M 31 (NGC 224)

This nebula, being visible to naked eye, was known since ancient times. The Persian astronomer Abd al-Rahman al-Sufi described it as a "little cloud" in his "Book of the Fixed Stars" in 964 A.C.; Messier, clearly not informed of it, attributed the discovery of M31 to Simon Marius, who described it in 1612, after a telescopic observation.

G. B. Hodierna (1597-1660), sicilian astronomer in the court of Giulio Tomasi e Caro, first "Gattopardo", indipendently discovered and described it in his work "De Systemate Orbis Cometici, deque Admirandis Coeli Characteribus", printed in 1654 in Palermo.

It was known, for long time, as "the Great Andromeda Nebula": till the beginning of the XX century, it wasn't certain at all that M31 would lie out of our Milky Way. William Herschel belived, and obviously he mistaked, its distance from us about 2,000 times as big as the distance of Sirius. Immanuel Kant first supposed its possible extragalactic kind, in his work "General history of the Nature and theory of the Sky". In this work, the great philosopher hypothesized that some nebulae were realy stellar disks roughly similars, as to dimensions and shape, to our Milky Way.

William Huggins first observed the spectral differences between true gaseous nebulae and the galaxies. In 1923 the 2.5 m. Mt. Wilson's telescope resolved it, at least partially, into stars. Some of these stars was cepheids, whose period allowed to calculate the distance. From then on, there is no doubt on the extragalactic nature of M 31.

M31 lies about 1° W from the star n Andromedae (mag. 4.52).

M31 is visible to the naked eye. Its angular extension (its major axis is about 6 times the apparent size of the Full Moon) makes the observation less spectacular in a telescope than in a good pair of binoculars: under a very dark sky, a 7x50 gives an unforgettable sight.

M 31 is the giant galaxy closest to us; it forms, with our Milky Way and others, M 32, M 110 and M 33, the so called Local Group.

M 31 is certainly the more studied outer. It allows to study from the outside, indeed, all the features of a galaxy that we cannot observe in our Milky Way owing to the absorption of the interstellar matter. There are studies on the spiral pattern, globular and galactic clusters, interstellar matter, planetary nebulae, supernovae remnants, galacit nucleus, and more.