Re: Surviving your star's red giant phase
- From: James Burns <burns.87@xxxxxxx>
- Date: Tue, 29 Aug 2006 09:05:14 -0400
sigidunum@xxxxxxxxx wrote:
Let's assume an alien race living on a mostly earthlike planet around a
mostly sunlike star. There's one big difference: the star is about to
leave the main sequence and become a red giant.
For simplicity's sake we'll say the star is pretty much exactly like
the Sun, except older. (The Sun will leave the main sequence in about
another 6.5 billion years.) (1) This means it's also brighter than
the Sun... main sequence stars grow slowly brighter with age, and by
the time the Sun finishes its main sequence core-hydrogen-burning
period, it'll be more than double its current luminosity. Let us
assume that the alien race evolved relatively recently, so that their
star's current luminosity seems "normal" to them. (2)
So their sun is leaving the main sequence. What now?
My understanding is that leaving the main sequence doesn't involve
anything obvious and dramatic. The first sign is a very gradual
reddening, combined with a somewhat faster brightening. In the case of
our Sun, it will redden down to a surface temperature of about 4900
degrees, which would be about the temperature of a typical late-K
orange dwarf. However, this will be a very slow process... the initial
reddening will take about 700 million years. Over this same period,
the brightness will increase by about 20%, from about 2.2 to 2.7 solar
luminosities. Assuming that the aliens evolved recently, the reddening
and brightening will have started far back in their planetary history,
so there will have been plenty of time for the biosphere to adjust.
The next step is that the star's long, slow increase in luminosity
suddenly accelerates as it departs the main sequence and becomes a
subgiant. In the case of our sun, it will brighten from about 2.7 times
its current luminosity to 34 times -- about an eightfold increase -- in
maybe 500 million years. Assuming this increase is more or less
linear, the homeworld will be uncomfortable in just a few million
years, probably uninhabitable within ten or twenty million. Still,
there's time to work on the problem.
Step three... now it gets tough. Helium burning kicks in and the star
becomes a no-kidding red giant. Over the next 80 million years, it
swells up to 160 solar diameters, cools down to 3100 degrees Kelvin
(lower middle M class) and peaks at a whopping 2300 solar luminosities.
Yikes.
Of course, since we said the aliens are comfortable at 2.7
luminosities, this is "only" a 900-fold increase. But still. -- Oh,
and also the solar wind increases something like a millionfold.
Probably the least of their problems, but worth mentioning.
After that... well, more stuff happens, including helium flashes that
will briefly raise luminosity to over 4000; and then the white dwarf
stage. But let's see if we can survive the initial ascent of the red
giant branch.
Assume for argument's sake that the aliens are homebodys who would
prefer to keep their planet. Assume further no god-tech. Yes, I know
that one man's god-tech is another's reasonable extrapolation of
straightforward engineering. So let's say the aliens have a plausible
late 21st century level of technology. Say further that they're
politically united and can make long-term plans over geological time.
In terms of resources they have a Solar System very similar to ours.
Our Solar System may look a bit different 6.5 billion years from now,
and then there's the metallicity question (2), but I handwave those
issues away [handwave]. Let's keep this simple.
They want to keep the homeworld habitable as long as possible. What
can they do?
Two obvious answers: (1) move the planet; (2) shade the planet. There
may be others. But note that "keeping the homeworld habitable" means a
fully habitable shirtsleeve surface environment for as long as
possible. Digging underground is not favored.
I think that shading might work for the subgiant phase, but by the time
we reach the giant phase it's probably time to start packing. In our
solar system, the surface of the Sun will expand to within a few
million miles of the Earth's orbit. The immense red Sun will fill half
the sky, and it will be obvious that we're skimming around it in a very
low orbit. Shades or not, this seems too close for comfort.
Moving an inhabited planet is tricky, but over millions of years it
ought to be doable. Tens of thousands of momentum-adding asteroid
flybys?
see earlier thread about a Clever Idea for re-engineering
the solar system. includes refs
.
AFAIK there's been one SFnal treatment of this: Niven's _World Out Of
Time_. That had the Sun prematurely pushed into a subgiant phase, and
the Earth parked in orbit around Jupiter. Niven had a fairly dramatic
method of moving the Earth: an enormous fusion ramjet floating in the
atmosphere of Uranus. Mobile Uranus could be used to fling other
planets around in a rather Velikovskian manner. I recall that this
made me go "hmmm", but I don't know if anyone has shown it impossible.
Thoughts?
Doug M.
(1) All assumptions taken from this paper:
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1993ApJ...418..457S
Note that this is from 1993, and may be slightly dated.
(2) As ours seems to us. The Sun has brightened by about a third over
the history of life on Earth, and the biosphere has adapted so far. At
some point solar warming will tip the Earth into runaway greenhouse;
estimates of that date vary, though the latest is "just" a couple of
billion years away.
(3) A solar system 6.5 billion years older than ours will almost
certainly have much lower metallicity, and may not even have rocky
planets.
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- Surviving your star's red giant phase
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