Extraterrestrial Civilizations - Isaac Asimov [45]
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