Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84; site lsuc.UUCP
Path: utzoo!utcs!mnetor!lsuc!msb
From: msb@lsuc.UUCP (Mark Brader)
Newsgroups: net.astro
Subject: Re: Question about Sirius
Message-ID: <987@lsuc.UUCP>
Date: Thu, 19-Dec-85 14:06:13 EST
Article-I.D.: lsuc.987
Posted: Thu Dec 19 14:06:13 1985
Date-Received: Thu, 19-Dec-85 16:26:53 EST
References: <283@infinet.UUCP> <900@psivax.UUCP>
Reply-To: msb@lsuc.UUCP (Mark Brader)
Distribution: net
Organization: Law Society of Upper Canada, Toronto
Lines: 60
Summary: reference to Asimov given

Jim Barnes (barnes@infinet.UUCP) writes:
> 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.  The question posed is whether a
> red giant star could collapse to a white dwarf star in 1500 years
> or so.  This seems to be much faster than what the current theory
> of stellar evolution would allow.

This matter is discussed by Isaac Asimov in the essay "Siriusly Speaking",
which can be found in the collection "The Sun Shines Bright".  The book is
dated about 1982 (plus or minus 2) and should be easily findable**.  I don't
have a copy at hand so I'll just very briefly summarize what I remember.

The conclusion is that the historical references are not good evidence.
Some appear to be metaphorical.  One important one is from ancient Egypt,
where the heliacal rising of Sirius (i.e. the first day it was visible at
dawn) happened to synchronize with the Nile's annual flood and formed the
basis for the calendar; here the crucial thing is that Sirius was most
important to the Egyptians on that particular day, and *when rising*,
*any* object is reddened.  So Sirius was red *when it mattered*.

Sirius B could not have gone supernova in historical times, because there
is no supernova remnant (like the Crab Nebula).  Nor, as was said, does
current theory allow for a red giant to turn dwarf in such a short time.
Conclusion: the historical accounts are misleading.

Now the question is: since I read all this in a book that was at least
a year or two old (it was in paperback already), *and* since the alleged
redness is reasonably accounted for ... what is the Boston Globe doing
printing this in December of this year?

Meanwhile, Stanley Friesen (friesen@psivax.UUCP) writes:

> ... 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 ...

Well, no it doesn't.  I remember a Scientific American article some
years ago that explained this phenomenon.  It was probably on double stars
in general.  Say B was originally more massive.  Then it becomes a red
giant first.  It gets so big that some of its mass spills over into A.
Because the activity that makes it a red giant is still going on, this
keeps happening for a while.  After it has happened, A is more massive,
turns blue, starts evolving faster than it was, and is therefore
really older than it looks.  (I don't think that "inversely proportional"
should be taken literally, by the way.)

I don't know the separation between Sirius A and B, and I don't have
a reference.  Any of the experts want to correct or confirm this?

Mark Brader
**The essays were originally in his science column in the Magazine of
  Fantasy and Science Fiction, but don't look up the original appearance,
  because the material was updated for book publication.