Astronomer Edwin Hubble was studying an object in the autumn
constellation Andromeda known as the Andromeda Nebula. Viewed
through a telescope, it looks like a pinwheel, with bright
streamers wrapping around a big bulge in the middle. At the
time, most astronomers thought the Andromeda Nebula and similar
objects were bright pockets of matter inside the Milky Way. But
on October 6, 1923, Hubble noticed a particular type of star
inside the Andromeda Nebula (M31). Hubble realized that the star
was a Cepheid variable, a type of star that astronomers use to
measure distances in the universe. Hubble found that this
Cepheid -- as well as others that he saw on other photographs --
was far outside the Milky Way. When Hubble reported his findings
the following year, astronomers realized that they had misnamed
the Andromeda Nebula. It's not a nebula at all. Instead, it's a
galaxy -- the first confirmed "city of stars" beyond the Milky
Way. In the decades since Hubble's discovery, astronomers have
found that galaxies com