Extraterrestrial Civilizations - Isaac Asimov [42]
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