Andromeda Galaxy

The Andromeda Galaxy

Andromeda Galaxy
Jose Juan Gutierrez's image for:
"The Andromeda Galaxy"
Caption: Andromeda Galaxy
Image by: Adam Evans
© CC-BY-2.0 via Wikimedia Commons

The Andromeda Galaxy (M31/ NGC 224) is a spiral galaxy, lying in the Andromeda constellation, 2.5 million light-years from Earth. The Andromeda Galaxy is the most prominent galaxy in the local group, containing twice the number of stars of those in the Milky Way Galaxy.  M31 is the closest neighbor to the Milky Way Galaxy, and it´s also the farthest astronomical object that can be observed with the naked eye; with an apparent magnitude of 3.4, the Andromeda Galaxy can be easily observed during moonless, clear nights. It´s been estimated that the Andromeda Galaxy will collide with the Milky Way Galaxy some billions of years from now.


The Andromeda Galaxy is thought to have been formed when it merged with an existing galaxy some 5-9 billion years ago, creating its metal-rich galactic halo and extended gas and dust disk. The Andromeda Galaxy is known to be headed on a collision with the Milky Way Galaxy some 4.5 billion years from now. M31 is the closest galaxy to the Milky Way, and it´s moving closer each day, at an approximate speed of 300 km/s (190 miles/s). Collisions between galaxies are not rare in the universe; in fact, they´re considered normal in the evolution of the universe. Both the Andromeda and the Milky Way are believed to have collided with other galaxies in the past.


M31 is located in the Andromeda Constellation and appears as a fuzzy patch in the sky during very dark, non-polluted nights. It´s the nearest spiral galaxy, located at 2.5 million light-years away from Earth. The Andromeda Galaxy is the largest galaxy in the local group; however, it is not the most massive, as the most up to date discoveries reveal; the Milky Way contains more dark matter, making it the most massive in the local group. The Andromeda Galaxy contains an approximate one trillion stars, which accounts for twice the number of stars contained in the Milky Way, with 200-400 billion stars.

First observation

In 964 AD, Abd al-Rahman al-sufi, a Persian astronomer, was the first to describe the galaxy as a small cloud in a work called ¨Book of fixed stars.¨ In 1612, Simon Marius, a German astronomer, gave the first description of the galaxy based on telescopic observation; however, when Charles Messier catalogued the galaxy as M31, in 1764, he mistakenly credited Simon Marius as Andromeda Galaxy´s discoverer. First telescopic observations mistakenly confounded it with a gaseous nebula or solar system formation. Later, it became the center of a debate between two astronomers, but in 1925, Edwin Hubble convincingly determined that the Andromeda Galaxy was a distinct galaxy.

Separate galaxy

During the 1920s, the Andromeda Galaxy became the focus of a great debate between Harlow Shapely and Heber Curtis, two American astronomers. In those years, the astronomical community believed that the Milky Way comprised the whole universe and the fuzzy patches inside it were part of that universe. While Shapely believed M31 to be part of the Milky Way, Curtis strongly alleged it to be a separate galaxy due to some dark lanes, resembling nebulous clouds. For a long time, since its first observations, the Andromeda Galaxy was believed to be a nebula residing in the Milky Way Galaxy, but in 1922, Ernst Opik placed M31 in a new perspective at 1.5 million light-years away; however, in 1925, Edwin Hubble, using Cepheid variable stars, demonstrated that M31 was a separate galaxy located outside of the Milky Way Galaxy.

Andromeda and Milky Way collision

The Andromeda Galaxy is getting closer to the Milky Way Galaxy at an approximate speed of 300 km/s (190 miles/s) The two galaxies are expected to merge into a single giant elliptical galaxy in about four billion years from now. These galactic events are common among galaxies. It is not known what the fate of the solar system will be, although by the time of the collision, the sun will have already swollen into a red giant and swallowed the Earth.

At a 3.4 magnitude, and an apparent angle of 3.18º in the sky, the Andromeda Galaxy is one of the brightest sky objects, and can be easily observed with the unaided eye during dark nights away from polluted cities. Although its diameter is six times that of the moon, only the central part is visible, even with the use of a small telescope. According to, M31 can be observed even under suburban skies; however, with the use of binoculars, you can enhance the observation of this galaxy.

More about this author: Jose Juan Gutierrez

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