pcmcgeer (11/26/82)
I have been pondering a rather Sirius matter of late (not my pun, Larry Niven's), and I thought I'd post it to the net. Now, according to what I know of the theory of stellar formation, all constituents of a stellar system form at roughly the same time (cosmically speaking). In fact, if any constituents of the system form earlier than others, then the larger ones form first. Now, the Sirius system consists of two stars: a hot, bright, young blue star (Sirius-A) and a white dwarf companion (Sirius-B). The white dwarf is a burned-out corpse of a G or K class star. The problem is that the smaller companion should have had a longer lifetime than Sirius-A, since hot blue-white stars burn their fuel much faster than cooler yellow or red ones. Therefore, if the two stars formed at the same time (which they should have), Sirius-A should have become a black hole or a neutron star long before Sirius-B left the main sequence. And yet it didn't. I can see three possibilities: first, that stellar systems don't congeal at roughly the same time; or, second, that one of the two stars is a capture. The third possiblity, which I discount, is that the white dwarf burned faster. In any case, *I* don't know the answer. Do any of you? Rick McGeer. decvax!watmath!pcmcgeer (USENET)
ltn (11/29/82)
Question:
Did Sirius A, now a young, hot, blue star, and its companion, Sirius B, a
white dwarf and presumably the dead hulk of a G or K star (which should have a
much longer life than its hotter companion) form at the same time, or if not,
how did the system form?
Answer:
The two stars did form at the same time, in the same place. Originally,
Sirius B burned much hotter and brighter than its companion. Millions of years
ago, the Sirius system appeared much brighter from Earth than it does today.
Sirius B quickly used up its hydrogen (and then its helium, too) and 'died.'
I'm not sure, it may have gone nova and lost some material that way, too.
But it was not massive enough to collapse into a black hole or neutron star,
and did not go supernova. A star doees *not* have to be a G or K type (yellow
dwarfs, like our Sun) to end up as a white dwarf. Only the most massive stars
will end up as black holes or neutron stars.
Les Niles
Bell Labs, Murray Hill (aluxz!ltn)CSvax:Pucc-H:Physics:els (12/09/82)
The way I understand it is:
1) The two stars formed, one a rather largish(sp?) star, the
other a smaller, but longer living, star;
2) Big one reaches end of lifetime, with two likely possibilities,
a) becoming a red giant, the larger one's outer atmosphere
comes within capture range of the little one, which captures
enough hydrogen to be 'rejuvenated' into a B or A class star,
after which the former red giant collapses into a white dwarf
(now lacking the mass to become something more exotic)
b) becoming a nova, the larger one ejects material, then
the rest of a) repeats.
3) The end result is a binary consisting of an A or B (possibly
an O, but these don't hang around on the main sequence very long) star,
and a white dwarf, a combo much like Sirius.
Hope this clears things up,
els[Eric Strobel]
pur-ee!pur-phy!elspcmcgeer (12/13/82)
Thanks, Eric. Your reply does clear things up a lot. However, it adds fascinating new questions. We know that Jupiter almost became a star: it sill radiates more heat than it receives from the sun. When the sun goes into the red giant stage, is it possible that Jupiter could borrow enough mass from the sun to become a G or K star with a companion white dwarf (remnants of the sun)? Or would Jupiter's orbit be enclosed by the red giant? If it would, is there any chance that Saturn could pull that sort of trick? If so, and if we can move the Earth when the Sun goes into the Red Giant stage, then the Earth might outlive the Sun (at that point, of course, our interest in the Earth is likely simply to be a museum of the past, but we might still want to save it). Rick.
djb (12/15/82)
Question: How much mass does Jupiter have to gain in order to be stellar in mass? Quick trip to the bookshelf. Let's see. The smallest known true stars are the two components of the binary system L 726-8 in Cetus. They are red dwarfs of about 0.04 solar mass. Jupiter is about 0.001 solar mass, so it needs to multiply its mass by a factor of 40. It's possible that some other objects are stars, but they haven't been identified or photographed yet: Object Mass (Sun = 1.0) Comment ******** ************* **************************** L 726-8A 0.04 Proven star - photographed L 726-8B 0.04 Proven star - photographed Companion to WZ Sagitta 0.03 star - Uncertain mass figure Lalande 21185 0.03 star - Uncertain mass figure Companion to Lalande 21185 0.01 unseen, possible star (?) 61 Cygni C 0.008 unseen, possible star (?) Companion to Barnard's Star 0.0015 unseen - assumed planet (?) Jupiter 0.001 known planet If 61 Cygni C turns out to be a star, then Jupiter is only a factor of 8 away from stellar mass. David Bryant cbosg!djb I suppose there must be some hard and fast value of critical mass for starhood. Anybody out there know what the minimum recipe is for making a star?
REM@MIT-MC@sri-unix (12/16/82)
From: Robert Elton Maas <REM at MIT-MC>
Oh boy, you've repeated the fallacy I've heard so many times I've just
got to reply!
We know that Jupiter almost became a star: it still radiates more heat
than it receives from the sun.
What do you mean by "heat"?
If you mean infrared: No, Jupiter emits microwave, not infrared,
mostly. Same with any other planet.
If you mean thermal convection: Obviously not, it's surrounded by a vacuum.
That leaves "total radiant energy", which is what I always assume
you-all mean when you make this claim. But I claim every planet, Earth
et al, does the same, radiates more energy than it receives.
A planet receives mostly near-infrared and some visible light (Sun is
radiating blackbody at somewhee around 5000K with minor absorbtion
lines that don't significantly affect things and minor X-ray and other
emissions that are almost as insignificant), and depending on its
temperature a planet re-radiates, mostly in microwave with some deep
infrared, every Joule of energy it receives, plus a little extra it
generates internally from radioactive decay and gravitational
collapse, plus a little more as its insides gradually cool towards the
surface temperature. Thus every planet emits a teensy more energy than
it receives. The same is true of just about every massive object in
the universe. Deep space absorbs all this waste heat, and would get
hotter over the billions of years if the Universe weren't expanding so fast.
What you-all really mean to say is that Jupiter gives off more than
twice as much heat as it receives, i.e. its internal source of heat
(gravitational collapse and cooling of interior mostly) is more than
its external source of heat, so that its radiated energy is more due
to internal than external sources. Equivalently its personal
contribution to radiated energy is greater than what it receives from
the Sun. This is a significant statement that I'e never heard stated
correctly by anyone who could make that statement with authority. (It
may even be false! How am I to know??) But the statement you made
(quoted above) and that I've heard elsewhere, is a triviality that is
literally true of Earth et all, even the Moon!REM@MIT-MC@sri-unix (12/16/82)
From: Robert Elton Maas <REM at MIT-MC>
Date: 12 Dec 82 17:34:48-PST (Sun)
From: decvax!utzoo!watmath!pcmcgeer at Ucb-C70
When the sun goes into the red giant stage, is it possible that
Jupiter could borrow enough mass from the sun to become a G or K
star with a companion white dwarf (remnants of the sun)? Or would
Jupiter's orbit be enclosed by the red giant?
What I heard/read was that the Sun's red-giant stage would about reach
the orbit of Mars, maybe a little further. I doubt Jupiter could yank
gas from such a distance, and the solar wind would be rushing by so
fast and Jupiter would be such a small target I doubt much gas would
be collected by Jupiter. Perhaps Pournelle or Koolish has more info
about current theories of stellar evolution as applied to Sun.
If so, and if we can move the Earth when the Sun goes into the Red
Giant stage, then the Earth might outlive the Sun ...
As the Sun expands we'll have to gradually move the Earth away or else
put a shade "tree" between Earth and Sun. Eventually we'll have to
move or totally-shield the Earth if we want to preserve it as a
historical landmark. (Gee, literally a little chunk of land in the
vastness of Dyson spheres and space-mobile homes and recreational
space-vehicles and solar-wind sailboats and Bussard ramjets etc.)
We may want to move Venus much earlier to let it cool off so we can
make manned landings on it to learn its geology and to experiment with
its climate.) Adding to its orbital energy by tugging an asteroid just
ahead of it in orbit would seem to be a way to move it out without
physically stressing it too much. A slight continuous accelleration
over a million years ought to do the trick in very-plenty of time
before the Sun's nova stage 5-10 billion years hence.REM@MIT-MC (12/19/82)
From: Robert Elton Maas <REM at MIT-MC> If you define a "star" to be something that glows in the night, then Jupiter and Earth are "stars". We're microwave stars, something hotter is an infrared star, and something yet hotter is a red star, etc. I think we have to define a star as something that is undergoing nuclear fusion rather than merely using gravitational collapse to convert potential energy into knetic energy. A star big enough to undergo nuclear fusion but which hasn't yet collapsed enough to start burning, is a pre-star. Also a smaller object that is getting more massive due to matter flowing from a companion, such that eventually there'll be enough mass to start nuclear fusion, should perhaps be called a pre-star also. Anything too small and not getting new mass to eventually be big enough, is a non-star, i.e. a simple planet or gas cloud. So, the question is, how much mass is needed to sustain Hydrogen-->Helium fusion, the only kind of nuclear fusion possible initially when a star is mostly Hydrogen? I think this has already been figured out and published somewhere but I don't know for sure where. Anything that we detect as an infrared source, that we measure and find too small to be burning hydrogen, we just have to call a non-star. If we call it a star just because it's hot enough to glow, our definition becomes too weak, as I explained above. -- What <should> we call it? "Non-star" is so general, as is "compact infrared source". How about a "dar" (contraction of "Dark stAR")? We could use that term also for former-stars that have burned their hydrogen and helium and are now black dwarfs.