Originally posted by marcus
If Alles picture needs correction or clarification please post same! There is some ambiguity about what fraction of the mass of the star undergoes fusion during the explosion. Here is page 13 from his essay
http://www.fire.biol.wwu.edu/trent/alles/Origin_of_Elements.pdf
The Evolution of Double Stars and Supernovae of Type Ia
Almost half the stars in the sky are double or multiple. If the two stars are close together then they can have dramatic effects on each other. The more massive of the two stars will evolve faster and when it becomes a red giant it may be so big that gravity draws its outer atmosphere across to the companion star. The transfer of material can lead to all kinds of interesting and exciting effects, depending on the properties of the two stars.
Stars that have lost their atmospheres to their companions are identical to the white dwarves in the center of planetary nebulae. The less massive companion star, assisted by the extra mass it has gained, eventually becomes a red giant and starts to transfer material back onto its white dwarf companion. This can have the
effect of increasing its mass beyond a critical limit of 1.4 times the mass of the Sun, known as the Chandrasekhar limit. When this happens the carbon-oxygen core can suddenly explode, converting half the mass by nuclear fusion into elements like chromium, manganese, iron, cobalt and nickel. This is called a Type Ia supernova. Because they are very bright and we think they always explode releasing about the same amount of energy, they are used as standard brightness light sources. The recent discovery that the expansion of the universe is accelerating, was made by observing these supernovae in galaxies 5,000 million
light years away. Type Ia supernovae are also a major source of iron and other heavy elements.
I am not sure of the point of this thread. Are you thinking out loud, or are you looking for something specific.
Either way, I feel compeled to "clarify" a point or two that
always gets oversimplified. I hope writers (and websites) do this to keep things "basic" or more for the layman. Alles' information was interesting, but again it just mentioned the "
Chandrasekhar limit, assuming that we all know that number, or may all think about the 1.44 Solar Masses (M
s) we read so much about and learned in school..
Chandrasekhar calculated the 1.44 M
s limit having in mind a small star, such as our sun, that had burned most Hydrogen in the core and then ignighted He as it swelled and expelled mass. Such a star would not have enough mass (= gravity) to create a core of elements heavier than He, so the core simply remained as a White Dwarf, no more nuclear reactions going on.
If an He core had more than 1.44 M
s, the gravity would overcome the electron degeneracy pressure and the dwarf would collapse to a more compressed state. That is why it is called a
LIMIT, no more than 1.44 M
s to stay a White Dwarf. But,
this is only for a Helium core. Chandrasekhar also calculated other "limits", later confirmed by many other physicists. For example, if a star has an
Iron core, the limit is 1.79 M
s, not 1.44. Chandrasekhar also calculated an upper limit for any star/material of 3.2 M
s above which any object (Neutron Star) would further collapse; Black Hole I suppose.
So, the point is that there are different "Chandrasekhar limits" for accumulations of matter, each based on the composition of whatever the hell it may be. It has often been stated that "
a White Dwarf that accumulates (by accretion) matter and exceeds the "Chandrasekhar limit" will explode as a Type Ia Supernova".
That statement is about 95% total bunk!
If your (anyone's) goal is to calculate the energy output of a supernova, you would have to know the chemical conditions necessary to cause the supernova in the first place, and you would need the mass of the matter involved in the explosion/fusion process that makes the "heavier elements" we are made of. It was calculated as early as 1956 (I think) that a Type Ia supernova, the topic of this discussion, can only occur in a White Dwarf with an initial composition of mainly Carbon and Oxygen. It has also been known that the
Chandrasekhar limit for a carbon-oxygen core is not 1.44 M
s,
it is 1.39 Ms.
Also, most White Dwarfs that form do not have the correct chemical composition, or in correct proportions, to go supernova whether accumulating more mass by accretion or not. A Type Ia is a very, very rare end for any White Dwarf. Most of them do nothing but cool, or if there is infalling matter, they flare-off in a surface nuclear reaction commonly known as a
NOVA, not supernova.
As of now, the most common problem (question) confronting physics with respect to Type Ia supernovae, is whether the carbon-oxygen "core" detonates or propogates, by what is known as Carbon deflagration (burning). The energy output by which Type Ia supernovae is used as a "standard candle" is all in the
visable part of the EM spectrum. Debate still survives as to the far larger energy output by way of neutrinos.
Marcus; I am not trying to dump water on your campfire, it is just that it has bugged me for years that the "Chandrasekhar limit" is always referred to as 1.44 M
s, and that so many people think (and are taught) that any old White Dwarf will explode as a biggie if more mass is dumped on it. Even the most active boys in the field, Wheeler, and especially J.C. Niemeyer and S.E. Woosley, don't call the 1.39 M
s a "Chandrasekhar limit", they call it the
Chandrasekhar mass.
