The Astronomy Nexus

All the stars in The Encyclopedia of Suns have one feature in common: They all have a luminosity, or total energy output, similar to that of the Sun. More specifically, they have a luminosity that is consistent with a star that can produce, and then maintain, life-supporting conditions on planets orbiting it. The range of luminosities for stars in the Encyclopedia is 10% to 300% that of the Sun. Details behind the choice of the upper and lower limits are given below.

Brighter stars / Upper luminosity limit

Stars produce energy from nuclear reactions, primarily by converting hydrogen to helium. For most of a star's "life", it burns hydrogen stably, changing relatively little in size, temperature, or brightness. This stage is called the "main sequence". Once a star has burned enough hydrogen, it becomes much cooler, brighter and larger -- a so-called "red giant". A red giant can be more than 100 times brighter than it was as a main-sequence star. By this point, any life-supporting planets around the star will have been sterilized, if not destroyed altogether.

As a rule, brighter stars spend less time on the main sequence than dimmer ones. Such stars have more mass and hence more nuclear "fuel", but luminosity increases much faster than mass (roughly as the 4th power of mass), so brighter stars tend to evolve much more quickly. Earth's history shows that life needed at least several billion years to become complex. The Cambrian epoch, which marked a widespread proliferation of complex multicellular life, occurred about 600 million years ago, or 4 billion years after the Sun and Earth formed. Thus stars with lifetimes in the 4 billion year range, and which have planets on which life evolves much as it did on Earth, are capable of supporting fairly complex forms of life before becoming red giants.

A 4 billion year lifetime corresponds to a luminosity of approximately three times solar, so I have adopted 3x solar as my upper limit. Given the uncertainties in our understanding of biology, this is not an absolute limit, but it is a convenient one -- stars dimmer than 3x solar luminosity can have planets whose history is similar to the Earth's.

Dimmer stars / Lower luminosity limit

Stars dimmer than the Sun pose no problems with stellar evolution, but do pose their own unique problems. The most serious is tidal locking. Planets orbiting closely enough to a dim star to be Earthlike (e.g. to have liquid water, etc.) experience much stronger tides than the Earth experiences. Because of friction, tides slowly decrease a planet's rotation -- i.e., they make its day longer. Over a long enough time the rotation slows so much that one side of the planet always faces the star, much as one side of the Moon always faces the Earth. Under such circumstances, a planet will become intensely hot on the star-facing side and
intensely cold on the dark side, making the development of life extremely difficult or impossible.

Although tide-locking is a known phenomenon, it's hard to figure out exactly when it becomes a serious problem. Stephen Dole, in Habitable Planets for Man, estimates that tide-locking affects all potentially Earthlike planets around stars below 0.72 solar masses, which works out to stars with less than 25% of the solar luminosity. More recently, Kasting et al. (Icarus 101, 1993)
suggest that tide-locking becomes a serious problem for planets orbiting stars between 0.5 and 0.6 solar masses (depending somewhat on the planet's orbit). Those mass limits correspond to luminosities of 6% and 12% of the Sun's, respectively.

The newer calculations are more quantitative and so I have leaned toward them. For simplicity's sake, I have roughly split the difference and taken a luminosity of 10% for my lower limit. Stars appreciably below 10% solar luminosity are likely, based on present knowledge, to tide-lock prospective habitable planets before complex life can evolve on them.

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