We may expect that a star needs to be stable over a significant period to foster life on its
planets, so we make the assumption that only planetary systems around
main sequence stars are good places for life to evolve. Since planetary orbits in binary star
systems tend to be unstable except for special choices, we may also guess that stars not in binaries are
better candidates for planets with life.
The preceding considerations indicate that the habitable zone around a star depends on its
surface temperature. Higher temperatures give larger habitable zones and, like cold campers
crowding around a small fire, lower temperatures give smaller habitable zones because lower
temperature main sequence stars are less luminous than those with higher temperatures.
Time on the Main Sequence
But that is not the only factor. We must also consider that the more luminous main sequence stars
live only for short periods before leaving the main sequence. The top right table summarizes the
approximate main sequence lifetime of stars for different spectral classes in units of a billion years.
Since it took as long as a billion
years for life to evolve on the Earth, we may take as a first guess that only stars with main sequence
lives of that length or longer are strong candidates for life-bearing planetary systems. This
eliminates main sequence stars around spectral class A and hotter.
The Optimal Classes
Taking these factors together, and considering our own experience on Earth, we surmise
that spectral class
M is probably too cool to have much chance for habitable planets and spectral class A and hotter do not live
long enough on the main sequence to evolve life. Thus, we conclude that spectral classes K-G-F are the
most likely stars to both have large habitable zones and live long enough for life to evolve in those zones.