The Milky Way

On a dark night we may see a band of light stretching across the sky. With binoculars or a small telescope this band is partially resolved into stars, as illustrated in the image on the right (Img Source). We call this band the Milky Way; it is composed primarily of stars that are too faint to be resolved so that we see their combined light as a faint glow. The Milky Way is an example of a gigantic collection of stars, gas, and dust that we call a galaxy.

Until the 1920s, it was fiercely debated whether the Milky Way was the entire Universe, or whether the "spiral nebulae" such as the Great Nebula in Andromeda were galaxies in their own right external to our own. This debate was finally resolved when Cepheid variables were used to measure distances to the globular clusters surrounding our own galaxy and to galaxies like Andromeda (M31). These measurements showed that Andromeda was much further away than the globular clusters, indicating that it was a separate galaxy.

We cannot of course view our galaxy from the outside, but the image below of the galaxy M100 is probably approximately what a top view of the Milky Way would look like. Another spiral galaxy that is probably similar in appearance to the Milky Way is M83.

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