Andromeda Galaxy (M-31) was discovered by Simon Maurius in 1612. This galaxy also observed earlier by the ancient persian astronomer Al-Sufi and called it "the little cloud". At 2.2 million years away from the earth, andromeda is the closest major galaxy. Even we can see this galaxy in the night sky without a telescope.
Andromeda galaxy contain of billion of stars. Some of them located in the edge of the galaxy. In 1991, Hubble Space Telescope photograph the andromeda galaxy and revealed that a double nucleus lies at it's center.
Andromeda galaxy share many characteristic with our milky way. It has the same age and shape, and also contain many of the sane type of object. The similarity with milky way bring this galaxy to be very interesting object. Astronomers study about this galaxy to seeking clue about our galaxy. It's very difficult to study our galaxy because from earth position (middle of milky way) many part of our our galaxy are blocked by dust.
Source: Phillis Engelbert an Diane L Dupuis, The Handy Space Answer Books, Visible Ink Press, 1998