On a dark, clear evening the human eye can distinguish several thousand distant suns, all of which lie in the Milky Way (our home galaxy). Floating freely in Earth orbit our senses would be assaulted by the light of nearly six times as many stars. The Palomar 200" optical telescope — now the second largest in the world — has the light-gathering power of a million human eye balls and extends our vision to several billion celestial objects in this galaxy alone. And about ten billion galaxies are observable with present-day astronomical equipment, the farthest (3C 123) lying eight billion light-years distant.¹⁹⁵²

How big is the universe? An important clue was uncovered by the American astronomer Edwin Hubble back in the early 1920s, when he was measuring the atomic spectra emitted by various galaxies.

The farther away the object, he found, the more its spectral lines appeared to be displaced towards the lower frequencies of light. This curious phenomenon, which became known as the "redshift," was interpreted to be a kind of Doppler effect for photons.

• Much the same as a receding siren seems to be putting out lower and lower pitched sounds as it passes by, so do galaxies seem to emit redder light as they travel away from us.
• Since the most distant objects are seen to possess the greatest redshifts, the simplest explanation is that they are receding from Earth at velocities approaching the speed of light.
• Nearby galaxies are moving at a far more leisurely pace. Conclusion: The universe is slowly expanding.

This is not to say that Earth has the extraordinary good fortune to lie at the exact geometric center of all creation, simply because most all astronomical objects appear to be heading away from us.

• More correctly, our galaxy is like a spot of India ink on the surface of a spherical balloon.
• We are surrounded by billions of similar spots. As the balloon inflates, every point on its surface moves away from all adjacent points.
• From the chauvinistic viewpoint of each galaxy, all others will look like they’re flying away at various speeds depending on distance.
• Each will view itself as the "center" of the universe! This idea that the cosmos will appear roughly the same from any position is called the Cosmological Principle.

The latest measurements of galactic redshifts seem to indicate that the speed of recession increases about fifty-five kilometers per second for each megaparsec* of distance from Earth.¹⁹⁵³ (This number is called the Hubble Constant.) Since the maximum velocity of recession which can be detected is the speed of light, then the outermost shell of the swelling universe should lie about eighteen billion light-years from Earth.

Of course, if the "balloon" deflates, points on its surface will rush together again. Using our value of the Hubble Constant, we can mathematically run time backwards and extrapolate to Time Zero — the creation event. The inverse of the Hubble Constant is in units of time and represents the age of the universe assuming a constant rate of expansion. This works out to a period of eighteen eons!**

But scientists believe that the expansion of the universe has not been constant; on the contrary, it has probably decreased with the passage of time because of gravity. Taking this into account, the true age of the universe will not be quite so large, and is now usually set at about sixteen billion years.^1953,1983 Our estimate of the radius of the universe, likewise corrected, drops down, to sixteen billion light-years.

Any theory of cosmology that purports to explain the mechanics of the cosmos must, at the bare minimum, be able to account for Hubble’s redshift phenomenon. The one proposal which has been most successful in this regard is the Big Bang hypothesis.

* 1 megaparsec(Mpc) = 10³ kiloparsecs(kpc) = 10⁶ parsecs(pc) = 3.26 × 10⁶ light-years(ly) = 3.07 × 10²² meters(m).

** An eon is one billion (10⁹) years.

According to this leading view of cosmic evolution, the universe began as a highly compact fireball of pure energy and infinite density.

• After perhaps a millionth of a second this density dropped off to nuclear values as the ylem or "cosmic egg" exploded outward.²⁰⁶²
• The overall temperature may then still have exceeded 1013 K.¹¹⁹²
• The stuff of the universe started to change from pure energy into matter, primarily neutrons.

When half an hour had passed most of the neutrons were gone, replaced by a mixture of 60% hydrogen ions and 40% helium ions (by mass), as well as a smattering of deuterium (heavy hydrogen).¹⁸¹³

Using this model, it can be calculated that at the one hour mark, the temperature was down to about 250 million degrees; after the passage of a quarter of a million years, it had fallen off to the present temperature at the surface of Sol. And 170 K was reached after 250 million years following the big bang.

This turned out to be a red-letter date in the evolution of the universe, because for the first time in history the density of matter became greater than the mass density of radiant energy.

• Protons and electrons must have coalesced into de-ionized H, He, and D atoms, leaving no more than 0.1% still in the plasma state.¹¹⁹²
• This signaled a dramatic change in the behavior of material. No longer was matter incessantly sloshed and stirred by the overpowering radiation field, which had kept it permanently osterized in a thin, gaseous form.
• Once radiation refrained from dominating the scene, matter was free to gravitationally condense into relatively huge, massive aggregates — supergalaxies, galaxies, and stars.¹⁸¹³

There are two variations of the Big Bang scenario upon which most discussion has focused. The first of these is known as the closed, or pulsating universe model.

• According to this thesis the universe is a gravitationally "bound" system.
• That is, some thirty eons or so from now the fragments from the original ylem explosion will cease their outward rushing and commence to fall back together again like the dots on the surface of a deflating balloon.
• Cosmic evolution occurs in a series of alternate expansions and contractions.
• At the very end, the Final Moment, everything is destroyed, the slate wiped clean in preparation for the beginning of the next eighty-billion-year cycle.

Besides giving rise to philosophical nihilism, this has interesting consequences for the development of life. During an expansion phase light is redshifted to the relatively harmless lower frequencies. However, during the contraction phase the intensity of dangerous high frequency radiation might become unbearable — due to a blueshift effect. If the pulsating model is correct, then we are lucky to be alive during the half of the cycle most likely to be hospitable to life. In the second half, the development and expansion of biology would be severely restricted. Are we, asks Carl Sagan, "trapped in a vast cycle of cosmic deaths and rebirths?"²⁰

The second variant of the Big Bang theory is the open, or expanding universe model, which suggests that the cosmos will never stop enlarging and ultimately will disperse to infinity.

• In this view, all matter reached the "escape velocity" of the universe at the time of the ylem explosion: The cosmic radius increases indefinitely.
• This is in sharp contrast to the pulsating model, in which the radius oscillates between some maximum value and zero.

There is evidence to support the Big Bang theories. For instance, it will be recalled that the fireball cooled rather rapidly as it expanded. If this rate is extrapolated from the Bang to the present, sixteen eons later, the temperature should be down around a few degrees above absolute zero. This early prediction from evolutionary cosmology was verified in 1965 with the discovery of microwave radiation which fills the entire universe perfectly isotropically. The energy corresponds to a constant, uniform temperature of 2.7 K. This actual relic of the primeval ylem superexplosion strongly affirms the Big Bang theories, and appears to verify the Cosmological Principle mentioned earlier.

How can we decide whether the universe is open or closed? It turns out that if the mean density of the cosmos is less than about 5 × 10^-30 gm/cm³ (only about three atoms per cubic meter — intergalactic space is very nearly empty), then there is insufficient mass to hang onto the galaxies gravitationally. The universe would be open and must disperse to infinity.

Measurements of the actual density are difficult to make. However, based on the latest data, revised Hubble Constant, and such parameters as the observed density of galaxy-clusters locally and the abundance of deuterium in space,¹⁹⁵⁹ astronomers have reached a tentative conclusion: The universe is open.¹⁹⁵³

Another class of cosmological theories which has persisted for decades in various forms is the steady-state model, which suggests that the universe is not thinning out at all despite the apparent recession of galaxies. According to a typical model of this variety, neutrons appear suddenly out of nowhere in the interstellar void, roughly one particle per cubic meter every few eons or so. The local density is thus maintained at a constant level, the outflowing mass exactly balanced by the spontaneously generation of matter within the included volume.

Figure 4.1 The Hoyle-Narlikar cosmology Full Size

The most recent attempt to forge a more plausible steady-state model is the Hoyle-Narlikar cosmology (Figure 4.1).¹⁹⁵⁶

This is based on the concept of a "mass field," which is such that the mass of a chunk of matter is dependent upon its spatial and temporal location in space-time.

• The universe consists of a checkerboard pattern of normal and reversed mass fields.

While mass never becomes negative, its value does vary from zero at a boundary to some maximum value defined by the field.

• Einstein’s relativistic cosmology is said to represent a special case,¹⁹⁵⁵ valid near a boundary but not across it or very far away from it.
• Astronomer Hoyle infers that we are close to such a boundary.

Figure 4.2 The Local Universe Full Size

If mass has been steadily increasing since we crossed the border some sixteen billion years ago, then galaxies we passed along the way — which lie nearer the boundary — should have older, less massive atoms. If less massive atoms also have less massive electrons, these electrons should lie in larger atomic orbits and generally emit spectral lines of lower frequency. That is, the spectra should be redshifted. The observed redshift of galaxies is explained, not by their headlong flight, but because the electrons comprising their atoms are lighter in weight.¹⁹⁵⁴

Hoyle also has an explanation for the 2.7 K background radiation. It is known that light is most efficiently scattered by particles of low mass. Hence, the boundary at Time Zero (where mass goes to zero) must completely scatter all radiation coming from previous cells. The background is just the smeared out starlight emitted by galaxies on the other side of the border. Hoyle calculates that such galaxies still exist prior to Time Zero as far back as 150 eons.¹⁹⁵⁶

Many other unusual theories have been proposed from time to time, including:

• The Klein-Alfvén matter-antimatter cosmology.¹¹⁹²
• The universe-as-a-black-hole idea.¹⁹⁶³ (and black holes as accretion nuclei for elliptical⁶⁵³ and spira1⁹⁶⁴ galaxies)
• The Everett-Wheeler "splitting universe" scheme.^1982,3683
• And other multiple universe ("multiverse") theories.^{1512,1957,1958}

The cosmological problem has not yet lent itself to a definitive solution. Perhaps it never will until we are able to ask ETs, situated elsewhere in distant space, for their observations and ideas.

Figure 4.3 The Local Supergalaxy Full Size

If the Big Bang theories of the universe are essentially correct, then it was not long after Time Zero on the cosmic time scale that matter began to condense gravitationally. Any small nonuniformities in the density of the heretofore homogeneous gas would be aggravated, and local condensations could begin to occur.

Today we bear witness to what astronomers believe is the end product of that grand condensation process: Stars. These giant plasma balls glow by the energy of intense thermonuclear fusion reactions, at temperatures reaching many hundreds of millions of degrees in their cores. These incandescent globes are collected into great structures called galaxies, which exist in many shapes and sizes. It is now known that galaxy-clusters also exist, assemblages of a few to as many as thousands of individual galaxies. More than 80% of all nearby galaxies belong to such clusters.^1974,2150 The spaces between them are virtually devoid of stars, gas, and other matter.

A few astronomers today are of the opinion that order exists in the cosmos on an even larger scale. They claim to have discovered monstrous aggregates of galaxy-clusters possessing literally millions of individual galaxies, with masses ranging from 10¹⁶-10¹⁷ M_sun each.^{20,399,1191,1271,1974,1985,3676}

More than twenty nearby "supergalaxies" have been tentatively identified, having diameters from thirty to ninety megaparsecs.^1974,1985 However, less than a dozen can be identified in much detail within about 160 Mpc (~500,000,000 light-years).³⁹⁹ (For comparison, the radius of the entire universe in the Big Bang cosmologies is roughly 4900 Mpc.) The spaces between supergalaxies is incredibly empty, even more so than between galaxies and clusters of galaxies.

We are believed to be embedded in the Virgo Supercluster, otherwise known as the Local Supergalaxy (Figure 4.3). The Local Supergalaxy is a squat, roughly cylindrical collection of nearby galaxy-clusters with a total mass of perhaps 10¹⁵ M_sun.²⁰²⁵ It is about forty megaparsecs in diameter and twenty megaparsecs thick.³⁹⁹ Its radius is thus about 0.8% that of the known universe, which is about 10^-7 of the total "volume" of the cosmos.

The Supergalaxy rotates counterclockwise as viewed from the North Super-galactic Pole, about once every hundred billion years. Since the origin of the universe, it has yet to make so much as a quarter-turn!

Figure 4.4 The Local Group1974,2025 Full Size

We are situated in a galaxy-cluster, called the Local Group (Figure 4.4), some twelve megaparsecs from the center of the Supergalaxy near Virgo I.

We are rotating with the Supergalaxy at about 2.5 million kilometers per hour, roughly 0.2% the speed of light.

The drawing depicts the approximate extent of the Local Supergalaxy as it is presently understood, along with the thirty-one largest galaxy-clusters* in this hemisphere. It should be noted that our own Local Group is the smallest of these.

* Galaxy clusters seem to form "clouds" within the main corpus of supergalaxies. The Local Group is a member of the Local Cloud of clusters. The Cloud is tilted about 140° with respect to the Supergalactic Plane.¹⁹⁸⁴ The Local Cloud also possesses rather large regions of high-velocity neutral hydrogen gas clouds. These intergalactic gas clouds (IGCs) mass about 10⁸-10⁹ M_sun, and sport about 100-1000 atoms per cubic meter.¹⁹⁸⁶ In the Local Cloud of galaxy clusters, there are about twenty IGCs per cubic megaparsec.¹⁹⁸⁶

Table 4.1 The members of the Local Group1945,1974,2025 Full Size

Galaxy-clusters range from as little as fifty kiloparsecs in diameter (Stephan’s Quintet, four member galaxies) to more than eight megaparsecs (Coma cluster, several thousand members). Ours is a modest-sized cluster, with twenty-one member galaxies and a diameter of 800-900 kpc.^1974,2025 The Local Group is somewhat flattened in shape, with most components (Table 4.1) in the southern hemisphere of our Milky Way Galaxy.* Eleven intergalactic tramp globular star clusters have also been spotted, the lone gypsy wanderers of the forbidding intergalactic void.¹⁹⁷⁴

* The largest Local Group member — Galaxy Andromeda — has an apparent velocity towards Sol, believed due to our rotation about the center of the Milky Way.¹³³⁷ Andromeda is also tilted about 150° to our angle of view.²⁰²⁵

Table 4.2 Characteristics of galaxies1945,1974,2025

There are basically three kinds of galaxies: Irregulars, spirals, and ellipticals (Table 4.2). Irregulars are small, formless collections of stars, containing perhaps 10⁹ M_sun. These galaxies consist of about 10-50% neutral hydrogen gas and dust²⁰ and have very few old reddish stars and very many young blue-white stars.¹⁹⁷⁴ Much of the matter that could be utilized in the construction of stars hasn’t been used up yet.

Spiral galaxies have consumed far more of their hydrogen — only about 1% of the original amount remains, on the average. There are a fair number of both old and young stars. The typical spiral has three major components: The halo (a spheroidal volume of space with very old stars in highly elliptical orbits), the nuclear bulge or "core," and the galactic disk (which contains the spiral structure and most of the mass). Great dust lanes are usually very conspicuous throughout.^20,1976

Elliptical galaxies are generally ellipsoidal in shape. Virtually all of the neutral hydrogen has been depleted, and there is little dust. Most of the building materials for stars are gone. Few suns have formed in very recent times; consequently, the stars tend to be very old.

The above taxonomy is not believed to be an evolutionary sequence, say, from youth to senility. Each of the three types of galaxy is thought to have originated in much different ways. For instance, if we start out with a very low mass protogalaxy, the hydrogen density will be low and stars cannot form very fast. An irregular galaxy is the result, such as the Large Magellanic Cloud in our own Local Group. If the mass is large but rotation is slow, then most of the hydrogen has a chance to condense into stars before the contraction causes angular momentum to rise prohibitively. The matter is consumed immediately, leaving none for later on. An elliptical galaxy is the end product of this process. Finally, if mass is high and rotation is fast, star formation will proceed with greater restraint. Stars may continue to form for many tens of billions of years. Such is the probable history of a spiral.^20,1974

Current estimates of the abundance of galactic types run as follows: Spirals 60%, ellipticals 30%, irregulars 10%.^{1973,2150,2475} There are two subclasses of spirals, normal and barred. The arms of bar spirals attach to a thick girder of stars passing symmetrically across the center of the galaxy. (The Milky Way itself is believed by some to have a small football-shaped, bar-like structure at its center.¹⁹⁷⁶) Normal spirals with spheroidal cores are twice as abundant as the barred variety. About a million large galaxies lie within a few hundred megaparsecs of Sol.²⁰

About half of all galaxies are "dwarfs."¹⁹⁴⁵ Dwarf ellipticals and irregulars exist; probably for dynamical reasons, there are no dwarf spiral galaxies.¹⁹⁴⁵ Roughly 5% of all galaxies form physical pairs ("binary galaxies") or multiple systems, and at least 1% show some "marked visible peculiarity".¹⁹⁷³

Figure 4.5 Great Ursa Major Spiral:M 101 / NGC5457

Figure 4.5 Great Ursa Major Spiral: M 101 / NGC5457. Nearby spiral galaxy, yet outside our Local Cluster. (Mount Wilson and Palomar Observatories, Plate VIII from Broms1191)

Which galaxies are most likely to harbor intelligent life? One of the prerequisites for life as we know it is a planetary environment in which to flourish. Perhaps an atmosphere and oceans are also required, along with an abundance of various carbonaceous chemical substances. It appears fairly safe to conclude that "heavy elements" (carbon, oxygen, silicon, etc.) must be present if life is to arise. Primordial hydrogen and helium alone won’t do.

Scientists believe that heavies are generated as a natural product of stellar evolution. Normal thermonuclear processes in stars produce elements that run the gamut from lithium to iron, and stellar supernovae generate still heavier atoms (iron through uranium). A single, good-sized supernova explosion may inject as many as 10³ Earth-masses of heavies into the interstellar medium.

Over a period of billions of years, the stuff from which stars are born has become more and more enriched with heavy elements. Ultimately, this has made possible both planets and the development of life. But where are these heavy atoms most abundant?

It is generally agreed that dwarf galaxies are extremely metal poor.^1816,1818 Consequently, we may immediately eliminate about half of all galaxies from contention.

We also know that virtually all stars in elliptical galaxies were formed at least ten billion years ago, soon after the Big Bang.¹⁹⁷⁴ Although there is some evidence that the heavy element deficiency is small or negligible compared to our Galaxy,¹⁸¹⁶ if the theories of stellar nucleogenesis are correct then elliptical galaxy stars appeared long before the interstellar medium was impregnated with heavies. So ellipticals probably contain fewer habitable worlds.

Spectroscopic data for irregular galaxies indicate a marked deficiency in heavy elements,²⁰ as much as 30% less than in our Galaxy generally.¹⁸¹⁶ Irregulars are slow starters — the ambient gaseous medium probably has not been sufficiently enriched to produce as many planetary systems. Furthermore, the available mass in irregular galaxies tends to run a couple orders of magnitude less than that available for star-building in ellipticals and spirals.¹⁹⁴⁵ We would therefore expect somewhat fewer sites for life than in our own Galaxy.

It appears that the best place to look for biology is in the spiral galaxies (Figure 4.5),²⁰³² a conclusion tentatively affirmed by our presence in one. This is indeed fortunate, since these comprise a majority of all giant galaxies.

Figure 4.6 The Milky Way galaxy(schematic only)1945,1961,1976,1780,1816 Top View of the Milky Way galaxy Side View Figure 4.6 Top View of the Milky Way galaxy Full Size   Figure 4.6 Side View of the Milky Way galaxy Full Size

Figure 4.7 Composite picture of the Milky Way (Lund Observatory), from Shapley 1878

Our Galaxy is a rather typical spiral, consisting of three distinct regions (Halo, Core, and Disk) and four distinct components (stars, gas, dust and high energy particles) (Figure 4.6).

The Halo is a rather thin distribution of very old stars, spread out roughly spherically to a radius of twenty-five kiloparsecs or more from center. Probably about 5% of the entire mass of the Milky Way lies in the Halo¹⁷⁸¹ (~17% of all stars¹⁸¹⁶). The Core is several kiloparsecs in radius, and stellar densities rise to values millions of times higher than near Sol. This closely-packed nucleus of our galaxy contains perhaps 10% of all stars.^57,1976 The main disk of stars is a bit more than fifteen kiloparsecs (50,000 ly) in radius and averages about one kiloparsec thick. Sol is located only ten parsecs above the Galactic Plane,^20,57 and about ten kiloparsecs from the center.^20,1945,1976

The gross mass of the Milky Way is about 1.5 × 10¹¹ M_sun (3 × 10⁴¹ kg), representing a total of perhaps 250 billion stars of various types (Figure 4.7). Its aggregate energy output is roughly 5 × 10³⁷ watts, and it rotates once every 240 million years in the clockwise direction as viewed from the North Galactic Pole. The Milky Way has made some fifty revolutions since its initial condensation twelve billion years in the past, and Sol has traveled nearly twenty full circuits since the origin of the Solar System about 4.6 eons ago.

The stars to be found in each of the three regions of the Galaxy are of distinctly different character. The Halo "Population II" suns are very old, reddish stars with heavy element abundances hundreds of times less than in the vicinity of Sol and in the Disk generally.^1945,2032 These stars have highly elliptical orbits around the Core, and appear to be a relic of an earlier evolutionary stage of the Milky Way. Both individual stars and giant spherical collections (called globular clusters) inhabit the Halo. Globulars usually have 10⁵-10⁶ old Population II stars, and run 20-100 parsecs in diameter.^1556,1973

The Disk "Population I" comprise the bulk of the stars in our Galaxy. Sol and most of our stellar neighbors are members of this population, although there are certainly a few Halo stars kicking around in the Disk (only about 3-5% of all stars near Sol¹⁸¹⁶). Disk stars have nearly circular orbits about the Core, and are pretty well confined to a layer one kiloparsec from the Galactic Plane.¹⁷⁸⁰

It is believed that the Core is also comprised of Disk Population I stars, but there are some peculiar differences. The Core suns tend to be very old, reddish objects much like the Halo population, and yet the abundance of heavy elements appears to be at least six or seven times higher than in the Disk, near Sol.¹⁸¹⁸

Like the stars themselves, interstellar gas is composed mainly of hydrogen (about 60% by mass) and helium (about 40% by mass). These gases, whether neutral or ionized, occur in discrete patches several parsecs wide in concentrations of more than ten atoms per cubic centimeter. A few exceptionally small, concentrated clouds exist with densities well above 1000 atoms/cm³ — as in the Orion Nebula and the Horseshoe Nebula.¹⁸¹⁶ In the Milky Way there is an estimated 6 × 10⁷ M_sun of ionized hydrogen and 1.4 × 10⁹ M_sun of neutral hydrogen, for an overall density of about 0.6 atoms/cm³.¹⁹⁴⁵

Both gas and interstellar dust (dust mass ~10⁶ M_sun or less) lie flat in the Disk, confined to within two hundred parsecs of the Galactic Plane.¹⁸¹⁶ The presence of this dust (10^-3-10^-4 cm particles) obscures visibility along the Plane by absorption and scattering of light — which limits our view of the Galaxy in the optical spectrum to a few thousand parsecs along the line of sight.^20,1972

But it is important to keep in mind how truly thin this dust is dispersed. While we find at least one atom of hydrogen in each cm³ of interstellar space, there is only one tiny dust flyspeck in a cube of space twenty kilometers on an edge.

New stars are constantly forming in these dust clouds, as well as larger grains of "dirty ice."¹⁹⁷² Blast waves from novae and supernovae, galactic winds and wakes,^1151,1960 and density waves that may be responsible for the spiral arms all propagate in this tenuous "galactic atmosphere."

Figure 4.8 The nature of the spiral arm feature of the Milky Way Galaxy

Figure 4.8a Figure 4.8b Full Size Full Size

We have until now neglected what is probably the most interesting feature of the Milky Way — the spiral arms. Contrary to common belief,⁶⁰⁷ they are not concentrated regions of stars. The brilliant arms of spiral galaxies have less than 5% more stars than interarm regions (which is where Sol is).⁵⁷ The most visible among these few extra stars are the class O and class B suns, the gas-guzzling "cosmic Cadillacs" of the Galactic showroom. These showy white stars are very massive and very young, and they consume all their hydrogen fuel in a relatively brief time. The spiral arms are regions of much higher gas density, marking off the boundaries of the maternity wards of the Milky Way. All normal Disk stars, as well as O and B classes, are born there.

Were we to photograph the Disk so as to eliminate the 0.1% or so of ostentatious O and B suns, we would see an almost flat, featureless distribution of normal stars (Figure 4.8). It is only because of a very few bright stars that we have any visible spiral structure at all. Hence, the Galaxy really appears to be a solid dish of common yellow and red stars with a very light sprinkling of hot, white ones in a generally spiral pattern.

Since O and B stars have such short lifetimes, most of them die before their orbits can carry them very far from where they were born. A few do escape, however, and become mixed in with the rest of the stars. (Regulus, in the constellation Leo and about 26 parsecs from Sol, is one such escapee.) O and B suns are largely confined to within 70 parsecs of the Galactic Plane,¹⁷⁸⁰ and have virtually perfect circular orbits around the Core in the Plane. These stars are sometimes referred to as "Extreme Population I."

There is one minor complication to the view of the Milky Way presented thus far. The concentration of hydrogen in the spiral arms is a stable feature of the Galaxy, and is thought to represent a wave of greatly increased gas density traveling across the Disk. Where density is highest, the hot O & B stars can be formed, trailing from what amounts to a galactic density-shockwave.

Figure 4.9 Position and orientation of Earth and Sol in the Milky Way Galaxy

Position Orientation Position of Earth and Sol in the Milky Way Galaxy Full Size   Orientation of Earth and Sol in the Milky Way Galaxy Full Size

We recall that Sol orbits the Core (Figure 4.9) approximately once every 240 million years. The problem is that the spiral density wave circles the Galaxy at a much slower rate, about once every 400 million years. Consequently, the bright spiral arm stars trail forward,* not backward, from the leading edge of the bow shock.¹⁹⁷⁶

The distribution of life in the Milky Way is intimately connected with Galactic evolutionary history.¹⁸¹¹ The Halo population is the oldest sub system, remnant of the first stars and star clusters formed from the original virgin hydrogen cloud — the protogalaxy — 12 eons ago. As the cloud gravitationally condensed and began to rotate faster, it flattened out and became more dense.¹⁸⁰⁹ It has been estimated that about a hundred million years were required for the gas to fall ten kiloparsecs to the Galactic Plane.¹⁸²⁷ During this time the Disk population was formed (after the Halo), which soon found itself rich in heavy elements.¹⁸⁰⁷ The spiral arm population is the youngest subsystem of the Milky Way (10⁵-10⁹ years old), is also rich in heavies, and is closely restricted to the Plane.¹⁹⁴⁵

* Sol's forward motion should carry us into the Orion Arm in 10⁷ years or so.

Figure 4.10 Heavy element abundance and distribution in the disk of the Milky Way Galaxy57,1972

Figure 4.10a - Abundance 4.10b - Distribution Figure 4.10a Heavy element abundance in the disk of the Milky Way Galaxy Full Size   Figure 4.10b Heavy element distribution in the disk of the Milky Way Galaxy Full Size

The abundance of heavy elements increases markedly toward the center of our Galaxy (Figure 4.10). The concentration in the Core is an order of magnitude higher than near the rim of the Disk, and as much as three orders of magnitude greater than in the Halo. Based on the distribution of heavies, where might we expect to find planets and life?

It is difficult to avoid excluding the oldest Halo stars, orbiting high above the plane of the Galaxy.³³ For the most part, these stars are extremely metal-poor. In addition, they are few in number, widely dispersed, and exceedingly dim because of their distance, Halo population II stars generally are not a good place to look for biology.^312,1633

Globular clusters are conspicuous collections of hundreds of thousands of individual suns. There may be as many as 2000 such clusters in the Galaxy,¹⁹⁷³ but at present only about 200 are known to exist for certain.^1807,1945 Since the component stars are population II, they, like the lone Halo objects, are exceedingly poor in metals. This alone would be enough to rule out all but the slimmest chance of finding life,¹⁶³³ but there are other problems. For instance, stars in these clusters are so tightly packed that encounters between them may become important inasmuch as the stability of planetary systems is concerned.³⁵² Also, a large number of the stars have left the "main sequence" (see below) and have become red giants. This stage of their evolution is marked by large variations in luminosity and dramatic increases in stellar radius.^20,1556,1973 It would appear that globular clusters are not fruitful places to search for intelligent lifeforms.

Table 4.3. Star Densities at the Galactic Core1821

If not in the Halo, how about the Core? As discussed above, the central regions of the Galaxy are more metal-rich than anywhere else in the Milky Way. The potential exists, therefore, for a vast multitude of terrestrial-like planets and planetary systems. There are probably also large quantities of organic and inorganic molecules near the Core — just what’s needed to start the ball of life rolling.^1816,1961

One quick objection to life at the Core might be that with such an immense concentration of stars in such a small volume (Table 4.3), the radiation flux might be too intense. However, simple order-of-magnitude calculations reveal that this is not a problem. Even in the innermost recesses of the nucleus, the total radiation received by a habitable planet will be no more than 0.06% in excess of that received from its primary. This should not be incompatible with an otherwise stable environment.

However, more serious objections to Core life may be raised. For example, we know that on the average about one supernova occurs every fifty years in a typical spiral galaxy.¹⁹⁶² In general, a hundred light-years is considered the distance of minimum biological effect for supernovae^468,469,498 (Astrophysicists Krasovskii and Sagan have suggested that one nearby supernova event about 10⁸ years ago may have contributed to the extinction of the dinosaurs.²⁰) Since there are about 10⁴ stars within 100 light-years of Sol, the mean time between catastrophic events is about two billion years, a comfortably lengthy period of time.²⁰ On the other hand, there are more than two hundred million stars within 100 light-years of the center of the Galaxy. Assuming the same supernova rate, the mean time between damaging events would be reduced to 50,000 years. This may well prove intolerable to life.

Another argument against populating the Core is based on dynamical considerations. The grand game of stellar billiards, involving close encounters and collisions between stars once every million years or so,²⁰ could make life in the central regions quite impossible. As Dr. R. H. Sanders and Dr. G. T. Wrixen of the National Radio Astronomy Observatory put it: "It is doubtful that there would be any life on planets in the galactic nucleus, since with such high stellar densities close encounters between stars would be so frequent that planets would be ripped out of their orbit every few hundred million years."¹⁹⁶¹ But this argument loses much of its appeal if we consider the outer Core regions (say, from 1-3 kiloparsecs out) where the star density is only an order of magnitude or so above Sol-normal.

Finally, there are indications that violent events are occurring at or near the Core. Astronomers Burbidge, Hoyle and Lequeux have hypothesized that the expanding 3-kpc arm observed near the Core could be the result of an explosion that savagely ripped through the central regions a mere twenty million years ago.¹⁸¹⁶ More recently, this theory has been refined to the following numbers: Titanic explosions may occur every 500 million years, releasing some 10⁵³ joules of energy (equivalent to total conversion of half a million solar masses into pure energy), followed by an ejection of one billion solar masses of matter.¹⁹⁶¹ Needless to say, this would be an extremely disruptive event. [Note added in 2008: Many astrophysicists now believe there is a black hole at the center of the Milky Way Galaxy.]

It appears doubtful that life will have evolved in the nucleus of our Galaxy, although biology in the outer Core regions is entirely possible. Looking at it from an aesthetic point of view, the spacescape enjoyed by the in habitants must be fantastically beautiful. Hundreds of stars would appear brighter than Sirius, our brightest. Their most luminous suns would be an order of magnitude brighter than Venus is to us.¹³⁶⁰ Starlight filtering through thick, patchy dust clouds near the nucleus would produce interesting optical effects. The evening sky at the Core would be as bright as moonlight on a clear night on Earth — total darkness might be unknown to these extra-terrestrials. But stars would still look like mere points of light. Only within one parsec of the center of the nucleus would supergiant stars appear as distinct globes to the naked human eye.

Where else can life exist in the Milky Way? Scientists believe that the most likely place for life will be in the Disk, both in the interarm and spiral arm regions. Heavy elements are plentiful there, and planets should be numerous.²⁰³² Stars are far enough apart to preclude close encounters, and supernovae are few and far between. Finally, most of the stars in the Galaxy may be found in this region.

There is every indication that extraterrestrial life will be abundant throughout the volume of the Disk.

Figure 4.11 Stellar number density near Sol, and stellar contraction time, as a function of stellar mass57,1808

Density vs. Mass Contraction Time vs. Mass Local Stellar Density vs. Mass57 Full Size   Stellar Contraction Time vs. Mass1808 Full Size

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.

* 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.

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

Figure 4.12 Hertzsprung-Russell Diagram Full Size

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.

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

Table 4.4 Typical Characteristics of Stars and Stellar Types Full Size

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 invisible 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.

* 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

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

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

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

Figure 4.14 Stellar ecospheres (habitable zones) Full Size

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.

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.

Table 4.5 General Planetary Orbital Parameters for Habitable Zone vs. Stellar Mass Full Size

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

Figure 4.15 Planetary surface temperature inside habitable zones Full Size

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).

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.

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