CHAPTER 5

The Stars

SUBSTARS

Having gone rather exhaustively through the Solar system, it would appear that although there may be life on several worlds other than Earth, even conceivably intelligent life, the chances are not high. Furthermore, the chances would seem to be virtually zero that a technological civilization exists, or could exist, anywhere in the Solar system but on Earth.

Nevertheless, the Solar system is by no means the entire Universe. Let us look elsewhere.

We might imagine life in open space in the form of concentrations of energy fields, or as animated clouds of dust and gas, but there is no hint of evidence that such a thing is possible. Until such evidence is forthcoming (and naturally the scientific mind is not closed to the possibility), we must assume that life is to be found only in association with solid worlds at temperatures less than those of the stars.

The only cool, solid worlds we know are the planetary and subplanetary bodies that circle our Sun, but we cannot assume from this that all such bodies in the Universe must be associated with stars.^* There may be clouds of dust and gas of considerably smaller mass than that from which our Solar system originated, and these may have ended by condensing into bodies much smaller than the Sun. If the bodies are sufficiently smaller than the Sun, say with only 1/50 the mass or less, they would end by being insufficiently massive to ignite into nuclear fire. The surfaces of such bodies would remain cool and they would resemble planets in their properties, except that they would follow independent motions through space and would not be circling a star.

All our experience teaches us that of any given type of astronomical body, the number increases as the size decreases. There are a greater number of small stars than of large ones, a greater number of small planets than large ones, a greater number of small satellites than large ones, and so on. Might we argue from that, that these substars, too small to ignite, are far greater in number than those similar bodies that are massive enough to ignite? At least one important astronomer, the American Harlow Shapley (1885–1972), has very strongly advanced the likelihood of the existence of such bodies.

Naturally, since they do not shine, they remain undetected and we are unaware of them. But if they exist, we might reason that there exist substars in space through an entire range of sizes from super-Jupiters to small asteroids. We might even suppose that the larger ones could have bodies considerably smaller than themselves circling them, much as there are bodies circling Jupiter and the other giant planets within our own Solar system.

The question is, though: Would life form on such substars?

So far I have suggested that the irreducible requirements for life (as we know it) are, first, a free liquid, preferably water, and, second, organic compounds. A third requirement, which ordinarily we take for granted, must be added, and that is energy. The energy is needed to build the organic compounds out of the small molecules present at the start, small molecules such as water, ammonia, and methane.

Where would the energy come from in these substars?

In the condensation of a cloud of dust and gas into a body of any size, the inward motion of the components of the cloud represents kinetic energy obtained from the gravitational field. When the motion stops, with collision and coalescences, the kinetic energy is turned into heat. The center of every sizable body is therefore hot. The temperature at the center of the Earth, for instance, is estimated to be 5,000° C (9,000° F).

The larger the body and the more intense the gravitational field that formed it, the greater the kinetic energy, the greater the heat, and the higher the internal temperature. The temperature at the center of Jupiter, for instance, is estimated to be 54,000° C (100,000° F).

It might be expected that this internal heat is a temporary phenomenon and that a planet would slowly but surely cool down. So it would, if there were no internal supply of energy to replace the heat as it leaked away into space.

In the case of Earth, for instance, the internal heat leaks away very slowly indeed, thanks to the excellent insulating effect of the outer layers of rock. At the same time, those outer layers contain small quantities of radioactive elements such as uranium and thorium, which, in their radioactive breakdown, liberate heat in large enough quantities to replace that which is lost. As a result, the Earth is not cooling off perceptibly, and though it has existed as a solid body for 4,600,000,000 years, its internal heat is still there.

In the case of Jupiter, there seem to be some nuclear reactions going on in the center, some faint sparks of starlike behavior, so that Jupiter actually radiates into space three times as much heat as it receives from the Sun.

This long-lasting internal heat would be more than ample to support life, if living things could tap it.

We could fantasize life as existing within the body of a planet where nearby pockets of heat might have served as the energy source to form and maintain it. There is, however, no evidence that life can exist anywhere but at or near the surface of a world, and until evidence to the contrary is obtained, we should consider surfaces only.

Suppose, then, we consider a substar no more massive than the Earth; or a body that massive that is circling a substar somewhat more massive than Jupiter but yielding no visible light.

Such an Earthlike body, whether free in space or circling a substar, would tend to be a world like Ganymede or Callisto. There would be internal heat, but, thanks to the insulating effect of the outer layers, very little would leak outward to the surface; any more than Earth’s internal heat leaks outward to melt the snow of the polar regions and mitigate the frigidity of Earth’s temperatures.

To be sure, on Earth there are local leaks of considerable magnitude, producing hot springs, geysers, and even volcanoes. We might imagine such things on Earth-sized substars as well. In addition, there could be energy derived from the lightning of thunderstorms. Still, whether such sporadic energy sources would meet the requirements for forming and maintaining life is questionable. There is also the point that a world without a major source of light from a nearby star may be unfit for the development of intelligence—a subject I will take up later in the book.

The Earth-sized substar would be composed of a much larger percentage of volatiles than Earth itself, since there would have been no nearby hot star to raise the temperature in surrounding space and make the collection of volatiles impossible. Therefore, again as on Ganymede and Callisto, we might imagine a world-girdling ocean, probably of water, kept liquid by internal heat, but covered by a thick crust of ice.

Substars still smaller than the Earth would have less internal heat and would be even more likely to be icy, have less in the way of sporadic sources of appreciable energy, have smaller internal oceans or none at all.

If a body were small enough to attract little or no volatile matter even at the low temperatures that would exist in the absence of a nearby star, it would be an asteroidal body of rock or metal or both.

What about substars that are larger than Earth and therefore possess greater and more intense reservoirs of internal heat? Such a larger body is bound to be Jupiterlike. A large substar is certain to be made up largely of volatile matter, particularly hydrogen and helium; and high internal heat will make the planet entirely liquid.

Heat can circulate much more freely through liquid by convection than through solids by slow conduction. We can expect ample heat at or near the surface in such large substars and the heat may remain ample for billions of years. However, again the most we can expect on a large substar is intelligent life of the dolphin variety—and no technological civilization.

In short, the formation of substars would rather resemble the formation of bodies in the outer Solar system, and we may expect no more of the former than of the latter.

For a technological civilization, we need a solid planet with both oceans and dry land, so that life as we know it can develop in the former and emerge on the latter. To form such a world there must be a nearby star to supply the heat that would drive away most of the volatile matter, but not all. The nearby star would also supply the necessary energy for the formation and maintenance of life in a copious and steady manner.

In that case, we must concentrate our attention on the stars. These, at least, we can see. We know they exist and need not simply assume the probability of their existence as in the case of the substars.

THE MILKY WAY

If we turn to the stars and consider them as energy sources in the neighborhood of which we may find life, possibly intelligence, and possibly even technological civilizations, our first impression may be heartening, for there seem to be a great many of them. Therefore, if we fail to find life in connection with one, we may do so in connection with another.

In fact, the stars may well have impressed the early, less sophisticated watchers of the sky as innumerable. Thus, according to the Biblical story, when the Lord wished to assure the patriarch Abraham that, despite his childlessness, he would be the ancestor of many people, this is how it is described:

“And he [God] brought him [Abraham] forth abroad, and said, ‘Look now toward heaven, and tell the stars, if thou be able to number them’; and he [God] said unto him [Abraham], ‘So shall thy seed be.’ ”

Yet if God were promising Abraham that he would ultimately have as many descendants as there were stars in the sky that he could see, God was not promising as much as might be assumed.

The stars have been counted by later generations of astronomers who were less impressed with their innumerability. It turns out the number of stars that can be seen with the unaided eye (assuming excellent vision) is, in total, about 6,000.

At any one time, of course, half the stars are below the horizon, and others, while present above the horizon, are so near it as to be blotted out through light absorption by an unusually great thickness of even clear air. It follows that on a cloudless, moonless night, far from all man-made illumination, even a person with excellent eyes cannot see more than about 2,500 stars at one time.

In the days when philosophers assumed all worlds were inhabited and when general statements to that effect were made, it is not clear whether any particular philosopher truly understood the nature of stars.

Perhaps the first clear statement of the modern view was that of Nicholas of Cusa (1401–1464), a cardinal of the Church, who had particularly striking ideas for his time. He thought that space was infinite and that there was no center to the Universe. He thought all things moved, including the Earth. He also thought the stars were distant Suns, that they were attended by planets as the Sun was, and that those planets were inhabited.

Interesting, but we of the contemporary world are less sanguine concerning habitability, and cannot accept in carefree fashion the notion of life everywhere. We know there are dead worlds, and we know that there are others, which while possibly not dead, are not likely to bear more than simple bacteria life forms of life. Why may there not be stars around which only dead worlds orbit? Or around which no worlds circle at all?

If it should turn out that habitability is associated with only a small percentage of the stars (as life seems to be associated with only a small percentage of the worlds of the Solar system), then it becomes important to determine whether there are stars other than those we happen to be able to see and if so, how many. After all, the greater the number of stars, the greater the chance of numerous life forms existing in space even if the chances for any one star are very low.

The natural assumption, of course, is that only those stars exist that can be seen. To be sure, some stars are so dim that excellent eyes can just barely make them out. Might it not seem natural to suppose that there are some that are fainter still and cannot be made out by even the best eyes?

Apparently, this seemed to occur to very few. Perhaps there was the unspoken feeling that God wouldn’t create something too dim to be seen, since what purpose could such an object serve? To suppose that everything in the sky was there only because it affected human beings (the basis of astrological beliefs) seemed to argue against invisible bodies.

The English mathematician Thomas Digges (1543–1595) did espouse views like those of Nicholas of Cusa and in 1575 maintained not only infinite space, but an infinite number of stars spread evenly throughout it. Italian philosopher Giordano Bruno (1548–1600) also argued the same views, and did so in so undiplomatic and contentious a manner that he was finally burned at the stake in Rome for his heresies.

The argument over the matter ended in 1609, however, thanks to Galileo and his telescope. When Galileo turned his telescope on the sky, he immediately discovered that he saw more stars with his instrument than without it. Wherever he looked, he saw stars that could not be seen otherwise.

Without a telescope one saw six stars in the tiny little star group called the Pleiades. There were legends of a seventh that had dimmed and grown invisible. Galileo not only saw this seventh star easily once he clapped his telescope to his eyes, he saw thirty more stars in addition.

Even more important was what happened when he looked through his telescope at the Milky Way.

The Milky Way is a faint, luminous fog that seems to form a belt around the sky. In some ancient myths, it was pictured as a bridge connecting heaven and Earth. To the Greeks it was sometimes seen as a spray of milk from the divine breast of the goddess Hera. A more materialistic way of looking at the Milky Way, prior to the invention of the telescope, was to suppose it was a belt of unformed star matter.

When Galileo looked at the Milky Way, however, he saw it was made up of myriads of very faint stars. For the first time, a true notion of how numerous the stars actually were broke in on the consciousness of human beings. If God had granted Abraham telescopic vision, the assurance of innumerable descendants would have been formidable indeed.

The Milky Way, by its very existence, ran counter to Digges’ view of an infinite number of stars spread evenly through infinite space. If that were so, then the telescope should reveal roughly equal numbers of stars in whatever direction it was pointed. As it was, it was clear that the stars did not stretch out equally in all directions, but that they made up a conglomerate with a definite shape to it.

The first to maintain this was the British scientist Thomas Wright (1711–1786). In 1750, he suggested that the system of stars might be shaped rather like a coin, with the Solar system near its center. If we looked out toward the flat edges on either side, we saw relatively few stars before reaching the edge, beyond which there was none. If, on the other hand, we looked out along the long axis of the coin in any direction, the edge was so distant that the very numerous, very distant stars melted together into dim milkiness.

The Milky Way, therefore, was the result of the vision following the long axis of the stellar system. In all other directions, the edge of the stellar system was comparatively nearby.

The whole stellar system can be called the Milky Way, but one usually goes back to the Greek phrase for it, which is galaxias kyklos (milky circle). We call the stellar system the Galaxy.

THE GALAXY

The shape of the Galaxy could be determined more accurately if one could count the number of stars visible in different parts of the sky, and then work out the shape that would yield those numbers. In 1784, William Herschel undertook the task.

To count all the stars all over the sky was, of course, an impractical undertaking, but Herschel realized it would be quite proper to be satisfied with sampling the sky. He chose 683 regions, well scattered over the sky, and counted the stars visible in his telescope in each one. He found that the number of stars per unit area of sky rose steadily as one approached the Milky Way, was maximal in the plane of the Milky Way, and minimal in the direction at right angles to that plane.

From the number of stars he could see in the various directions, Herschel even felt justified in making a rough estimate of the total number of stars in the Galaxy. He decided that it contained 300 million stars, or 50,000 times as many as could be seen with the unaided eye. What’s more, he decided that the Galaxy was five times as long in its long diameter as in its short.

He suggested that the long diameter of the Galaxy was 800 times the distance between the Sun and the bright star Sirius. At the time, the distance was not known, but we now know it to be 8.63 light-years, where a light-year is the distance light will travel in one year.^* Herschel’s estimate, therefore was that the Galaxy was shaped like a grindstone, and was about 7,000 light-years across its long diameter and 1,300 light-years across its short diameter. Since the Milky Way seemed more or less equally bright in all directions, the Sun was taken to be at or near the center of the Galaxy.

More than a century later, the task was undertaken again by the Dutch astronomer Jacobus Cornelius Kapteyn (1851–1922). He had the technique of photography at his disposal, which made things a bit easier for him. He, too, ended with the decision that the Galaxy was grindstone-shaped with the Sun near its center. His estimate of the size of the Galaxy was greater than Herschel’s, however.

In 1906, he estimated the long diameter of the Galaxy to be 23,000 light-years and the short diameter to be 6,000 light-years. By 1920, he had further raised the dimensions to 55,000 and 11,000 respectively. The final set of dimensions involved a Galaxy with a volume 520 times that of Herschel’s.

Even as Kapteyn was completing this survey of the Galaxy, a totally new outlook had entered astronomical thinking.

It came to be recognized that the Milky Way was full of clouds of dust and gas (like the one that had served as the origin of our Solar system and, perhaps, of others) and that those clouds blocked vision. Thanks to those clouds, we could only see our own neighborhood of the Galaxy and in that neighborhood we were at the center. Beyond the clouds, though, there might well be vast regions of stars we could not see.

Indeed, as new methods for estimating the distance of far-off star clusters were developed, it turned out that the Sun was not in or near the center of the Galaxy at all, but was far off in the outskirts. The first to demonstrate this was Harlow Shapley, who in 1918 presented evidence leading to the belief that the center of the Galaxy was a long distance away in the direction of the constellation Sagittarius, where, as it happens, the Milky Way is particularly thick and luminous. The actual center was, however, hidden by dust clouds, as were the regions on the other side of the center.

Through the 1920s, Shapley’s suggestion was investigated and confirmed, and by 1930 the dimensions of the Galaxy were finally worked out, thanks to the labors of the Swiss-American astronomer Robert Julius Trumpler (1886–1956).

The Galaxy is more nearly lens shaped than grindstone shaped. That is, it is thickest at the center and grows thinner toward its edges. It is 100,000 light-years across and the Sun is about 27,000 light-years from the center, or roughly halfway from the center toward one edge.

The thickness of the Galaxy is about 16,000 light-years at the center and about 3,000 light-years at the position of the Sun. The Sun is located about halfway between the upper and lower edge of the Galaxy, which is why the Milky Way seems to cut the sky into two equal halves.

The Galaxy, as it is now known to be, is four times the volume of Kapteyn’s largest estimate.

In a way, the Galaxy resembles an enormous Solar system. In the center, playing the part of the Sun, is a spherical “Galactic nucleus” with a diameter of 16,000 light-years. This makes up only a small portion of the total volume of the Galaxy, but it contains most of the stars. Around it are large numbers of stars that follow orbits about the Galactic nucleus as planets do around the Sun.

The Dutch astronomer Jan Henrick Oort (1900–) was able to show in 1925 that the Sun was moving in a fairly circular orbit about the Galactic nucleus at a speed of about 250 kilometers (155 miles) per second. This speed is about 8.4 times the speed of the Earth moving around the Sun. The Sun and the whole Solar system revolve about the Galactic nucleus once every 200,000,000 years, so that in the course of its lifetime, so far, the Sun has completed perhaps twenty-five circuits about the Galactic nucleus.

From the speed of the Sun’s progress about the Galactic nucleus, it is possible to calculate the gravitational attraction exerted upon it. From that and from the distance of the Sun from the Galactic center, it is possible to calculate the mass of the Galactic nucleus and, roughly, of the entire Galaxy.

The mass of the Galaxy is certainly over 100 billion times that of our Sun, and some estimates place it as high as 200 billion times that of our Sun.

We might, quite arbitrarily, just in order to have a number to deal with, strike a point between the extremes and say (always subject to modification as better and more precise evidence is obtained) that the mass of the Galaxy is 160,000,000,000 times the mass of the Sun.

The mass of the Galaxy is distributed among three classes of objects. These are (1) stars, (2) nonluminous planetary bodies, and (3) clouds of dust and gas.

Although the nonluminous planetary bodies may conceivably be much more numerous than stars, each is so tiny compared to the stars that the total planetary mass must be small in comparison. Again, while the clouds of dust and gas take up enormous volumes, they are so rarefied that the total cloud mass must be small by comparison.

We can be sure that nearly all the mass of the Galaxy is in the form of stars. Although our own Solar system, for instance, contains but one Sun and innumerable planets, satellites, asteroids, comets, meteoroids, and dust particles circling it, that one Sun contains about 99.86 percent of all the mass of the Solar system.

The stars of the Galaxy may not make up so overwhelming a percentage of the total mass as that, but it is fairly safe to suppose that they may make up 94 percent of the mass of the Galaxy. In that case, the mass of the stars in the Galaxy is equal to 150,000,000,000 times the mass of the Sun.

Can that mass of stars be turned into the number of stars?

That depends on how representative the mass of the Sun is with respect to the mass of stars generally.

The Sun is a huge object compared to the Earth, or even compared to Jupiter. Its diameter is 1,392,000 kilometers (868,000 miles) or 110 times the diameter of the Earth. Its mass is 2 million trillion trillion kilograms, or 324,000 times the mass of the Earth. Nevertheless, it is not remarkable as stars go.

There are stars that are as much as 70 times as massive as the Sun and that shine a billion times as brightly. There are other stars that are only 1/20 the mass of the Sun (and are therefore only 50 times the mass of Jupiter) and that flicker with a light only one-billionth that of the Sun.

Roughly speaking, one must conclude that the Sun is an average star, about equally distant from the extremes of giant size and brilliance on one end of the scale and pygmy size and dimness on the other end of the scale.

If the stars were equally distributed all along the mass scale and if the Sun were really average, then we would assume that there were 150 billion stars in the Galaxy.

As it happens, however, the smaller stars are more numerous than the larger ones, so that it is fair to estimate that the average star is about half the size of the Sun in mass. (There are small stars in which matter is very compressed and which are very dense, but their mass is not unusually high and they do not affect the average.)

If, then, the total mass of the stars in the Galaxy is 150 billion times the mass of the Sun, and the average star is 0.5 times the mass of the Sun, then it follows that there are some 300 billion stars in the Galaxy. This means that for each visible star in the sky, each one a member of the Galaxy, there are 50 million other stars in the Galaxy that we cannot see with our unaided eyes.

THE OTHER GALAXIES

Have we now come to an end? Are 300 billion stars all there are in the Universe? To put it another way, is the Galaxy all there is?

Suppose we consider two patches of luminosity in the sky that look like isolated regions of the Milky Way, and that are so far south in the sky as to be invisible to viewers in the North Temperate Zone. They were first described in 1521 by the chronicler accompanying Magellan’s voyage of circumnavigation of the globe—so they are called the Large Magellanic Cloud and the Small Magellanic Cloud.

They were not studied in detail until John Herschel observed them from the astronomic observatory at the Cape of Good Hope in 1834 (the expedition that fueled the Moon Hoax). Like the Milky Way, the Magellanic Clouds turned out to be assemblages of vast numbers of very dim stars, dim because of their distance.

In the first decade of the twentieth century, the American astronomer Henrietta Swan Leavitt (1868–1921) studied certain variable stars in the Magellanic Clouds. By 1912, the use of these variable stars (called Cepheid variables because the first to be discovered was in the constellation Cepheus) made it possible to measure vast distances that could not be estimated in other ways.

The Large Magellanic Cloud turned out to be 170,000 light-years away and the Small Magellanic Cloud 200,000 light-years away. Both are well outside the Galaxy. Each is a galaxy in its own right.

They are not large, however. The Large Magellanic Cloud may include perhaps 10 billion stars and the Small Magellanic Cloud only about 2 billion. Our Galaxy (which we may refer to as the Milky Way Galaxy if we wish to distinguish it from others) is 25 times as large as both Magellanic Clouds put together. We might consider the Magellanic Clouds as satellite galaxies of the Milky Way Galaxy.

Is this all, then?

A certain suspicion arose concerning a faint, fuzzy patch of cloudy matter in the constellation Andromeda; a patch of dim light called the Andromeda Nebula. Even the best telescopes could not make it separate into a conglomeration of dim stars. A natural conclusion was, therefore, that it was a glowing cloud of dust and gas.

Such glowing clouds were indeed known, but they did not glow of themselves. They glowed because there were stars within them. No visible stars could be seen within the Andromeda Nebula. The light from other luminous clouds when analyzed, however, turned out to be completely different from starlight; whereas the light of the Andromeda Nebula was exactly like starlight.

Another alternative, then, was that the Andromeda Nebula was a conglomeration of stars, but one that was even more distant than the Magellanic Clouds, so that the individual stars could not be made out.

When Thomas Wright had first suggested in 1750 that the visible stars were collected into a flat disc, he theorized that there might be other such flat discs of stars at great distances from our own. This idea was taken up by the German philosopher Immanuel Kant (1724–1804) in 1755. Kant spoke of “island universes.”

The notion did not catch on. Indeed, when Laplace developed his notion that the Solar system had formed out of a whirling cloud of dust and gas, he cited the Andromeda Nebula as an example of a cloud slowly whirling and contracting to form a sun and its attendant planets. That was the reason the theory was called the nebular hypothesis.

By the time the twentieth century opened, however, the old notion of Wright and Kant was gathering strength. Occasionally, stars did appear in the Andromeda Nebula, stars that were clearly “novas”; that is, stars that suddenly brightened several magnitudes and then dimmed again. It was as though there were stars in the Andromeda Nebula that were ordinarily too dim to see under any circumstances because of their great distances, but that, upon briefly brightening with explosive violence, became just bright enough to make out.

There are such novas, now and then, among the stars of our own Galaxy, and by comparing their apparent brightness with the brightness of the very dim novas in the Andromeda Nebula, the distance of the Andromeda could be roughly worked out.

By 1917, the argument was settled. A new telescope with a 100-inch mirror had been installed on Mt. Wilson, just northeast of Pasadena, California. It was the largest and best telescope that existed up to that time. The American astronomer Edwin Powell Hubble (1889–1953), using that telescope, was finally able to resolve the outskirts of the Andromeda Nebula into masses of very faint stars.

It was the “Andromeda Galaxy” from that point on.

By the best modern methods of distance determination, it would appear that the Andromeda Galaxy is 2,200,000 light-years distant, eleven times as far away as the Magellanic Clouds. No wonder it was difficult to make out the individual stars.

The Andromeda Galaxy is no dwarf, however. It is perhaps twice as large as the Milky Way Galaxy and may contain up to 600 billion stars.

The Milky Way Galaxy, the Andromeda Galaxy, and the two Magellanic Clouds are bound together gravitationally. They form a “galactic cluster” called the Local Group and are not the only members, either. There are some twenty members altogether. There is one, Maffei I, which is about 3,200,000 light-years away, and it is just about as large as the Milky Way. The remainder are all small galaxies, a couple with less than a million stars apiece.

There may be as many as 1.5 trillion stars in the Local Group altogether, but that isn’t all there are either.

Beyond the Local Group, there are other galaxies, some single, some in small groups, some in gigantic clusters of thousands. Up to a billion galaxies can be detected by modern telescopes, stretching out to distances of a billion light-years.

Even that is not all there is. There is reason to think that, given good enough instruments, we could make observations as far as 12 billion light-years away before reaching an absolute limit beyond which observation is impossible. It may be that there are 100 billion galaxies, therefore, in the observable universe.

Just as the Sun is a star of intermediate size, the Milky Way Galaxy is one of intermediate size. There are galaxies with masses 100 times larger than that of the Milky Way Galaxy, and tiny galaxies with only a hundred-thousandth the mass of the Milky Way Galaxy.

Again, the small objects of a particular class greatly outnumber the large objects, and we might estimate rather roughly that there are on the average 10 billion stars to a galaxy, so that the average galaxy is of the size of the Large Magellanic Cloud.

That would mean that in the observable universe, there are as many as 1,000,000,000,000,000,000,000 (a billion trillion) stars.

This one consideration alone makes it almost certain extraterrestrial intelligence exists. After all, the existence of intelligence is not a zero-probability matter, since we exist. And if it is merely a near-zero probability, considering that near-zero probability for each of a billion trillion stars makes it almost certain that somewhere among them intelligence and even technological civilizations exist.

If, for instance, the probability were only one in a billion that near a given star there existed a technological civilization, that would mean that in the Universe as a whole, a trillion different such civilizations would exist.

Let us move on, though, and see if there is any way we can put actual figures to the estimates; or, at least, the best figures we can.

In doing so, let us concentrate on our own Galaxy. If there are extraterrestrial civilizations in the Universe, those in our own Galaxy are clearly of greatest interest to us since they would be far closer to us than any others. And any figures we arrive at that are of interest in connection with our own Galaxy can always be easily converted into figures of significance for the others.

Begin with a figure that deals with our Galaxy and divide it by 30 and you will have the analogous figure for the average galaxy. Begin with a figure that deals with our Galaxy and multiply it by 3.3 billion and you have the analogous figure for the entire Universe.

We start then with a figure we have already mentioned: