protostars, supernovas and brown dwarfs

[Colfusion]

i was wondering if you could explain to me a few objects. First what is a protostar. What's the difference in a type 1 supernova and a type 2. Lastly what a brown dwarf is. Thank you for any help you can give me.

[K6-III]

A protostar is a star in formation where fusion has not yet begun.

A brown dwarf is a white dwarf that has dissipated its energy, and is dim...

[smiley]

A brown dwarf is a star that is not large enough to initiate fusion reactions.

A type 1 supernova consists of a red giant and a white dwarf, orbiting such that matter is being pulled off the red giant onto the white dwarf. At a critical point, the white dwarf will no longer be able to support the additional mass without collapsing. The collapse triggers the explosion.

A type 2 supernova is a massive star that has become a red giant. Its nuclear fuel supply has run out, triggering the collapse and explosion.

[Steve Langford]

Type I supernovae do not have hydrogen lines in their spectra. Type II supernovae show prominent hydrogen lines. Type II supernovae are produced by the collapse of massive stars, when the nuclear fuel is exhausted in their cores. In most cases, only 10% of the hydrogen in a star’s atmosphere is available for burning. There is not enough mixing in a star to burn much more. When a massive star goes supernovae, there is usually enough hydrogen in the surrounding atmosphere to produce hydrogen absorption lines.

However, this is not always the case. A massive star can loose most of its hydrogen during the red giant stage of its evolution. In these stars, strong solar winds remove the outer 90% of the star before the core collapses. It is also possible that a binary partner can steal most of the hydrogen from the atmosphere of a dying massive star. When these massive stars subsequently go supernovae, the emitted light shows weak (if any) hydrogen absorption lines. These are also Type I supernovae, specifically Type Ib and Type Ic supernovae, depending on the details of the spectra.

Only the Type Ia supernovae are due to the collapse of a white dwarf. In this case, a stable white dwarf gradually steals mass from a binary partner until its mass reaches the limit of stability, the Chandrasekhar mass. Then it goes supernovae. White dwarfs normally have little hydrogen, and thus hydrogen absorption lines are not normally observed. It is believed that Type Ia supernovaes completely destroy the original white dwarf. Type Ia supernovae are believed to have produced about 70% of all the iron in interstellar space. This iron is available for planet formation, among other things.

White dwarfs are amazing things, typically with 10^56 electrons compressed into a space the size of the earth. The Heisenburg uncertainty principle states that you cannot know the position and the velocity of a particle with arbitrary precision. To put that many electrons in such a small space (low uncertainty in position), the uncertainty in their velocity must be very high. This is only possible if their average velocity (and therefore kinetic energy) is very high. As a white dwarf’s mass increases, gravity actually pulls the star in so that it gets physically smaller, and thus the electron kinetic energies get larger. Eventually, some of the electrons have so much kinetic energy that they can reduce their energy by reacting with protons to become neutrons. This reaction proceeds catastrophically, leading to a Type Ia supernova.