© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

4.4  The Stars

Stars come in many sizes, brightnesses, shapes and colors. In Orion we find the beautiful orange-red Betelgeuse keeping company with the brilliant bluish-white Rigel. In constellation Auriga lies the Sol-like familiar yellow star Capella. The brightest star in Libra, named Zubeneschamali, is naked-eye green in color and is best seen low in the midnight summer sky.¹¹⁹¹

More than two-thirds of all stars form multiple systems -- double stars, triple stars and more. With a telescope one can observe the gold and blue splendor of g Andromedae, the twin red and green suns of a Hercules, and the exquisite orange, yellow and blue of zCancri.⁴⁹ The stars in eclipsing binaries are often extremely near to one another, so close that the tidal force pulls the smaller sun into an ellipsoidal shape. Gigantic beautiful whorls and ribbons of luminous matter flow from one to the other in complex patterns so faint they can only be witnessed visually by the local inhabitants of these systems. Even with our most powerful telescopes we cannot actually see these processes but must infer them from indirect evidence.²⁰

Besides color and shape, stars differ markedly in their relative luminosity. This property varies among suns across more than eight orders of magnitude -- as much as a hundred thousand times brighter, to more than a thousand times dimmer, than Sol.

If the spectra of a large number of stars are compared, however, certain regularities immediately become apparent. All stars can be divided into relatively few groups whose spectra all look pretty much the same. These are the classes O, B, A, F, G, K, and M. (There are a few others -- R, N, S -- but these are of lesser importance.)*

We’ve already seen that the O and B stars are the hot, short-lived, young and massive suns of spiral arm fame. Classes A and F are less hot and have longer lifetimes. Sol is class G. But the majority of all stars fall into the two classes K and M. These are relatively feeble, undistinguished objects, yet they burn little fuel and live extremely long lives -- more than ten thousand times longer than their O and B counterparts. Luminosity, then, is a rough index of both the rate of fuel consumption and the life span of a star.³²

Numbers from zero to nine are used to further subdivide the spectral classes. For instance, a G0 sun is more luminous than a G5, which in turn is brighter than a G9 -- the dimmest in the G class. The next-faintest star, of course, would be K0. M suns are the feeblest of all.

The brightest star on record is class O5, since objects from O0 to O4 have not been found. Stars with numbers between zero and four are often referred to as "early," while those with higher numbers are considered "late." Sol, technically a G2 sun, would thus be viewed as an "early spectral class G star."

Stellar mass, in contrast to luminosity, is restricted to within relatively narrow limits (Figure 4.11). Few stars have masses beyond an order of magnitude more or less than Sol’s. There is good reason for this.

Figure 4.11 Stellar number density near Sol, and stellar contraction time, as a function of stellar mass^57,1808

A star in the process of formation is a battleground for two opposing forces which struggle constantly to gain the upper hand. Gravity, which tries to collapse the ball of gas into a small volume with high density, is counteracted by radiation pressure, which grows more intense as the star’s thermonuclear furnace kindles and catches. The protostar shrinks to the point where radiation and gravity exactly balance each other, and relative stability is achieved.

Below about 0.01 M_sun the ball of gas just sits there, big and cold. Gravitational forces predominate. Internal pressures are just too low for nuclear fires to ignite. Dr. Hong-Yee Chiu at the NASA Institute for Space Studies calculates that stellar mass must be greater than about 0.02 M_sun for fusion reactions to be initiated.¹³¹⁴ This prediction squares well with observations. The lightest stars known -- of M9 class -- are all at least 0.05 M_sun or more. Jupiter, the gas giant planet and possible arrested protostar, masses only 0.001 M_sun

In the direction of higher mass, Chiu calculates that if the body exceeds about 30 M_sun the radiation pressure must be so great it would literally blow the star apart. Indeed, the largest stars known mass very close to this value.¹³¹⁴

The Hertzsprung-Russell diagram (Figure 4.12) is a plot of luminosity as a function of stellar class. About 91% of all stars fall neatly onto a narrow strip running diagonally from top to bottom. This is known as the main sequence.

Figure 4.12. Hertzsprung-Russell Diagram
Stellar Luminosity (Sol = 1.0) (Solar Luminosity Lsun = 3.84 x 1026 joules/sec)

The main sequence is not an evolutionary track, and is perhaps best thought of as a "house" in which a star resides for most of its life. It is believed that the earliest stages of stellar evolution involve the condensation of a giant cloud of gas and dust many light-years in diameter and massing perhaps 1000 M_sun.¹⁹⁴⁵ As contraction proceeds, the material fragments into many smaller globules until only tiny pieces remain. These units contain a few M_sun of matter and measure about a light-year across.

As the protostar shrinks its gravitational potential energy is converted to heat, and after millions of years the object has drawn itself together as a warm cloud about the diameter of our solar system (say, 40 AU). At this point, energy resources are shifted to ionizing instead of heating the gas. The protostar shrinks down to less than 1 AU in perhaps twenty years or so.¹⁸⁰⁸

A star suddenly appears in the midst of the whirling gas. We see that the actual contraction phase is very short, lasting less than one percent of the sun’s total main sequence lifetime.⁵⁷

These T Tauri stars are stellar newborns, and their luminosity fluctuates erratically with time.²⁰ Another peculiar feature of such objects is the blowing off of prodigious quantities of matter. It has been estimated that the original protostar loses from 30-50% or more of its starting mass in this fashion.^85,473,1945 Hydrogen burning begins as the T Tauri stage draws to a close, and the star enters the main sequence as a full adult.¹⁸⁰⁸

Naturally, not all stars of the same mass cease contraction at the same position on the H-R diagram. Those protostars which are deficient in heavy elements -- such as might be the case in globular clusters -- arrive at the main sequence at a considerably lower luminosity than most Disk stars. These are called the subdwarfs.^20,1945

For most normal suns, however, the mass determines both the point of entry onto the main sequence and the length of time of residence there (Table 4.4). Large O and B stars enter high on the sequence, and remain only a few tens of millions of years; the bantamweight K and M suns enter near the bottom and stay for tens of eons. Luminosity on the main sequence increases only very slightly with the passage of time. Sol, for example, has grown only 20% hotter since it left the T Tauri stage five eons ago.²⁰

Stars are evicted from the main sequence only when all or most of their hydrogen fuel in the car has been exhausted. With the sharp reduction in radiation pressure the core contracts. Hydrogen gas in the outermost shell begins to burn. Collapse of the core raises the temperature there, so that helium-, carbon-, and ultimately oxygen-burning become possible. The star thus separates into two rather distinct components -- diffuse burning shell and dense, hot core.

In this "red giant" stage, the shell of hydrogen may be gradually driven outward leaving a brilliant white core behind. (Stars which have left the main sequence remain red giants for perhaps 1% of their total lifetimes.) This "white dwarf " soon finishes off the remainder of its fuel and all fusion reactions cease. A white dwarf slowly cools to become an in visible black dwarf. Life for Sol-sized stars ends as inauspiciously as it began -- as cold, dark matter.

More massive suns have more spectacular deaths. Stars about 30% heavier than Sol go supernova, leaving behind a small, dense object called a neutron star -- essentially a gigantic atomic nucleus, perhaps ten kilometers in diameter, spinning furiously in space.^1214,1314 Densities run about 10¹⁴ times higher than that of lead. The pulsar in the Crab Nebula is one of many such objects observed by astronomers in the last decade or so.

Suns with initial masses of 3 M_sun or more also supernova, but instead of neutron stars these titanic explosions create spherical nuggets of gravitationally collapsed matter that have come to be known as black holes.** These holes in space represent such a high local mass density that light itself moves too slowly to achieve escape velocity at the surface. Observational astronomers think they’ve detected one "BH," probably a couple kilometers in diameter, located in the constellation Cygnus.¹⁹⁷⁰

When a star leaves the main sequence, so much energy is released that any life present is probably destroyed. Consequently, as far as the search for extraterrestrial life is concerned, only main sequence stars need be considered as possible candidates for habitable extrasolar systems.³²⁸ T Tauri objects, giants and supergiants, white and black dwarfs all may be eliminated from consideration. Fortunately, this still leaves us with about three-quarters of all suns in the Galaxy as putative abodes for life.

We know that life required 4.6 eons to arrive at its present stage of development here on Earth. Even if a certain margin of variation is allowed to account for differing speeds of evolution on different planets, the first fossil records of marine invertebrates don’t appear until the opening of the Cambrian Period a mere 600 million years ago. It is plausible to conclude that at least three or four eons -- the so-called "genesis time" -- may be required on any planet for intelligent life to gain a foothold.²¹⁴

If this is indeed the case, then life will be restricted to stars of class F5 and later.^57,328 Suns of earlier classes remain on the main sequence for less than the critical genesis time of several billion years, rendering improbable the emergence of intelligence.

Another argument in favor of class F5 as the early cutoff point is based on measurements of stellar rotation among the various classes of stars. There appears to be a sharp break at F5 in the amount of angular momentum possessed by suns (Figure 4.13). This conspicuous phenomenon can reasonably be explained by invoking the presence of planets.¹²⁷⁸

Figure 4.13 Stellar rotation vs. spectral class (Main Sequence stars only)^20,328

It is suspected that the birth of planetary systems is closely linked to the contraction and evolution of the primary. Approximately 98% of the angular momentum of our solar system is carried by the planets -- which represent only 0.2% of the total mass!

The hotter, fast-rotating stars are thought to be devoid of planets because they still retain the high initial rotation rate caused by the condensation of the original protostar. Cooler stars, later than F5, appear to have lost this great rotation somehow. One reasonable interpretation is that, like Sol in our system, these stars invested most of their angular momentum in their planets during the process of solar system formation.³²⁸

What is the smallest star that can harbor life? To answer this question we must briefly consider the concept of habitable zones or stellar ecospheres (Figure 4.14). An ecosphere is that region of space surrounding a sun where the radiation is neither too strong nor too feeble to support life. Too close to a star and a planet will fry; too far away, and it will freeze. The habitable zone lies between these two extremes.

Dr. Stephen H. Dole of the Rand Corporation has defined the limits of ecospheres so as to ensure that at least 10% of the surface of a world remains habitable all the time.²¹⁴ Dole estimates that to accomplish this the radiation from the primary must be within 35% of Earth-normal. (This may be too pessimistic^57,600 or too optimistic^1907,2031 to suit some, but it’s a good first guess.) Of course, the size of the ecosphere will vary from star to star, the less massive dim suns having much smaller zones of habitability than the more massive, brighter ones (Table 4.5). And planets must huddle closer to cooler stars to keep warm; the ecospheres of F stars will lie at considerably greater distances than the zones surrounding, say, class K suns.

Figure 4.14 Stellar ecospheres (habitable zones)

Another argument frequently advanced is that since K and M stars have relatively close ecospheres, planets within these habitable zones will become partially or totally tidally locked to their primary. That is, such planets would rotate extremely slowly; worse, they might become one-face worlds, always presenting only one side to the sun for heat. This could result in the atmosphere freezing out on the cold side^57,214,1908 or other environmental severities.²⁰

Stars massing less than 0.7 M_sun may have ecospheres so narrow and close as to possess no havens from such rotational arrest.²¹⁴ This corresponds roughly to stellar class K3. On the other hand, K2 and earlier stars should have at least a small region within their habitable zones in which tidal braking is much less severe.

* Ecosphere is that region of space surrounding the star in which insolation is ±35% Earth-normal. According to Dole, this will leave at least 10% of the planet’s surface habitable.²¹

Dr. S.I. Rasool at NASA has also suggested that the atmospheric evolution of planets may be critically dependent on the amount of ultraviolet radiation emitted by the primary.³⁷⁶ A deficiency in the UV could mean that the hydrogen and helium in the primeval solar system might not have a chance to dissipate from even the innermost planets, which would remain large, gaseous, and quite jovian. (Also, it is believed by some that M suns may be "flare stars," which emit sudden blasts of deadly UV at random intervals.^57,1775)

But there are more serious complications involved in the ultraviolet problem. The steady-state intensity of UV radiation at the surface of the primitive Earth was at least an order of magnitude greater than the next most abundant source of energy.¹⁰¹⁷ An ultraviolet deficit might greatly slow or even preclude the origin of life and early biochemical evolution.

It would appear that class K stars radiate at least an order of magnitude less UV than class G, although this has been disputed by some.^57,1775 Class M suns are even more niggardly, emitting less than 1% as much UV as Sol at equivalent locations within their ecospheres. The evidence, while far from conclusive, seems to rule out stars later than early K as possible abodes for life.^214,1018

As a first approximation, then, we choose to limit ourselves to population I stars in classes F5 through K2 on the main sequence -- perhaps 11% of all Milky Way suns (Figure 4.15).

Figure 4.15 Planetary surface temperature inside habitable zones

There is one further restriction on our selection of life-supporting stellar environments.²¹⁴⁸ About one-third of all stars occur in pairs (binary stars), and some two-thirds occur in multiples of all kinds (binaries, trinaries, hexastellar systems, etc.).²⁰ There should be less chance of finding habitable worlds in multiple star systems because of the relatively large variations in planetary surface temperatures (due to the peculiar convoluted orbit traced by a planet circling many suns).^50,1020,1053 The danger of "slingshot" ejection must also be reckoned with.

Calculations reveal that if the components of a binary star system follow relatively circular orbits and are either very close together or very far apart, stable orbits and moderate planetary temperatures are possible.²¹⁴*** Dr. Su-Shu Huang, formerly a physicist at NASA’s Goddard Space Flight Center in Washington, made a preliminary determination of habitable orbital configuarations near binaries whose components are roughly equivalent in mass.¹⁰²⁰

If good planetary orbits are to exist, the two stars must lie either less than 0.4L^½ AU apart or more than 13L^½ AU apart, where L is solar luminosity in Solar units, L_sun.

Of course, if either component of a binary system is class F4 or earlier, then both are unlikely to have been around sufficiently long for intelligent life to have arisen (though planets and simple lifeforms are not precluded). We also must reject population II binaries, as well as those which have a red giant, white dwarf, neutron star or black hole as one member of the pair.¹⁰¹⁸

Dr. T.A. Heppenheimer at the Center for Space Science in California has completed some simple calculations on the formation of planets in binary systems.¹³⁰⁰ His preliminary results indicate that, taking into account the typically large orbital eccentricity (e ~ 0.5) found in binary star systems, the components must actually be separated by more than 30 AU if they are to provide suitable habitats for biology. Apparently about one-third of all F5-K2 binaries within five parsecs of Earth satisfy this requirement.^{575,1300,2029}

In conclusion, our quest for life on other worlds should be limited to perhaps 5% of all stars in the Galaxy. The basic search therefore encompasses some ten billion suns, most of which lie in the Disk and outer Core regions of the Milky Way.

* Traditional mneumonic: "Oh Be A Fine Girl, Kiss Me Right Now. Smack!" Suggested non-sexist mnemonic: "Out Beyond Andromeda, Fiery Gases Kindle Many Red New Stars."²¹¹¹ The modern version doesn’t seem to be catching on.

** The properties of black holes are fascinating, and many excellent reviews have been written, including those by Thorne,^{1965,1966,1967} Penrose,¹⁹⁶⁸ Kaufmann,¹⁹⁷¹ Ruffini and Wheeler,¹⁹⁶⁹ and Hawking.²⁰²¹

*** It has been suggested that the Trojan points of double stars might be a good place to look for habitable planets.⁶⁰⁷

Last updated on 6 December 2008