Extraterrestrial Civilizations (13 page)

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 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.
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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.