The Andromeda Galaxy M31 is a giant spiral galaxy quite close to our Milky Way. Although it is easily visible to the naked eye it was described in the year 964. This observation of the Persian scientist Abd-al-Rahman Al Sufi was lost over the years and a telescope was needed to re-discover M31 by Simon Marius.
With a diameter of about 160,000 light-years M31 is much larger than our galaxy. Nevertheless, new studies claim that its mass may comparable to that of the Milky Way. Even with small amateur telescopes it is possible to photographically resolve single stars in M31 – the brightest Chepeid is about 16mag. M31 is surrounded by hundreds of globular clusters and many other objects like HII regions, open clusters and planetary nebula can be detected within its spiral arms. The two largest satellite galaxies of the Andromeda Galaxy are M32 and M110.
The current distance to our galaxy of about 2,5 million light-years will decrease over the time and it is believed that both galaxies will collide In several billion years forming a large elliptical galaxy.