We’ve seen that one of the biggest hurdles astronomers faced was coming up with a way to measure the absolute distance to a star. While the siriometer provided a relative method, since the distance to Sirius wasn’t known, all distances were relative to the unknown distance to Sirius. There are a number of heroes and some heroines in the tale of how astronomy finally was able to come up with absolute distances, but probably the major technical advance that allowed calculation of distance was photography.
Looking at the sky, whether with the naked eye or through a telescope, discloses some stars but most are probably too faint to be seen. When light hits the eye, it is processed by the brain and discarded. No matter how long you stare at a star, the light that reaches you does not become cumulative. However, the longer a photographic plate or film is left exposed to light the more the light shows up. Over time what can’t be detected by the eye becomes a bright image on a plate. By studying the plates instead of the night sky, astronomers were able to detect thousands, even hundreds of thousands, more stars. But more importantly for determining distance, photographic plates of the same star taken over several days revealed that some stars change in brightness. While scientists had been aware of variability, in the past it could only be described in vague terms, such as “Alpha Hydrae much inferior to Gamma Leonis.” Photography allowed for objectivity.
One type of variable star, a Cepheid variable, proved most interesting. The graph of the variation in brightness of a Cepheid has a distinctive shark fin shape, increasing in brightness quickly and declining rather more slowly. The great thing about Cepheids is that they can be used to determine distance. Henrietta Leavitt, who began as an unpaid volunteer at Harvard’s observatory, made a breakthrough discovery. She focused her attention on the Small Magellanic Cloud, a cluster of stars that were known to be only “far away.” By studying 25 Cepheids she identified she was able to determine their absolute brightness, not just relative brightness.
Some years later, Edwin Hubble identified a Cepheid in the Andromeda Nebula. At the time the great debate over whether the Milky Way was all there is and nebulae were part of it, or whether nebulae were galaxies in their own right still raged. By using Leavitt’s work, Hubble was able to calculate the distance to the Andromeda Nebula at approximately 900,000 light years. Since it was known that the Milky Way is about 100,000 light years across, this conclusively proved that the Andromeda Nebula was so far away that it had to be a galaxy of its own.
Hubble was so astounded by his calculations that he delayed releasing them for several months. When he finally did, he wrote to Harlow Shapely, the main proponent of the nebulae-as-part-of-the-Milky Way view. Shapely replied by saying “here is the letter that ruined my universe.”
For the most part, nebulae were re-classified as galaxies. A few nebulae (which originally meant a cluster of celestial objects having a cloud-like appearance) were identified as clouds of gas and interstellar dust. Thus, with apologies to Star Wars, which made frequent use of “nebula” as a destination, that term really describes a gaseous dirt bag, hardly a proper place for a princess.
The universe had suddenly grown from the Milky Way to something that contained hundreds of thousands, if not millions, of Milky Ways.