by Robert Gendler
Distance: 2.5 million Light Years
M31 has played a pivotal historical role in astronomy. Early observers saw the soft, foggy patch of glowing light as just another spiral nebula but weren’t yet equipped with the knowledge to appreciate its nature.
The true nature of M31 began to became clear in 1923. In that year Edwin Hubble, using the just completed 100 inch telescope at the Mount Wilson observatory, made his monumental discovery of Cepheid Variable stars in M31 and in one stroke forever changed the astronomical paradigm of the universe as we know it.
Appropriately interpreting the cepheid data, Hubble was the first to appreciate the faint nebula in Andromeda as an “island universe”, an immense galaxy in its own right, similar to our Milky Way.