Re: Why Would Aliens Hide?

Robert J. Bradbury (
Wed, 24 Nov 1999 01:14:53 -0800 (PST)

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.