Peering upward on late September evenings, the sky is dominated by the Milky Way, which arches from the southwest to the northeast. High overhead, we can easily view the Great Rift, an area within the Milky Way but, seemingly, almost completely devoid of stars. In fact, this absence of stars is due to the gas and dust, common in spiral galaxies such as our own, which obscures the stars beyond.
Almost everything we see in the night sky, with just our eyes, is part of our own Milky Way galaxy. Almost, and therein lies a tale.
After the moon has set, and the sky is very dark, if you scan east of the zenith, the point in the sky directly overhead, you will eventually come to a hazy spot, about one third the distance toward the horizon, and larger than the Moon. It may even look like a broken off piece of the Milky Way. That diffuse object is the Great Andromeda Galaxy, so you are viewing, with just your eyes, another galaxy two and a half million light years away. Yet, a hundred years ago most people, including most astronomers, believed that the entire universe consisted solely of our own galaxy. In fact, back then, this galaxy was referred to as the Andromeda Nebula.
Astronomer Edwin Hubble, for whom the Hubble Space Telescope is named, in 1923, observed Cepheid variable stars within the Andromeda galaxy. Such stars are referred to by astronomers as “standard candles” because there is a strong, direct relationship between their period of variability and their brightness. And, once a star’s intrinsic or “true” brightness is known, its distance can readily be determined. Although a better understanding of Cepheid variables has established that the Great Andromeda Galaxy is even further away than Hubble thought, his measurements clearly placed it at a distance which must be well beyond our own Milky Way.
If you have the chance to attend a star party this Fall, perhaps with the Black Canyon Astronomical Society, ask to view the Great Andromeda Galaxy. Look, especially, for its dark, obscuring bands of gas and dust, just like our own galaxy’s Great Rift.
You’ve been listening to Western Slope skies, produced by members of the Black Canyon Astronomical Society. I’m Bernie Forman.