© 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

4.2  Galaxies

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.

Figure 4.3 The Local Supergalaxy³⁹⁹

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!

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.

Figure 4.4 The Local Group^1974,2025

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

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

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 4.5 Great Ursa Major Spiral: M 101 / NGC5457

Nearby spiral galaxy, yet outside our Local Cluster. Note heavier, well-knotted arms and smaller core as compared with spirals of Plates I and IV. (Mount Wilson and Palomar Observatories, Plate VIII from Broms¹¹⁹¹)

* 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

** 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

Last updated on 6 December 2008