Extraterrestrial Civilizations - Isaac Asimov [56]
The most massive stars known are some 70 times the mass of the Sun, but they are 6 million times as luminous. On the other hand, a star with only 1/16 the mass of the Sun (65 times the mass of Jupiter) might be just massive enough to glow a dull red heat, and it would only be one-millionth as luminous as the Sun.
What would it be like for a planet circling a star at such extremes?
Suppose, for instance, Earth were circling a star 70 times as massive as the Sun.
Of course, if Earth were circling this giant star at the same distance at which it circles the Sun, the star would appear forty times as wide in the sky as the Sun does to us, and it would deliver 6 million times as much light and heat. The Earth would be a ball of red-hot rock.
We can easily imagine, however, that every star has a shell around it at some distance, within which a planet could circle and be heated by the star it circles to Earthlike standards of comfort. For a large star this shell, or “ecosphere,”* would be farther away than for a small star. In the case of the 70-time-Sun giant, the ecosphere would be at a distance of hundreds of billions of kilometers from the star.
Suppose, then, that the Earth circled the giant star at a distance of 366 billion kilometers (227 billion miles). This would be a distance 2,450 times the distance of the Earth from the Sun and 62 times as far as Pluto is from the Sun. At such a distance it would take 14,500 years for the Earth to revolve about the star.
From that magnificent distance, the giant star would seem very small, so small that it would show no visible disc, but would shine merely like a star, but not like the stars we see. It would be extraordinarily bright because its temperature would be so much higher than that of the Sun (50,000° C as compared to a mere 6,000° C) that even though the giant star was so distant and so small in appearance, it would deliver as much light and heat to the distant planet as the Sun does to Earth.
To be sure, the giant star’s temperature alters the nature of its radiation. At the distance we have imagined for Earth, the star would deliver the same total amount of energy that the Sun delivers now, but a much larger fraction of the giant star’s energy would be in the form of ultraviolet light and x-rays, and a much smaller fraction would be visible light.
Human eyes are adapted to respond to visible light so that the light of the giant star would seem dimmer than that of the Sun. On the other hand, the flood of ultraviolet and x-rays would be deadly to Earth life.
Yet perhaps this is not a fatal objection. The Earth’s atmosphere protects us against the energetic radiation of our Sun and we can imagine Earth moved still farther from the giant star. The decline in total radiation and the amount stopped by a possibly thicker atmosphere might then be suitable for the development of life at the price of somewhat lower planetary temperatures than we are used to.
There is, however, a more vital objection to the giant star, one that can’t be countered by adjusting the planetary place within the ecosphere or by fiddling with the planetary atmosphere.
A star is not an adequate incubator for life throughout its existence. It cannot supply the energy necessary for life, for instance, while it is condensing and forming out of the primal nebula. It must first condense to the point where the nuclear fires start at the center and it begins to radiate light. Eventually, the condensation reaches a stable stage and the radiation, having reached some maximum figure, remains there.
The star is