Stars are classified both by their spectral classes; O, B, A, F, G, K and M, and their luminosity classes. This information can be obtained through a careful study of the spectral of these stars, and certain atomic lines which are sensitive to the luminosities of the stars. The Luminosity classes are:
Class..........Type..............Luminosity.... I Supergiants 100,000 suns II Bright Giants about 700 suns III Giants about 70 suns IV Sub-giants about 5 suns V Dwarfs about 1 sun .................................................
where the absolute luminosities I give above reflect those of stars near spectral type G. At other spectral types, the luminosities may be very different in absolute magnitude.
A partial list of bright, G-type sub-giants is:
Star.......................Spectral class..............Magnitude Beta Hydra G1 +2.8 94 Ceti G0 +5.0 34 Lyncis G8 +5.4 Phi Virginis G2 +4.8 Zeta Herculis G0 +2.8 Mu Herculis G5 +3.4 Delta Pavonis G6 +3.6 Kappa Delphinus G2 +5.0 ................................................................
Subgiants are not necessarily more massive than their Main Sequence, dwarf counterparts because luminosity depends on the star's surface temperature and radius. These stars are all roughly the same temperature since they are mostly early G-type stars, so the fact that their luminosities are 5 - 10 times that of the Sun could just mean that they are 2 - 3 times larger which makes their surface areas 5 - 10 times that of the Sun.
These stars would also be expected to have habitable zones around them which would be a bit further out from these stars than the distance from the earth to the Sun. For a star 10 times the luminosity of the Sun, the zone would have a mean radius of about twice the Earth-Sun distance. But there is a catch to this.
The only problem is that these stars are on the way to their eventual demise as red giants, and within a few hundred million years, a major change will happen in the location of the life zones in their environment which will be lethal for any organisms that had managed to evolve there in previous billions of years. Our own Sun will become a sub-giant in a few billion years, and in a far shorter time than that, our biosphere will be incinerated by a runaway greenhouse effect by 500 million years hence.
The figure above shows the limits to the habitable zone compared to our solar system. For a sub-giant of the same mass as the Sun, the zone is displaced outwards and moves to the right. For the Sun with 10 times its current output, the zone is about 3 times further out. This means that the Earth will find itself outside this life zone with a toasted biosphere!
Copyright 1997 Dr. Sten Odenwald
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