Iamon lyme: Awww drat! I did it again! I'm getting ahead of myself... The metal content has nothing to do with lifespan, it has to do with a star system having enough of the heavy elements for creating an earth type planet. Our sun is expected to last a total of about 10 billion years, burning hydrogen steadily on its main sequence... so in this case it IS size that matters.

"Metal" content was about another point I wanted to make, how not just any old star or star system is able to have life just because a star is the right size or has the right luminosity. Drake and Sagan beamed a message to a large concentration of stars called globular cluster M13. The theory was because there are lots of stars in that region there was a higher probability of communicating with intelligent life.

Fat chance of that happening, since globular clusters are the worst places to go looking for life. They are among the most ancient things in the universe, which means their stars have a very low abundance of heavy elements... they're made up almost entirely of hydrogen and helium. The heavier elements are needed for building terrestrial planets. In globular clusters you are more likely to find only dust or grains or maybe boulders, but nothing like an Earth type planet that can serve as a platform for life to exist (much less develop). But here's the kicker, at the time Drake and Sagan sent their message this was already known.