Re: [Phys-l] astrophysics question

["Michael Edmiston" <edmiston@bluffton.edu>]

Now that Mark has added more information about his question, I would say the 
proposed process does not usually happen.  A high mass star does not engage 
in helium flash.  Above roughly 2.5 solar masses helium fusion initiates 
more smoothly and does not give rise to the instabilities that cause a 
smaller star to end up with the violent helium flash that blows away the 
outer portion of the star, leaving a white dwarf.  Instead, for stars above 
2.5 solar masses, and especially around 8 solar masses and above, the star 
is headed for a supernova and therefore no white dwarf will form.

Perhaps an exception to this could arise if the very massive star is part of 
a binary system.  If a very massive star is paired with a smaller star, the 
larger star will evolve faster and reach a red giant stage sooner.  If the 
separation is correct, this could allow the smaller star to steal matter 
from the larger star.  There are situations in which we believe the original 
smaller star actually ended up being the larger star.  If the large star 
losses enough mass to the companion quickly enough that it gets below 2.5 
solar masses before it proceeds into a supernova event, then it will have 
lost enough mass that it could end up with a helium flash and then become a 
white dwarf.  Maybe?

I am not sure how likely this is, because the very massive stars move 
rapidly from hydrogen fusion to helium to carbon and are often fusing carbon 
even before the hydrogen and helium are "burned out."  Although I teach 
astronomy, I am not an astrophysicist.  I was unable to find any sources in 
my library that indicate a massive star at the upper left of an H-R diagram 
can actually become a white dwarf by any known process.  If it should 
supernova of either type, there will not be a white dwarf.  Therefore the 
star would have to lose a lot of mass before it reached the supernova phase. 
Aside from mass loss to a companion, I am unaware of any other process.

Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
1 University Drive
Bluffton, OH 45817
419.358.3270
edmiston@bluffton.edu


--------------------------------------------------
From: "Mark Sylvester" <mark.sylvester@spin.it>
Sent: Thursday, July 03, 2008 4:11 AM
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Subject: 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.
>
> Mark.