Extraterrestrial Civilizations - Isaac Asimov [43]
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