M31 - Andromeda Galaxy

Object image


On one side of Andromeda’s dress is a patch of light called M31 or the Andromeda galaxy. The great galaxy in Andromeda shines at mag. 3.5. This beautiful spiral galaxy is the most distant object visible to the naked eye, yet the closest major galaxy to the Milky Way, about 2,200,000 light years away.

Nearby are two small satellight galaxies, M32 (Elliptical galaxy), mag. 8.2. The other is NGC 205 (Elliptical galaxy), mag. 9.4.

All the stars that are visible to the naked eye lie within our own Milky Way galaxy. This means that the stars that make up all the constellations, including Virgo and Leo and Coma Berenices, are stars of our own galaxy. The galaxies we see in these constellations are not actually located in the constellations, they are only viewed along the same line of sight. Were we to actually go to another galaxy, even the Andromeda galaxy (which is the closest major galaxy to the Milky Way, only 2.2 million light years away), then the stars of the Milky Way would not be distinguishable. It makes no sense to talk of the constellations of Andromeda or Virgo as seen from another galaxy, since from another galaxy any observer would see all of the Milky Way stars together, just as we see a small patch of fuzzy light when we look at the Andromeda galaxy. It is not that Andromedans would see our constellation Andromeda differently; they would not distinguish it from the Milky Way at all.

The Andromeda galaxy has been known from early times. Al-Sufi described it as a "little cloud" in 964 AD. Simon Marius observed it in 1612 through a telescope, and described it as like a flame of a candle. It was not easy for astronomers to understand what a galaxy looks like. Early viewers of the Andromeda galaxy did not imagine that it was a star system like our own. Christian Huygens (HOY-gens) thought it was a hole in the heavens through which we might peer into the luminous regions beyond. Edmond Halley agreed, suggesting that the light came from a region of perpetual day, a shining ether filled with the light that originated on the first day of creation, before the formation of the Sun, Moon and stars. In 1845 Lord Rosse, using his great reflecting telescope, first resolved it into stars. In the 1920s Edwin Hubble--using the 100-inch telescope at Mount Wilson--used Cepheid variable stars to show that the Andromeda galaxy lay beyond the Milky Way. Thus Hubble established that the "nebulae" or cloudy spots which could be resolved into stars are actually external galaxies.