msb@lsuc.UUCP (Mark Brader) (03/10/86)
Pardon the long inclusions here, but I'm adding a group to this... Robert Elton Maas wrote the following as a tangent to another topic being discussed in net.space (and the corresponding ARPA mailing list, whatever it's called): > > By the way, in latest Sky&Telescope there's a note on Sirius being > > reddish in recorded history (in fact about 600 AD), because at that > > time its white-dwarf companion was in the red giant stage. It's hard > > to believe the conversion from red giant to white dwarf could occur so > > quickly, although with Sirius gobbling most of the loose hydrogen as > > fast as it is shed, and ionizing & light-pressure-shoving the rest of > > the emitted hydrogen, I could imagine it within the realm of > > possibility. Any news since S&T publishing date on that topic that you > > know of? Jim Barnes replied: > I recently (two months ago?) read an article in the Boston Globe > that said roughly the same thing - Sirius was reddish within recorded > history. I posted the same question to net.astro. The general response > received was that the redness was due to the time of day when the > observations were made. i.e. Sirius sets near the sun during the dog > days of the summer, hence the reddish color is due to the sunset, not > the color of the star. Current theories of stellar evolution do not > allow for a star to evolve that rapidly. Well, as one of the people who replied to Jim's previous query in net.astro, I feel obliged to correct myself. See, this hypothesis about Sirius being red has been proposed and put down before, and I quoted an article a few years old. (I also said, "Why is this in the Boston Globe now?", and no one replied.) The reason it was in the Boston Globe is that there's new evidence. I don't read Sky & Telescope, but it was in Scientific American in February. (Page 59, in the "Science and the Citizen" column). They quoted a letter in Nature, but did not mention which issue; presumably the S&T article refers to the same letter. I will now summarize the Scientific American item in my own words. (The only reason I didn't do this sooner, by the way, was that I was sure someone else would cite it!) 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 (Bochum, W. Germany) studied an astronomical almanac that was compiled by Gregory of Tours about 580 AD. Only one copy exists. The almanac was written to help monasteries to find the time at night, so it give month-by-month lists of rising times for various constellations. Schlosser and Bergmann examined representations of the 6th century sky at a planetarium. Gregory refers to a star Rubeola or Robeola, which had been thought to be Arcturus; but this is impossible and it must actually have been Sirius. But the name means red or rusty. The latter to Nature suggested that the explanation is that Sirius B was a red giant that recently. The transition from red giant to white dwarf is supposed to take very much longer than 1,500 years, and is generally accompanied by cataclysmic explosions. It is conceivable that the slightly elevated level of metals in Sirius A reflects an explosion in Sirius B, though unobserved, but the apparent speed of the transition is unexplained. One thought occurs to me. The previous explanation could still be correct, i.e., that Sirius was important to the ancient Egyptians only when it was rising (when any star appears red), and that the Greek and Roman references to color are metaphorical. See, I figure it could be that the Egyptians started a tradition of calling Sirius reddish, and that this tradition led to the Greek and Roman metaphorical references, and that they in turn led to the name Rubeola, and during all that time nobody cared to look at the actual color of the star. (How old is the Latin-sounding name Sirius anyway?) Personally, I find both possibilities hard to believe. Isn't science fun? Mark Brader
gsmith@brahms.BERKELEY.EDU (Gene Ward Smith) (03/14/86)
In article <1146@lsuc.UUCP> msb@lsuc.UUCP (Mark Brader) writes: >> > By the way, in latest Sky&Telescope there's a note on Sirius being >> > reddish in recorded history (in fact about 600 AD), because at that >> > time its white-dwarf companion was in the red giant stage. It's hard >> I recently (two months ago?) read an article in the Boston Globe >> that said roughly the same thing - Sirius was reddish within recorded >> history. I posted the same question to net.astro. The general response > 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 "There are no differences but differences of degree between degrees of difference and no difference"
clyde@reed.UUCP (Clyde Bryja) (03/16/86)
> 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."