° This column represents the percentage of all stars. Stars not on the main sequence (primarily giants) make up the remaining 0.77 per cent. For these stars, spectral class and mass are not directly related.
The giants, or stars that have left the main sequence, are seen to account for less than 1 per cent of all stars, although they are extremely prominent in the night sky because of their great luminosity. Those stars of major interest to us as possible primaries in a system containing habitable planets are relatively inconspicuous objects having absolute visual magnitudes in the range from about +7 to +3. Absolute magnitude is a measure of the appearance a star would have if viewed at a distance of 10 parsecs, or 32.63 light-years. The absolute magnitude of our Sun, for example, is +4.8, while the faintest stars normally visible to the naked eye in the night sky are of +5 or +6 apparent magnitude. On the other hand, of the one hundred brightest stars in the sky, all but one have absolute magnitudes in the range -f~3 to —7. The exception is Alpha. Centauri
* One parsec equals 3.26 light-years; one cubic parsec equals 34.7 cubic light-years.
(absolute visual magnitude, +4.5), which appears bright to us only because of its nearness. From Table 12 it may be seen that the most important stars (in the present context), those main-sequence stars in spectral classes F, G, and K, account for about 25 per cent of all stars. They are not at all rare objects.
Since the mass of a star also determines its lifetime in the main sequence and has a bearing on the size of its ecosphere, the prevalence of stars in the proper mass range Ns must be considered in conjunction with several other parameters to determine the number of habitable planets in the Galaxy.
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