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Re: [Phys-l] astrophysics question

John pointed out several Wiki references that speak to Mark's question, but according to my understanding, John's first short answer (yes) to Marks' question (can a supernova remnant be a white dwarf) is incorrect.

A Type I supernova does not result in a white dwarf. It results in no central object remaining. Type I is also called "carbon detonation." The carbon core engages in a runaway fusion process that more or less takes place throughout the whole core at once. The core explodes and obliterates the star. There is nothing really left at what was the core.

A Type II supernova is a core collapse (as opposed to a detonation). This results in a neutron star that can further collapse to a black hole.

In either case (Type I or Type II) the star is gone. That is, it will not "rekindle.' It's either a neutron star or black hole (II), or it is obliterated (I).

On the other hand, a white dwarf results from an explosion that does not occur in the core itself. This explosion occurs in a shell outside the core of a red giant, most commonly when that shell (which is mostly helium surrounding a carbon core) reaches a rate of fusion sufficient to blow away the outer portion of the star. Material outside this shell gets blown away, perhaps into a "planetary nebula" and material inside the shell becomes the white dwarf. If this is the first time such an event has occurred, it is typically called a "helium flash" because it is typically a shell of helium whose helium-fusion rate undergoes oscillations that are unstable and grow in magnitude until one of the oscillations ejects the outer portion of the star.

The helium flash increases the luminosity of the star, but not sufficiently to refer to it as a supernova or even a nova.

If the white dwarf has a companion star from which it can steal hydrogen, the star can grow in mass until there is sufficient hydrogen to rekindle hydrogen fusion. This results in a very faint white dwarf jumping in luminosity by quite a bit, so we observe an apparent new star where we did not see much before, and this is a nova (as opposed to supernova). The nova will extinguish once the hydrogen is fused, but it can rekindle if more hydrogen is gained from the companion star. So nova events can occur over and over.

Bottom line, the events that leave a white dwarf behind are not supernova events. Rather, they are the "helium flash" of a red giant, or they are the rekindled hydrogen fusion (hydrogen gained from a companion star) which is called a nova.

Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
1 University Drive
Bluffton, OH 45817