The Galaxy and the Stars-That-Are-Suns

Everyone agrees that the universe is vast and old and loaded with galaxies and stars. Almost nothing in science is more obvious. And because of this, and the foundation stone faith of science in the "Uniformity of Nature," almost no intuition is stronger than that the universe is filled with life. There are many people for whom all that is required to settle that question is one good look at the night sky. The methods and attitudes of science are more slow afoot, however, yet perhaps more sure. The factor in the Drake Equation which takes "one good look at the night sky" is R_*.

R_*, the rate of star formation in our galaxy, seems a straightforward matter, and in fact there is very little debate. If we have a reasonable understanding of star birth, we can look to likely galactic locations and make a direct estimate. Or, if we have a reasonable history/timescale of the galaxy and a good star count, we can divide stars by time and get another estimate. Both approaches have been taken and the results are given with an aura of confidence: our galaxy has averaged about 25 star births per year, and has perhaps slowed down to between 1 and 10 starbirths per year in its current mature stage of development.

This author prefers to alter the meaning of R_* to remove some of the confusion which enters later factor-analyses in the Drake Equation. Because some stars are never suitable for life-formation, and others become unsuitable as their life histories progress, it seems appropriate to settle the "star question" all at one at the beginning, and to eliminate unsuitable categories of stars now. This amounts to changing the concept R_* to R_<, the rate of "sun-formation" in the galaxy. "Sun" is here defined in its limited sense as a star possessing the proper lifespan, metallicity, and force-environment (re: Luminosity; stability; companion stars) such that a life-advancing timescale and planet-formation were at least possible.

How many proper stars or suns are born in the galaxy per year? The question is less difficult than it may seem. In fact there is also little debate about it in the literature. The key assumptions are regarded as conservative:

1. Life in advanced forms needs a long time to evolve, perhaps 2 to 6 billion years. Any proper star should have a lifetime at least that long;
2. Life in advanced forms needs a planet to develop upon. Any proper star should have arisen from a molecular cloud rich in heavy elements so as to make planet formation at least possible;
3. Life in any form needs a hospitable energy environment, not involving wild energy swings and radiation bursts. Any proper star should allow stable orbits for rotating planets and planets beyond radiation flare zones.

Assumption "a" eliminates all fast-and-hot burning blue giant stars of the so-called O, B, A, and upper-F classes. Assumption "b" eliminates all so-called first generation stars, stars arising early in the history of the galaxy from the only available elements of that era: hydrogen and helium. Forming as they did before the building and dispersal (by supernovas) of the heavy elements, there was no heavy material to initiate planetary cores, ergo no planets, no base upon which to evolve ecologies.

Assumption "c" eliminates several categories of stars. No stars close to the galactic center are candidates due to extreme violent energy environments throughout the nucleus area. In fact it has been postulated that the nucleus occasionally erupts violently in extreme forms of radiation outbursts, the waves of which would scour at least the near-nuclear systems of life s1Clarke 1981. Such outbursts could be violent enough to destroy ecologies galaxy-wide unless their systems were shielded in the galactic arms when the "killer wave" passed by. On the other hand such shock waves could be the impetus for new star-system condensation and be ultimately a "biogenic" wave instead. Either way, the concept of the Milky Way as an occasionally explosive Seyfert galaxy brings an unknown but potentially time-synchronizing element into the discussion about the level of advancement of galactic ecologies.

Other stars are eliminated by assumption "c" as well. No small cool red-dwarf stars or so-called M and Lower-K classes are proper suns. Their relatively dim heat sources require planets so close as to be at risk from solar flaring and to be gravitationally locked (one face always roasting while the other freezes). A third category, multiple star systems, might be eliminated due to the planetary formation and orbital destabilization problems caused by the gravitational dynamics between the close stars. Many multiple star systems have been shown to all stable close-in planetary orbits, however, and the estimates of acceptable multistar systems vary from 10 to 90% s2Ksanfomality 1986 s3Gilette 1984 s4Dole 1964 s5Harrington 1977.

When we take our "good look at the night sky" with these restrictions in mind, we find that our galaxy has about 250 billion stars. Eliminating the mass at the nucleus and the non-heavy-metaled star systems of the halo, we are left with about 100 billion disk stars. Getting rid of the few large bright stars and the many small dim ones, and about half of the remainder which exist in multiple systems (keeping the other 50% of the sun-like multiple partners), we are graced with a total of about 6 to 15 billion "proper stars," or suns.

These are the later generation stars of the lower F, G, and upper K classes, most single but some in permissible double-star arrangements, and all in the galactic disk. If these stars formed at a somewhat regular rate across galactic history, there would have been about one per year. Because we are interested in the formation rate far back into the past (5 billion years ago when our solar system was being born) so as to estimate civilizations of our level of advancement or greater, perhaps this would be the most accurate figure to accept. Our system formed about halfway into the current lifespan of the galaxy. The use of R_< = 1 is, if anything, conservative, as there was certainly an initial period in galactic history when no high-metallicity stars formed whatever, and so the proper stars we count are probably more bunched toward our own time frame. But, R_< = 1 is an acceptable starting point... and 6 to 15 billion sun-like environments.

Such a beginning springboard of the imagination could lead a prominent scientist such as Philip Morrison of MIT to state it is both timely and feasible to begin a serious search for extraterrestrial intelligence, while almost simultaneously declaring about ufology: I have now, after a couple years of fairly systematic listening and reading, no sympathy left for the extraterrestrial hypothesis (quoted in Ridpath 1975).

As this is on the surface of things an extremely puzzling dichotomy of positions, and yet one which seems to accurately reflect establishment scientific thinking, we must proceed on in search of some explanation.