barnes@infinet.UUCP (Jim Barnes) (12/09/85)
***
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?
--
-------------------------
decvax!{cca!emacs,wanginst}!infinet!barnes Jim Barnesandrew@cadomin.UUCP (Andrew Folkins) (12/13/85)
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 It seems to be corroborated by historical references from Sumerian, Egyptian and (I think) Greek writings from several hundred years BC.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.
friesen@psivax.UUCP (Stanley Friesen) (12/13/85)
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. > 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. Also, a supernova that close would have all sorts of bad side effects here on Earth, it would actually be brighter than the Sun for months! And then there would be some really heavy radiation. Yet a star as light as the current dwarf companion could *not* have evolved into its current state that quickly, certainly not before the more massive visible component achieved the same state. So the question is even more puzzling. How could this configuration even come about? At least unless it has been this way for enough millions of years that the expelled mass has been dispursed. Anyone have a more definitive answer?? -- Sarima (Stanley Friesen) UUCP: {ttidca|ihnp4|sdcrdcf|quad1|nrcvax|bellcore|logico}!psivax!friesen ARPA: ttidca!psivax!friesen@rand-unix.arpa
jeff@utastro.UUCP (Jeff Brown the Scumbag) (12/15/85)
> >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
wyatt@cfa.UUCP (Bill Wyatt) (12/16/85)
> > >... 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. > > > > > That Sirius B is now less massive than Sirius A is not that unusual; > ... > The problem with the Sirius system, to my mind, lies in the assumption > that something has happened to it in historical times. ... > ... 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 > Astronomy Department, U. of Texas, Austin I completely agree with everything Jeff said, but there may be one (apparent) way a star changes color - by rising from the horizon. The ancient Egyptians identified the start of summer as the first time Sirius was visible rising in the dawn sky. I believe this was near the summer solstice, and is also where we get the term "dog days" [i.e. hot summer days. We don't see it that early anymore, because of ~4000 years of precession (Thuban, in Draco was the pole star back then). The point is that a star on the horizon will be red (and twinkling like mad), just as the sun reddens at sunrise and sunset. In a dusty climate, the reddening would also be enhanced. My speculation is that the reference to Sirius as a red star refers to its appearance just as it rises, which is when the ancient Egyptians would have been most interested in it. Bill Wyatt -- Bill UUCP: {harvard,genrad,allegra,ihnp4}!wjh12!cfa!wyatt Wyatt ARPA: wyatt%cfa.UUCP@harvard.ARPA
friesen@psivax.UUCP (Stanley Friesen) (12/18/85)
Thank-you for your response, it has cleared things up considerably. In article <174@utastro.UUCP> jeff@utastro.UUCP (Jeff Brown the Scumbag) writes: >> >> 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. > >That Sirius B is now less massive than Sirius A is not that unusual; > 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. I had heard of this phenomenon but I was unsure whether the Sirius system was a close enough binary for this kind of mass exchange to occur. Then as you mention below there is the mass loss to planetary nebulae, which would still be visible if Sirius B had been a red giant in historical times. Certainly we both agree that if Sirius B *had* been a red giant in historical times there would still be easily observed remnants of that condition in the Sirius system. > > 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.) > I know that *one* type of supernova leaves a neutron star, but I believe there are at least *three* different types of supernova, each with a significantly different mechanism. So is it really established that *none* of the types of supernova could possibly leave a white dwarf ?? (Note: my main recent source is the article in Scientific American a few months ago - I may have to reread the article to refresh my memory on the different types of supernova) >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. To say the least, given the absence of any observable anomaly in the system other than the Algol Paradox! -- Sarima (Stanley Friesen) UUCP: {ttidca|ihnp4|sdcrdcf|quad1|nrcvax|bellcore|logico}!psivax!friesen ARPA: ttidca!psivax!friesen@rand-unix.arpa
msb@lsuc.UUCP (Mark Brader) (12/19/85)
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.