Aspiring young life forms seeking a safe habitat in the Milky Way should find a good stable star of 0.8 to 1.2 solar masses, settle down, avoid crowds, and above all, stay away from hot, young, heavenly bodies.
The fate of intelligent life in the Galaxy may be in many ways analogous to the fate of an individual on Earth. He must survive the embryonic stage and the early years; then, if war, famine, pestilence, or just plain bad luck do not get him, the natural evolutionary process of aging and death will. Similarly, if complex life is to develop and thrive on a given planet, there must be no miscarriage because of too much or too little starshine. It must survive supernovae, giant flares, interstellar dust clouds, and large-scale galactic convulsions, only to succumb eventually when the parent star burns out.
My purpose is to discuss these cosmic vicissitudes of life and to show that, in spite of them, life should persist in abundance in the Galaxy for essentially the same reason that communication with that life will be so difficult: the vast interstellar distances that at the same time protect and imprison us.
Broadly stated, the critical cosmic conditions for the rise of intelligent life on a planet are that the planet must receive the proper amount of energy from the parent star for the proper length of time. Calculations of model atmospheres (Hart, 1978, 1979, and references) show that the proper energy  flux can, with some degree of confidence, be assumed to be the flux received at Earth from the Sun, , plus or minus 10% give or take a few percent.
The parameter that determines the evolution of a star is its mass. For stars having masses in the range 0.4-7 solar masses (), the luminosity during the hydrogen burning, or main-sequence phase, varies with approximately the fifth power of the mass: The energy supply E of a star is proportional to the star's mass, so the main-sequence lifetime tMS is of the order Since the Sun with mass has a main-sequence lifetime of about 10 billion years, we find
for stars in the mass range 0.4-7 .
At the end of this time, the core of the star, having exhausted its supply of hydrogen, collapses until it is stabilized by a new set of fusion reactions that convert helium into carbon. These reactions produce energy much more rapidly than do the hydrogen fusion reactions. To accommodate this increased flow of energy, the outer shell of the star must expand. The radius and luminosity of the star increase by a factor of a 100 or more. The star has become a red giant. The main-sequence phase has ended and, with it, in all likelihood, any life in orbit around the star.
In terms of the evolution of life we can say that, if intelligent life has not developed around a star by the end of the star's main-sequence lifetime, it never will. If we assume that the time for evolution of intelligence is at least 1.5 billion years, then by equation (1) it could only happen around stars with masses less than about 1.6 .
Stars much less massive than the Sun have very long main-sequence lifetimes, but they also have low luminosities. If life is to evolve on a planet around such a star, it must be very close to the star, say as close as Mercury is to the Sun, in order to receive an adequate amount of starlight. But if the planet is too close, the star will raise huge tides on the planet. These tides will tend to bring the planet's rotation into synchronism with its orbit so that the same side will face the star at all times. The oceans on the near side will boil, and on the far side they will freeze, making it impossible for intelligent life to evolve. This argument eliminates stars with masses less than about 0.7 (Dole, 1970; Project Cyclops, 1973).
More restrictive conditions have been derived from consideration of the evolution of planetary atmospheres. Computer simulations taking into account all the major processes affecting the bulk composition of the atmosphere and the mean surface temperature of the planet show that the parent star must evolve in a manner very similar to that of the Sun to avoid either a runaway greenhouse effect or runaway glaciation. Low-mass stars evolve too slowly. If the planet were far enough away from such a star to avoid over  heating early in its evolution, it would be too far away to avoid glaciation later since the luminosity of the star will not have increased sufficiently in the intervening eons. Conversely, massive stars evolve too quickly and produce too much ultraviolet light.
In summary, the only stars that have continuously habitable zones around them are ones in the range 0.8 to 1.2 . Bearing in mind the uncertainties in atmospheric models and the sensitivity of the results to the planet size, this range should perhaps be expanded somewhat, but it does reinforce our conviction that only stars similar to the Sun are likely to support the rise of intelligent life.
The evolution of a nearby star to the red giant phase will not significantly affect the overall energy budget of a planet in orbit around a mainsequence star. If Sirius, at a distance of 8 light years, were to become a red giant with a luminosity of a thousand Suns, then the flux incident at the top of our atmosphere would be changed by only about 30 millionths of 1%. To alter the total energy input significantly, a star would have to attain the luminosity of a thousand Suns and be located roughly at the orbit of Pluto. The only places where we find stars this close together are in binary systems or in the nuclear core of the Galaxy, both of which regions are unlikely to support life for other reasons. Of course, gross energy flux is not the whole story. The climatic stability and the continued good health of living things depend on the spectral distribution as well as the magnitude of the incident radiation. In particular, a large (thousand-fold) increase in high-energy radiation could be fatal either indirectly through the changes it would cause in the upper atmosphere and hence the climate or directly by radiation damage to living tissue.
The most probable cause of a large increase in high-energy radiation is a Supernova explosion, although giant stellar flares or explosions in the galactic nucleus are also possibilities. The cause of a supernova outburst is still the subject of intense investigation and considerable controversy, but it is generally agreed that the onset of the explosion is ultimately related to instabilities in the structure of the star that arise when the supply of nuclear fuel in the core is exhausted. These instabilities occur only in stars that arrive at the  end of the red giant phase with masses greater than about 1.5 . The density and temperature in the central core in such stars exceed the critical values beyond which stability is impossible. The star collapses, and a supernova explosion ensues.
For almost 2 weeks, the supernova radiates more energy than a billion Suns and ejects matter at velocities approaching the velocity of light. The expanding shell of debris creates a nebula of hot gas and high-energy particles that, for hundreds or even thousands of years, radiates vigorously in both the x-ray and radio regions of the spectrum. In addition, the cataclysm may leave behind a small, extremely dense core known as a neutron star. These stars have been detected as pulsating sources of radiation-pulsars.
About one star in a hundred will eventually become a supernova of type II (Type I supernovas seem to be associated with mass-exchange processes in close binary systems.) These are the hot, bright young stars that go through their evolution quickly because of their large mass (see eq. (1)). They are located primarily within the spiral arms of the Galaxy, and it is there that the frequency of supernova explosions is highest. Clark et al. (1977) have shown that a supernova is likely to occur within 30 light years of the Sun approximately once every 100 million years during the Sun's passage through a spiral arm.
In discussing the extent to which a nearby supernova explosion could change the ionizing radiation environment, I shall start with changes we can be sure of and work my way gradually out on a limb as I discuss more speculative possibilities.
The normal cosmic contribution to the ionizing radiation environment consists of cosmic rays, which come from the Galaxy and perhaps beyond, and ultraviolet, x, and gamma radiation, which comes from the Sun. Table 1 summarizes these contributions.
Type of radiation
Approximate flux at top of atmosphere, erg/cm2/sec*
Visible light (3000-6000 Å)
Ultraviolet (2000-3000 Å)
X-ultraviolet (100-1000 Å)
X rays (100-0.1 Å)
 The characteristic signature of a supernova remnant is the emission of polarized radio waves. It is well established that this radio emission is produced by very high-energy electrons trapped in the magnetic field of the remnant. From the observations we can obtain reliable estimates of the cosmic ray content of supernova remnants. For remnants between a few hundred and a few thousand years old, the cosmic ray content is in the range 1049 to 1050 ergs (Woltjer, 1972), assuming that 10 times as much energy goes into cosmic ray protons as into the electrons.
This estimate does not take into account the cosmic rays that have escaped the remnant. Considerations of galactic cosmic ray and gamma ray fluxes indicate that on the order of 1050 to 1051 ergs of cosmic rays are produced by each supernova during its explosion and subsequent evolution (Lingenfelter, 1969; Stecker, 1975; Higdon and Lingenfelter, 1975; Chevalier et al.,1976).
For a supernova occurring at a distance of 30 light years, the cosmic ray cloud would take 103 to 104 yr to sweep over Earth. During this period the cosmic ray flux on Earth would increase by a factor of 1000.
A more speculative possibility is that large quantities (3 x 1050 ergs) of cosmic rays are expelled during the early phases of supernova explosions, producing a relativistic blast wave that would sweep over Earth in about 100 years In this event the cosmic ray background would be increased by a factor of 10,000. This possibility is more speculative because as yet we have no confirmation of the theoretical models. However, in quasars and other violently active extragalactic sources, explosive events in which 1051 ergs of high-energy particles are released on a time scale of months are commonly observed. It is possible that these are just the early phases of supernova explosions, which have been suggested to occur at the rate of 1 to 10 per year in the nuclei of active galaxies (see Arons et al.,1975).
The biologic effects of a nearby supernova can be grouped into two broad categories: (1) climatic effects and (2) effects due to changes in the ionizing radiation environment on the surface of Earth.
The effects of an enhanced flux of high-energy radiation on the ozone layer have been estimated by a number of authors (Ruderman, 1974; Reid et al., 1978; Whitten et al., 1976). In particular, Reid et al. (1978) show that the ozone layer would be 90% depleted, and the column density of NO2 would be increased by a factor of 100, leading to decreased absorption of ultraviolet light and increased absorption of visible light. These changes m the radiation budget of the atmosphere could lead to a drop in the average surface temperature of about 3 K, a decrease in global precipitation, and a Worldwide drought, which could be catastrophic.
As a result of the depletion of the ozone layer, the biologically effective ultraviolet radiation dose would be increased by a factor of 10 or more. The ultimate biologic consequences due to cell deaths and increased mutation  rate are unknown, but a large-scale extinction of life on the exposed planet is certainly feasible.
The normal cosmic ray flux produces a radiation dose rate of about 0.03 roentgen/yr (Herbst, 1964), so a thousandfold increase in the cosmic ray flux would imply a dose rate of 30 roentgens/yr. The lethal dose for most laboratory animals is in the range 200-700 roentgens (Terry and Tucker, 1968, and references); so life would not be killed outright, but the accumulated dose could well be fatal. A 10,000-fold increase in dose rate would imply a dose of 300 roentgens/yr and 6000 roentgens in 20 years, which would almost certainly be fatal. As discussed above, such an increase could occur if a relativistic blast wave were produced by a supernova, or if the supernova were to occur at a distance of only 10 to 15 light years.
These numbers show that a nearby supernova could well cause a mass extinction of life on Earth. Perhaps if it were to occur now, we would have the technology to go underground and survive somehow. But if such an event had occurred a few thousand years ago, the human race probably would not have survived. Indeed it has been suggested by a number of authors dating back to Schindewolf (1954) that a supernova explosion was responsible for the mass extinctions that occurred 60 million years ago in connection with the disappearance of the dinosaurs (Krasovski and Shklovsky, 1957; Terry and Tucker, 1968; Ruderman, 1974; Russell and Tucker,1971; Reid et al., 1978).
More to the point, what are the implications for the search for extraterrestrial life? First of all, we could not expect civilizations to thrive in an environment where a nearby supernova occurred every 10 million years or so. Large radiation-sensitive species simply would not have time to establish themselves. Of course, it is possible that radiation-resistant populations such as insects might develop intelligence under these conditions. If we ignore this possibility, then we must eliminate those regions of the Galaxy where the density of massive stars and hence of supernovae is high: the spiral arms and the region within a few thousand light years of the galactic center. Stars similar to the Sun, which pass through the spiral arms only once every 100 million years, taking 10 million years to make the passage, would be expected to be just marginally safe.
The galactic nucleus may be an inhospitable place for more reasons than one. Compared to some other galaxies and to quasars, the power output of our galactic nucleus is unimpressive. However, the nucleus shows unmistakable evidence of a violent past history, the most direct evidence being  clouds of gas streaming out of the center at high velocity. The largest of these features, which is detectable some 10,000 light years from the galactic nucleus' implies an explosion 500 million years ago which released the energy equivalent of a hundred million supernovae (see Oort, 1977). If other active galaxies are a guide, this did not occur all at once, but m a thousand smaller explosions over the period of a few million years. Thus any x- or gamma-ray flash would have a luminosity of 1045 ergs/see at most, and would produce a noticeable but not catastrophic enhancement at Earth. Most of the cosmic ray energy would escape the Galaxy in a direction perpendicular to the plane, and so would have little effect either. In conclusion, explosions in the galactic nucleus could wreak havoc within a few thousand light years of the nucleus, but are unlikely to cause problems further out.
Reid et al. (1978) have shown that a very large flare on our Sun (100-1000 times the largest ever observed) would affect the ozone layer in much the same way as a supernova occurring at a distance of about 30 light years. Some stars do give off flares of this magnitude, but they have large turbulent convective zones that are presumably responsible for the greatly enhanced surface activity. Presently accepted solar models do not predict such large convection zones; but these models also predict that neutrinos should be emitted by the nuclear reactions in the core and that these neutrinos should be detectable at Earth (see Ulrich, 1975). They are not, which has led to speculation that the Sun undergoes periods of large-scale convective mixing, which could in turn allow the possibility of large solar flares. If the Sun does it, then so should other stars similar to the Sun, and these superflares should be detectable over distances of a 1000 light years. Eventually, data from x-ray observatories should allow a test of this hypothesis. Until then, I feel we cannot rule it out.
An encounter between the Solar System and a dense cloud of interstellar material during its passage through a spiral arm of the Galaxy might produce a climatic change by shielding Earth from the solar wind, thus changing the quantity of energetic particles reaching Earth from the Sun (McCrea, 1975; Begelman and Rees, 1976). It is doubtful, however, that the effects would be as catastrophic as those discussed previously.
Life on any given planet can be adversely and even fatally affected by a variety of cosmic catastrophies. My advice to any aspiring young life forms seeking a safe place in the Galaxy would be similar to the advice a father might give his son as he prepared to venture into the world: find a good stable star, settle down, avoid crowds, and above all, stay away from hot, young, heavenly bodies.
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