From: Robert J. Bradbury (bradbury@www.aeiveos.com)
Date: Wed Nov 24 1999 - 02:14:53 MST
On Tue, 23 Nov 1999, Eric Watt Forste wrote:
> Perhaps Universe is so young that we are the first kids on the
> block.
Doubtful, Kardashev points out in several of his articles that
the age of the universe is 2, perhaps 3 times the age of our
solar system.
>
> ... Lighter metals such as carbon and oxygen are produced in
> medium-size stars and distributed by planetary nebula ejection and
> by white-dwarf novas. The heaviest elements are made and distributed
> only in supernovas. There are several different kinds of events,
> and each has a different characteristic rate, and the rate for each
> event varies from galaxy to galaxy.
The heavy metals get produced very rapidly, within a few hundred
million years of the formation of the galaxies (high H gas cloud
density leads to rapid formation of stars with M > 10 M_sun
that go supernova after very short lives).
>From Astronomy Today (1997):
Star Class Mass Luminosity Lifetime
(M_sun) (L_sun) (years)
Rigel B8Ia 10 44000 20*10^6
Sirus A1V 2.3 23 1000*10^6
A. Centuri G2V 1.1 1.4 7000*10^6
Sun G2V 1.0 1.0 10000*10^6
P. Centuri M5V 0.1 0.6 1000000*10^6
So, the big stars burn very fast. Supernovas, I believe also
throw out a fair amount of C & O. I'm unsure of the relative
contributions of "life elements" by novas vs. SN but am fairly
sure that you get sufficient materials within the first
couple of billion years to form planetary systems that
can support life. A more serious constraint might be
the gradual decline in nearby SN that would fatally
damage proto-genetic material. But we have discussed that
extensively. And of course Deinococcus radiodurans does show
that life can be radition tolerant.
> I did find a textbook of
> cosmochemistry that looked promising, but if the numbers I'm looking
> for were in there, they were deeply buried in mathematics that I
> have yet to do the homework for.
The best book, IMO, is "Nucleosynthesis and Chemical Evolution of Galaxies"
by Bernard E. J. Pagel, and you are right that the numbers are probably
buried in the math and a collection of astronomy papers.
>
> Another way to estimate these rates is to look for a metallicity
> gradient in redshift. How much richer in metal are nearby (older)
> galaxies with respect to distant (younger) galaxies? I haven't yet
> done any research in this direction. I don't even know whether it
> is more difficult to measure metallicity from distant-galaxy spectra
> than from individual star spectra.
It is more difficult, because the lines will be weaker and you will
need longer observation times. You probably also have an interesting
problem of an assortment of star ages in a galactic "point source"
and the line doppler shifting caused by stars rotating toward you
and away from you (in side on) galaxies. You may be limited to
picking a handful of stars of a specific spectral class and measuring
the average metal abundances in them. But since stars in our
galaxy have some really wide ranges of metal abundances (probably
depending on local space region history) extracting an overall
accumulation of metal profile for a galaxy seems really difficult.
This type of work has been done in globular clusters that seem
to be old (because the metal content is low). But this fails
to consider whether the metal content is low because they might
have been mined. If you need "rare" elements and are planning
to be around for trillions of years, it might make sense to
construct "star-processing" stations. You then run simulations
of galactic orbital dynamics and locate good sling-shot candidates
whare a minor orbital tweek will send the star out of the galaxy to
one of the "star-processing" stations (aka globular cluster).
The stars we now see there are the leftovers.
>
> In the absence of this information, I'm comfortable with the
> assumption that most planetary systems formed before 4.6 gigayears
> ago (when ours formed) were below the metallicity threshold required
> for the spontaneous development of self-reproducing molecules.
Bad bet, IMO. I'd push the threshold for planetary systems back
to 8-10 Gyr. Keeping in mind that *if* star mining can occur
on a large scale, then the stellar age estimates based on metal
accumulation will be *underestimates*.
The problems the biotech people are going to have with the
right and left wing luddites are going to be small compared
to the problems the transhumanists are going to have getting
the astrophysicists to consider a "life-filled" universe.
Robert
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