M 31 - Andromeda Galaxy
The Andromeda Galaxy M 31 is a giant spiral galaxy quite close to our Milky Way. Although M 31 is easily visible to the naked eye it was first 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 M 31 by Simon Marius.
With a diameter of about 160,000 light-years M 31 is much larger than our galaxy. Nevertheless, new studies claim that its mass may be comparable to that of the Milky Way.
Even with small amateur telescopes it is possible to resolve single stars in M 31 photographically – the brightest Chepeid is about 16mag. M 31 is surrounded by hundreds of globular clusters and many other objects like HII regions, open clusters and planetary nebula. 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.