On Thu, Jun 24, 1999 at 04:31:29AM -0500, Eliezer S. Yudkowsky wrote:
> > As for the orbit of the Sun, its circularity prevents
> > it plunging into the inner Galaxy where
> > life-threatening supernovae are more common. And
> > its small inclination to the Galactic plane prevents
> > abrupt crossings of the plane that would stir up the
> > Sun's Oort Cloud and bombard the Earth with comets.
> > By being near the Galaxy's corotation radius, the Sun
> > avoids crossing the spiral arms too often, an event
> > that would expose it to supernovae, which are more
> > common there.
> > Because life-bearing stars have to be close to the
> > corotation radius, that rules out more than 95 per
> > cent of stars in the Galaxy in one fell swoop.
To our benefit, in the long term :-)
Note that there will be lots of stars with slightly less circular orbits where multicellular life never gets a chance to start because every two hundred megayears or so it gets toasted by the core supernovae. But this won't matter a lot to humans intent on finding lebensraum; just find a nice, sterile terrestrial planet orbiting a star that's heading up and out of the core and you've got, oh, the best part of a hundred million years to terraform it then live there in comfort. (Assuming that the rustic carbon-based way of life appeals to you.)
In short: probably not a lot of systems where multicellular life exists, but loads which a suitable for colonization.
(One thing most interstellar SF ignores is that colonizing a planet that already supports a complex biosphere may be a rather, ahem, interesting experience, immunologically speaking.)