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Posting-Version: version B 2.10.2 9/5/84; site cadomin.UUCP
Path: utzoo!utcsri!ubc-vision!alberta!cadomin!andrew
From: andrew@cadomin.UUCP (Andrew Folkins)
Newsgroups: net.astro
Subject: Re: Question about Sirius
Message-ID: <647@cadomin.UUCP>
Date: Fri, 13-Dec-85 13:24:06 EST
Article-I.D.: cadomin.647
Posted: Fri Dec 13 13:24:06 1985
Date-Received: Sat, 14-Dec-85 01:53:27 EST
References: <283@infinet.UUCP>
Reply-To: andrew@cadomin.UUCP (Andrew Folkins)
Distribution: net
Organization: U. of Alberta, Edmonton, AB
Lines: 52
Summary: 

In article <283@infinet.UUCP> barnes@infinet.UUCP (Jim Barnes) 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.
>
>Does anyone in net land have more information on this?  Anyone
>with opinions on whether the collapse of a red giant to white
>dwarf is possible in such a short time?
>

I also have heard this theory.  It seems to be corroborated by historical
references from Sumerian, Egyptian and (I think) Greek writings from several
hundred years BC.  The references describe Sirius as red, and brighter than
it is today, visible even in the daytime (!!).  

However, according to present astrophysical theories, there doesn't seem
to be a way for a red giant to collapse into a white dwarf this quickly
( < 2000 years).  The accepted view is that the red giant sheds it's
outer layers, forming a planetary nebula and leaving it's core behind as
a white dwarf. Two thousand years later, such a nebula would be easily 
detectable.  IRAS detected an infrared excess from Sirius, implying the 
existance of material of some kind around the star, but this is not what a 
planetary nebula would look like. 

One test that can be performed is determing the surface temperature of the
dwarf star (I believe that this has been done, but I don't know the temperature
obtained).  The hotter the dwarf, the younger it is.  If Sirius' companion
is *very* hot, then it would be possible that it is young enough.
 
From an astrophysical standpoint, an easier explanation is that the
ancient observations were in error, and that Sirius was seen as red due
to atmospheric effects : if the star were lower in the sky then (something
I'd have to check on), then it would appear redder (the same way the
setting sun appears red).  The problem is, I don't know if Sirius would
be that low.  On the equator today, it rises to about seventy degrees 
above the horizon.  For the atmospheric effects described above, the star
would have to be less than ten, *at maximum*, otherwise it would appear
as it does today.

-- 
Andrew Folkins        ...ihnp4!alberta!andrew    
 
All ideas in this message are fictional.  Any resemblance, to any idea,
living or dead, is purely coincidental.