Extraterrestrial Civilizations - Isaac Asimov [44]
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,