On clear nights, a faintly glowing smudge is visible without optical aid between Cassiopeia and the great square of Pegasus in the constellation Andromeda. This patchy oval of light is the neighboring Andromeda Galaxy which, at 2.5 million light years away, is actually the closest spiral galaxy to our own and the farthest object from Earth visible to the naked eye.

This patchy oval of light is the neighboring Andromeda Galaxy which, at 2.5 million light years away, is actually the closest spiral galaxy to our own and the farthest object from Earth visible to the naked eye. With an apparent magnitude of 3.4, the Andromeda Galaxy, or M31, is one of the brightest objects in the Messier catalog, a list of nebulae and star clusters compiled by French astronomer Charles Messier. It is believed to span 220,000 light years and is tilted toward us at about a 15 degree angle from the edge-on position, allowing a decent glimpse of its structure.

With an apparent magnitude of 3.4, the Andromeda Galaxy, or M31, is one of the brightest objects in the Messier catalog, a list of nebulae and star clusters compiled by French astronomer Charles Messier. It is believed to span 220,000 light years and is tilted toward us at about a 15 degree angle from the edge-on position, allowing a decent glimpse of its structure. A spiral galaxy consists of a hub, made of old red and yellow stars encompassed by spiral arms containing younger, brighter stars, along with a lot of dust and gas. M31 has many more stars than the Milky Way - about one trillion stars versus a few hundred billion - and the estimated luminosity of M31 is about 25 percent higher than that of our own galaxy.

A spiral galaxy consists of a hub, made of old red and yellow stars encompassed by spiral arms containing younger, brighter stars, along with a lot of dust and gas. M31 has many more stars than the Milky Way - about one trillion stars versus a few hundred billion - and the estimated luminosity of M31 is about 25 percent higher than that of our own galaxy. M31 is visible even from moderately light polluted areas, but only the bright middle section toward the core can be seen with the naked eye. Through a telescope, use the lowest magnification available to allow the widest possible field of view. Resolving individual stars won't be possible.

M31 is visible even from moderately light polluted areas, but only the bright middle section toward the core can be seen with the naked eye. Through a telescope, use the lowest magnification available to allow the widest possible field of view. Resolving individual stars won't be possible.Most galaxies are receding from one another, but M31 and the Milky Way galaxy are gravitationally attracted and speeding toward each other at about 70 miles a second. Scientists predict a collision in a few billion years with two possible outcomes: Our solar system could be knocked farther toward the edge of our galaxy, or the two could combine into one massive elliptical galaxy.

Most galaxies are receding from one another, but M31 and the Milky Way galaxy are gravitationally attracted and speeding toward each other at about 70 miles a second. Scientists predict a collision in a few billion years with two possible outcomes: Our solar system could be knocked farther toward the edge of our galaxy, or the two could combine into one massive elliptical galaxy.For nearly a millennium, M31 suffered a bit of an identity crisis. Its earliest recorded observation was in 964 AD by Persian astronomer Abd al-Rahman al-Sufi, who described it as a small cloud in his Book of Fixed Stars. Other star charts from that period label it in a similar manner.

For nearly a millennium, M31 suffered a bit of an identity crisis. Its earliest recorded observation was in 964 AD by Persian astronomer Abd al-Rahman al-Sufi, who described it as a small cloud in his Book of Fixed Stars. Other star charts from that period label it in a similar manner.The first photograph of M31 was taken by amateur astronomer Isaac Roberts in 1887. Because our galaxy was thought to be not just the only galaxy but also the whole universe at that time, he thought it was merely a spiral-shaped nebula.

The first photograph of M31 was taken by amateur astronomer Isaac Roberts in 1887. Because our galaxy was thought to be not just the only galaxy but also the whole universe at that time, he thought it was merely a spiral-shaped nebula.In 1923 Edwin Hubble discovered Cepheid variable stars in the M31 "nebula." A variable star's luminosity changes over a period of time due to being obscured by another object or from changes within the star itself. The period-luminosity relationship of Cepheid variable stars in particular generates very precise distance measurements, and by correctly interpreting M31's Cepheid variable data Hubble determined it was not a nebula but an entirely separate galaxy.

In 1923 Edwin Hubble discovered Cepheid variable stars in the M31 "nebula." A variable star's luminosity changes over a period of time due to being obscured by another object or from changes within the star itself. The period-luminosity relationship of Cepheid variable stars in particular generates very precise distance measurements, and by correctly interpreting M31's Cepheid variable data Hubble determined it was not a nebula but an entirely separate galaxy.In recent years, important discoveries have been made about M31 using various technologies and observation techniques. In 2009, for example, its first planet may have been discovered using a technique called microlensing, which is caused by the deflection of light by a massive object. The planet is believed to be about six times bigger than Jupiter with an even larger companion star.

In recent years, important discoveries have been made about M31 using various technologies and observation techniques. In 2009, for example, its first planet may have been discovered using a technique called microlensing, which is caused by the deflection of light by a massive object. The planet is believed to be about six times bigger than Jupiter with an even larger companion star.