Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/5/84; site reed.UUCP
Path: utzoo!watmath!clyde!cbosgd!hplabs!tektronix!reed!clyde
From: clyde@reed.UUCP (Clyde Bryja)
Newsgroups: net.astro,net.space
Subject: Re: Re: Was Sirius red after all?
Message-ID: <2797@reed.UUCP>
Date: Sat, 15-Mar-86 17:37:06 EST
Article-I.D.: reed.2797
Posted: Sat Mar 15 17:37:06 1986
Date-Received: Mon, 17-Mar-86 04:04:15 EST
References: <8603020306.AA00249@s1-b.arpa> <8603061510.AA04282@infinet.UUCP> <1146@lsuc.UUCP> <12364@ucbvax.BERKELEY.EDU>
Organization: Reed College, Portland, Oregon
Lines: 36
Xref: watmath net.astro:1482 net.space:6484

> In article <1146@lsuc.UUCP> msb@lsuc.UUCP (Mark Brader) writes:
> 
> 
> >   Ancient Babylonian, Greek, and Roman texts were known to consistently
> >   refer to Sirius as red.  But now there is medieval evidence for this as
> >   well.  Wolfhard Schlosser and Werner Bergmann of the Ruhr University
> 
>     One other problem with the Sirius B as red giant theory: Sirius B has 
> about the same temperature as Sirius A: about 12000 K if I remember right.
> This is *much cooler* than some other white dwarves, and seems to indicate
> a considerable cooling-off period. Also, where is there any sign of a left-
> over planetary nebula?
> 
> ucbvax!brahms!gsmith    Gene Ward Smith/UCB Math Dept/Berkeley CA 94720

Recent measurements of the *effective* temperature of Sirius B yield a
value in the range of 25 to 28 thousand kelvins.  The effective temperature
of a star is the surface temperature that the star would have if it were
a perfect blackbody radiating with the same luminosity.  For main sequence
stars, the effective temperature and the actual surface temperature are of
nearly the same value.  Not so white dwarf stars.  I don't know what the
actual surface temperature of Sirius B is, but I'm sure it's a lot less than
the effective temperature.  The point of typing all this is to say that
Sirius B radiates a lot of energy per unit surface area (much more energy
than would be implied by considering the actual surface temperature as a
measuring stick).  Incidently, the effective temperature of Sirius A is about
10,000 K.
	The high radiative power of Sirius B (and it is high compared to
most white dwarves, though lower than a few) would imply that it contracted
relatively recently.  I still feel 2000 years to be much too recent, though.
-- 
+++++++++++
"For Easter Day is Christmas time,		Clyde Bryja
 And far away is near,				Box 21, Reed College
 And two and two is more than four,		Portland, OR	97202
 And over there is here."