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The Andromeda galaxy, our neighbour

Repérez la galaxie d’Andromède (M31) en utilisant les étoiles de Cassiopée et Andromède.
Credit: Montage Louie Bernstein/Espace pour la vie; Stellarium.org
Repérez la galaxie d’Andromède (M31) en utilisant les étoiles de Cassiopée et Andromède.
The Andromeda galaxy, our neighbour

Autumn is the ideal season for observing our distant cosmic neighbor, the Andromeda galaxy. It’s the only galaxy that’s easily visible to the naked eye from the Northern Hemisphere. It’s also the most distant object visible without optical aid. The Andromeda galaxy is located 2.5 million light years (25 billion billion kilometers) away. 

Galaxies are vast groups of stars. The Andromeda galaxy has about 800 billion of them, arranged in a disk that measures 220,000 light years in diameter; our galaxy, the Milky Way, only contains about 300 billion stars in a disk that’s just 120,000 light years across. Andromeda is a spiral galaxy whose curved arms consist of young stars and dust, which give it a bluish colour in photographs. However, the spiral arms are somewhat less evident because we view the galaxy almost edge-on. Andromeda also has several dwarf galaxies in orbit around it.

The Milky Way and Andromeda galaxy are the two largest in the Local Group, which contains about 60 members. And the Local Cluster is part of a much larger cluster, the Laniakea or Local Supercluster, which has about 100,000 galaxies. But it’s nothing compared to all the galaxies in observable universe, which number over 1 trillion…

The Andromeda galaxy is approaching the Milky Way at about 120 km/second and a collision is inevitable. But don’t worry, it won’t happen for another 4 billion years! And even the term collision evokes the wrong image: the two galaxies will penetrate one another and deform gravitationally, merging to form a new galaxy - a process that will take hundreds of millions of years. 

For a long time, we thought the Universe consisted of the Milky Way, and that the “Andromeda nebula” was a cloud of gas contained within it. But in 1923, Edwin Hubble, and astronomer in the United States, proved that Andromeda was another galaxy located more than a million light years away (according to his first estimate). By this discovery, Hubble showed that the Universe extended well beyond the Milky Way.

Grab your binoculars

During autumn, the Andromeda galaxy is visible all night. To observe it, you’ll need a relatively dark sky, away from the city, with as little light pollution as possible. In the evening, you’ll find it directly overhead near the zenith. On our seasonal sky map the Andromeda  galaxy is identified by it’s number in Charles Messier’s catalogue: M31.  Under a dark sky, on a moonless night, it appears as a hazy elongated patch of light to the naked eye, larger than the Moon. Using binoculars or a small telescope, you can see its bright nucleus surrounded by a much larger faint region. A telescope also reveals its two main satellite galaxies, which appear as small faint patches on either side of the nucleus. And with a bit of practice and some patience,  you’ll even be able to discern the galaxy’s spiral contours. 

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