Andromeda Galaxy

The Andromeda Galaxy (pronounced /, also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda Nebula in older texts) is a spiral galaxy approximately 2,500,000 light-years (1.58×1011 AU) away in the constellation Andromeda. It is the nearest spiral galaxy to our own, the Milky Way. As it is visible as a faint smudge on a moonless night, it is one of the farthest objects visible to the naked eye, and can be seen even from urban areas with binoculars. It is named after the princess Andromeda (Greek: Ανδρομέδη – Andromédē) in Greek mythology. Andromeda is the largest galaxy of the Local Group, which consists of the Andromeda Galaxy, the Milky Way Galaxy, the Triangulum Galaxy, and about 30 other smaller galaxies.

At an apparent magnitude of 3.4, the Andromeda Galaxy is notable for being one of the brightest Messier objects,making it easily visible to the naked eye even when viewed from areas with moderate light pollution. Although it appears more than six times as wide as the full moon when photographed through a larger telescope, only the brighter central region is visible with the naked eye.

Observation history

The earliest recorded observation of the Andromeda Galaxy was in 964 CE by the Persian astronomer, Abd al-Rahman al-Sufi (Azophi),who described it as a "small cloud" in his Book of Fixed Stars. Other star charts of that period have it labeled as the Little Cloud.

In 1785, the astronomer William Herschel noted a faint reddish hue in the core region of the M31. He believed it to be the nearest of all the "great nebulae" and, based on the color and magnitude of the nebula, he incorrectly guessed that it was no more than 2,000 times the distance of Sirius.

William Huggins in 1864 observed the spectrum of M31 and noted that it differed from a gaseous nebula.The spectra of M31 displayed a continuum of frequencies, superimposed with dark absorption lines that help identify the chemical composition of an object.

In 1885, a supernova (known as "S Andromedae") was seen in M31, the first and so far only one observed in that galaxy. At the time M31 was considered to be a nearby object, so the cause was thought to be a much less luminous and unrelated event called a nova, and was named accordingly "Nova 1885".

The first photographs of M31 were taken in 1887 by Isaac Roberts from his private observatory in Sussex, England. The long-duration exposure allowed the spiral structure of the galaxy to be seen for the first time. However, at the time this object was commonly believed to be a nebula within our galaxy, and Roberts mistakenly believed that M31 and similar spiral nebulae were actually solar systems being formed, with the satellites nascent planets.

The result was the largest velocity recorded at that time, at 300 kilometres per second (190 mi/s), moving in the direction of the Sun.