While Einstein, Friedmann, Lemaitre and others were sitting around thinking things up, another group of scientists, the astronomers, were combing the night sky looking through larger and larger telescopes, digging deeper into space and farther back in time. If you look at the night sky with the naked eye it seems pretty much the same in any direction. But peer through a powerful telescope and an amazing world opens.
Galileo built the first telescope. In 1781, William Herschel, using a telescope he built himself, made a momentous discovery: the planet Uranus. In 1789 he built an even larger telescope, a monster with a mirror 1.2 meters in diameter. Unfortunately it was also 12 meters (over 40 feet) in length. One of his main projects was to try to determine the distances to hundreds of stars. He did this by making the assumption that all stars emit the same amount of light, so that the difference in brightness of stars is explained by how far away they are. In the 18th Century scientists knew that brightness decreases in inverse proportion to the square of the distance. Thus, if one star appears 1/4 as bright as another it is because that star is twice as far away. As a standard he used the star Sirius, the brightest star in the sky. A star only 1/9 as bright as Sirius must be three times as far away. He called this distance a siriometer, so the fainter star was 3 siriometers away from Earth. Herschel knew his assumption that all stars were equally bright was probably incorrect but, he figured that on average this was true and would result in a fairly accurate picture of the sky.
What Herschel discovered was that stars appeared to be clumped together in a disc, rather like a pancake. Imagine that we are in the middle of the pancake and that raisins are sprinkled throughout it. If we look down the length of the pancake we would expect to see many raisins or stars, but if we look up or down we would not expect to see too many. This conformed to what Herschel observed: a band of stars stretching across the sky. This had long been known to the ancients though they couldn’t tell that the milky band of light was a collection of stars. This soon became known as the Milky Way. At Herschel’s time the Milky Way was assumed to be the entire universe. It was thought there was nothing outside of it.
As telescopes became better astronomers saw more and more blobs of light, which they called nebulae. A debate arose over whether a nebula was part of the Milky Way galaxy or was an entire galaxy unto itself. The two main protagonists in this debate, Harlow Shapely, who favored the view that nebulae were part of the Milky Way, and Heber Curtis, who argued that nebulae were separate galaxies. Shapely believed that nebulae were clouds of gas where stars incubated. Curtis argued that nebulae were scattered more or less symmetrically around the Milky Way but couldn’t be detected because of all the stars and interstellar dust in the plane of the Milky Way.
The debate was formalized in April, 1920, when the two met at the National Academy of Sciences in Washington. At the end there was no clear-cut winner. It would continue for three more years until Edward Hubble, probably the greatest astronomer of his generation, was able to show that the Andromeda Nebula was in fact a separate galaxy, the Andromeda Galaxy, some 900,000 light years from earth.