Re: Need excuse for why a star supports life.

Hi Mike,

It is perfectly OK to have planets around an F5 star. Your information that stars with high metal content are more likely to have planets is correct, but F5 stars (or F stars in general) do not necessarily have low metal content. The star tau Bootis, a type F6, has a giant planet, for example. See the Extrasolar Planets Encyclopedia for more details.

As for the differences between G4 III and G5 III stars: the OBAFGKM sequence is a temperature sequence, so a G5 star is a little cooler than a G4 star. If these were stars on the main sequence (i.e., G4 V and G5 V stars), this would mean that the G5 star was a bit less massive and a bit less luminous. Luminosity class III stars are not on the main sequence; once they leave the main sequence, stars pass through several different spectral classes. So two class III stars of different temperatures might have started out with slightly different masses and compositions, or they could be nearly identical stars, just at slightly different times in their evolution. From looking at an H-R diagram, it looks like there's little luminsoity difference between class III stars, but because of the temperature difference, there will be a difference in radius between the stars. There could also be a density difference, but it depends on what the stars' original masses were.

Most college astronomy textbooks deal with this in quite some detail; looking through the section on `stellar evolution' should give you a good idea of what's going on.

Pauline