M 31 - Andromeda Galaxy

The Andromeda Galaxy, M31, is a giant spiral galaxy situated relatively close to our Milky Way. Although M31 is easily visible to the naked eye, it was first described in the year 964 by the Persian scientist Abd al-Rahman al-Sufi. However, this observation was lost over the years, and a telescope was needed to rediscover M31 by Simon Marius.

With a diameter of about 160,000 light-years, M31 is much larger than our galaxy. Nevertheless, new studies suggest 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 M31 photographically – the brightest Cepheid is about magnitude 16. M31 is surrounded by hundreds of globular clusters and many other objects such as HII regions, open clusters, and planetary nebulae. The two largest satellite galaxies of the Andromeda Galaxy are M32 and M110.

The current distance to our galaxy is about 2.5 million light-years, which will decrease over time. It is believed that both galaxies will collide in several billion years, forming a large elliptical galaxy.