4.3 - The Milky Way Galaxy

Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

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

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 x 10¹¹ M_sun (3 x 10⁴¹ kg), representing a total of perhaps 250 billion stars of various types (Figure 4.7). Its aggregate energy output is roughly 5 x 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.

Figure 4.7 Composite picture of the Milky Way (Lund Observatory, from Shapley¹⁸⁷⁸

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 x 10⁷ M_sun of ionized hydrogen and 1.4 x 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."

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.

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

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

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

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

Figure 4.10 Heavy element abundance and distribution in the disk of the Milky Way Galaxy^57,1972

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.

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.

Table 4.3. Star Densities at the Galactic Core¹⁸²¹
Radius from Center, R	Star Density	Stars within Radius R	Average Distance Between Stars	Irradiation by Surrounding Stars
(parsecs)	(stars/pc³)	(M_sun)	(parsecs)	(A.U.*)	(Earth/Sol = 1.0)
0.1	5.2 x 10⁷	6.0 x 10⁵	0.0034	700	6 x 10^-4
1.0	8.4 x 10⁵	8.8 x 10⁶	0.013	2700	9 x 10^-5
10.	1.3 x 10⁴	1.4 x 10⁸	0.053	11,000	6 x 10^-6
20.	3.8 x 10³	3.2 x 10⁸	0.081	17,000	4 x 10^-6
100	2.2 x 10²	5.0 x 10⁹	0.21	43,000	1 x 10^-6
(Nucleus) 800	20.0	2.0 x 10¹⁰	0.46	94,000	3 x 10^-7
(Core) 3000	3.0	-- -	0.87	180,000	9 x 10^-8
(Disk/ Sol) 10⁴	0.15	-- -	1.88	380,000	5 x 10^-9

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.


A - F - G - K - M Disk Population ~99.9% of all Main Sequence Stars Main Sequence lifetime > 200 million years	O - B Spiral-Arm Population I ~0.1% of all Main Sequence Stars Main Sequence lifetime < 200 million years

North Galactic Pole: RA = 0^h 49^m Dec = +27.4° (in Coma Berenices) South Galactic Pole: RA = 12^h 49^m Dec = -27.4° (in Sculptor)0° Galactic Longitude: RA = 17^h 43^m Dec = -28.9° (Sol -- > Galactic Center line)