PM
P: 268
I believe quark star are a hypothetical stars.


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