The Andromeda galaxy is the closest full size galaxy to the Milky Way because of this it is known as our nearest galactic neighbor. The Andromeda Galaxy is also the only galaxy visible to the naked eye in the Northern Hemisphere. At 2.9 million light-years from Earth, Andromeda is also the farthest object that can be seen from Earth. Andromeda is not only known as our closest neighbor but scientists also refer to Andromeda and the Milky Way as sister galaxies. Andromeda closely resembles the Milky Way in shape (spiral), structure, and distribution of chemical elements. But that is where their similarities end, for Andromeda dwarfs the Milky Way and contains around twice as many stars.
This galaxy is set among the stars of the constellation of Andromeda, the tiny misty blur that astronomers know by the catalog number M 31 is easy to miss. Yet despite its unassuming appearance, M 31 is immensely greater than the stars that surround it.
For centuries, astronomers thought that the Andromeda galaxy was nothing more than a nebula, a cloud of light-reflective dust and gas situated within the Milky Way. Then, in the 1880s, astronomer Isaac Roberts used a 20-inch telescope to take the first detailed photograph of Andromeda. For the first time, the spiral arms were revealed; but since no one could make out any individual stars, M 31 was still assumed to be a nebula.
Ideas changed after a 100-inch telescope, the world's largest, opened on Mount Wilson near Los Angeles in 1917. The great astronomer Edwin Hubble was able to see for the first time that the outer spiral arms of the Andromeda galaxy contained individual stars. Theses appeared similar to many found in the Milky Way, but were much fainter. Hubble located three novae. One...