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From: jeff@utastro.UUCP (Jeff Brown the Scumbag)
Newsgroups: net.astro
Subject: Re: Question about Sirius
Message-ID: <174@utastro.UUCP>
Date: Sun, 15-Dec-85 03:55:01 EST
Article-I.D.: utastro.174
Posted: Sun Dec 15 03:55:01 1985
Date-Received: Fri, 20-Dec-85 02:52:44 EST
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Organization: U. Texas, Astronomy, Austin, TX
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> >In the Boston Globe today (December 9) there is an article about
> >the dog star Sirius.  Sirius today is visible as a blue-white star 
> >but this article refers to historical accounts that refer to Sirius 
> >as a red star.  The speculation (theory?) presented in the article is
> >that the white dwarf companion star of the visible blue-white star
> >was a red giant in the recent past.
> >
> 	Well, not by ordinary, simple collapse, but a supernove would
> do it, assuming that some types of supernovas do not result in a
> neutron star. There is in fact quite a problem with the dwarf
> companion. According to standard stellar theory the lifetime of a star
> is *inversely* proportional to its mass. That is in order for the
> dwarf companion to have "died" before the currently visible member
> even left the main sequence it must have been enormously more massive
> than Sirius prime. It is now *less* massive. This certainly implies
> something like a supernova, yet there is *no* trace of a supernova
> remnant, nor of a planetary nebula, nor of any other sort of expelled
> mass around Sirius.

That Sirius B is now less massive than Sirius A is not that unusual;
there are lots of systems out there with the more massive member of a
pair being the less evolved.  The problem is, as mentioned above, that
the more massive a star is, the faster its evolution.  This is known
as the "Algol paradox" since Algol (another famous binary star) has the
same kind of trouble.  The way out is that the (now) less massive star was
more massive and has lost a lot of mass, possibly (maybe even probably)
dumping matter on the (now) more (but then less) massive star.  There
isn't much doubt that this kind of thing happens quite frequently, though
the details of binary star evolution are not all that well understood.
(Give us time, we're working on it.)

The problem with the Sirius system, to my mind, lies in the assumption
that something has happened to it in historical times.  I have no 
objection to the assertion that Sirius B was once more massive than it
is now and subsequently became a red giant en route to a white dwarf; I
object strongly to the suggestion that it was a red giant as little as
2000 years ago.  (Incidentally, that Sirius B is a white dwarf indicates
that it did *not* supernova; supernovae don't leave white dwarfs behind.
Also, you're right that a supernova that close to us would be Very Bad
News Indeed.)

Red giants at the end of their red-giant-hood are thought to shed
considerable mass in the form of a planetary nebula, with what they don't
lose ending up as a white dwarf once the nebula dissipates.  The planetary
nebula phase doesn't last long (astronomically!).  So, if you drop the
requirement that things have happened in the last 2000 years, there's
plenty of time for the red-giant-to-become-Sirius B to make a planetary,
become a nice white dwarf, and then drift to within a few parsecs of Sol
for us to see it, without appealing to anything exotic, and with all the
nebula nicely dissipated into the general interstellar medium.

I think most astrophysicists would agree with me that it would take a lot
more evidence than what we have to make us think it possible that the Sirius
system has undergone a real change in color in recent history.  There *are*
stars which do undergo impressive changes in color and brightness in times
that short.  We know enough about them and about Sirius to know that Sirius
is not one of them.  Until a detailed study of many historical documents
from several cultures indicates otherwise (and such a study has not yet
appeared) I feel justified in saying the burden of proof lies still with the
historians.

Jeff Brown the Scumbag
		{allegra,ihnp4}!{noao,ut-sally}!utastro!jeff
		jeff@astro.UTEXAS.EDU
Astronomy Department, U. of Texas, Austin