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Re: [Phys-l] astrophysics question

Thanks for replies, Michael and John.

My query arose from an examination question that asks under what circumstances a white dwarf can be the outcome of a main sequence star at the high mass upper left end. A colleague was asserting that the required mass loss would have to occur during the supernova, which didn't seem to make sense. Indeed, the helium flash/planetary nebula process is what I had in mind... but this avoids the supernova.

Thanks again.


At 16.01 01/06/2008, you wrote:
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

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

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