Andromeda Constellation

Andromeda constellation is located in the northern sky, between Cassiopeia’s W asterism and the Great Square of Pegasus.

The constellation was named after the mythical princess Andromeda, the wife of the Greek hero Perseus. It is also known as the Chained Maiden, Persea (wife of Perseus), or Cepheis (daughter of Cepheus). Andromeda was first catalogued by the Greek astronomer Ptolemy in the 2nd century.

Among other notable deep sky objects, Andromeda constellation contains the famous Andromeda Galaxy (Messier 31) and the dwarf elliptical galaxies Messier 32 (Le Gentil) and Messier 110.


Andromeda is the 19th biggest star constellation in the night sky, occupying an area of 722 square degrees. It is located in the first quadrant of the northern hemisphere (NQ1) and can be seen at latitudes between +90° and -40°. The neighboring constellations are Cassiopeia, Lacerta, Pegasus, Perseus, Pisces and Triangulum.

Andromeda contains three Messier objects – Messier 31 (Andromeda Galaxy), Messier 32 and Messier 110 – and has seven stars with known planets. The brightest star in the constellation is Alpheratz. The Andromedids meteor shower (also known as the Bielids) in mid-November appears to originate from this constellation.

Andromeda belongs to the Perseus family of constellations, along with Auriga, Cassiopeia, Cepheus, Cetus, Lacerta, Pegasus, Perseus, and Triangulum.

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Andromeda Constellation Map, by IAU and Sky&Telescope magazine


In Greek mythology, Andromeda was the daughter of King Cepheus of Ethiopia and Queen Cassiopeia, who offended the Nereids (sea nymphs) by claiming that she was more beautiful than they were.

The nymphs complained to the sea god Poseidon and he sent a sea monster, Cetus, to flood and destroy Cepheus’ lands as punishment for his wife’s boastfulness. When the king sought advice from the Oracle of Ammon on how to prevent complete destruction of his lands, he was told that the only way to appease the gods and nymphs was to sacrifice his daughter to Cetus. Subsequently, Andromeda was chained to a rock and would have been left to the monser if Perseus had not come along and saved her. The two were later married and had six children, including Gorgophonte, who fathered Tyndareus, the famous Spartan king, and Perses, who was an ancestor of the Persians.

In the story, it was the goddess Athena who commemorated princess Andromeda by placing her image among the stars, next to the constellations representing her husband Perseus and her mother Cassiopeia.


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Alpheratz, photo by Stockholm Observatorium

Alpheratz – α Andromedae (Alpha Andromedae)

The brightest star in Andromeda is Alpheratz, Alpha Andromedae. It is sometimes also known as Sirrah. It lies 97 light years from Earth.

Alpheratz is a binary star with an apparent visual magnitude of +2.06. It is a hot blue star classified as a B8 subgiant.

The brighter of the two stars forming the binary has an unusual chemical composition, with extremely high levels of mercury, manganese, and other elements. Its mass is approximately 3.6 solar masses and its surface temperature is about 13,800 K.

With a luminosity 200 times that of the Sun, Alpheratz is the brightest mercury-manganese star known.

The companion star is also more massive than the Sun and has a luminosity 10 times that of the Sun. The two stars orbit each other within a period of 96.7 days.

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Andromeda Galaxy, Alpheratz and the Great Square of Pegasus, photo by Betelguese

Alpheratz was once considered to be a part of the Pegasus constellation along with being designated Alpha Andromedae, and it had the second designation, Delta Pegasi.

Both of its names, Alpheratz and Sirrah, are derived from the Arabic phrase al surrat al-faras, which means “the navel of the horse.” The horse refers to Pegasus.

Arabian astronomers also call the star al ras al mar’ah al musalsalah, which means “the head of the chained woman.”

Alpheratz is the northeastern star of the Great Square of Pegasus.

The three other stars that form the square are Alpha, Beta and Gamma Pegasi (Markab, Scheat, and Algenib respectively).

Alpheratz connects the Andromeda constellation with Pegasus, the horse Perseus rode to Andromeda’s rescue.

Mirach – β Andromedae (Beta Andromedae)

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Mirach and NGC 404, The Ghost of Mirach, photo by NASA

Mirach, Beta Andromedae, has roughly same apparent magnitude as Alpheratz, as it varies from +2.01 to +2.10. It is classified as a suspected semi-regular variable star.

Mirach is a cool, bright red class M giant, approximately 200 light years distant. It is 1,900 times more luminous than the Sun and 3-4 times more massive. It has a magnitude 14 hydrogen fusing star for a companion. Mirach is part of an asterism called the girdle.

The name Mirach is a corrupted derivation from the Arabic word for “girdle,” mizar, referring to the star’s placement at Andromeda’s left hip.

Mirach lies only seven arc-minutes away from the galaxy NGC 404. The galaxy is sometimes called Mirach’s Ghost because its proximity to the star makes it difficult to observe and photograph. NGC 404 is a lenticular galaxy, more than 10 million light years away.

Almach – γ Andromedae (Gamma Andromedae)

Almach, Gamma Andromedae, is the third brightest star in the constellation and also another binary star. Its name derives from the Arabic al-‘anaq al-‘ard, which means “caracal,” or “the desert lynx.” Almach is approximately 350 light years distant.

The brighter component of Almach, Gamma-1, is a golden yellow giant, while the companion is blue. They lie approximately 10 arc seconds apart. The primary star is a class K bright giant. It has an apparent magnitude of 2.26 and is about 355 light years distant. It is 2,000 times more luminous than the Sun.

The fainter companion star, Gamma-2, is itself a binary star, consisting of fifth and sixth magnitude white dwarf stars. The brighter component, a spectrograph will reveal, is also a double star.

δ Andromedae (Delta Andromedae)

Delta Andromedae is a double star with an apparent magnitude of 3.28, approximately 101 light years distant from our solar system. The brighter component is a K-type giant, while the dimmer companion is either a G-type main sequence star or a white dwarf.

ι Andromedae (Iota Andromedae)

Iota Andromedae is a B-type main sequence dwarf, bluish white in colour. It has a magnitude of +4.29 and is 503 light years distant.

υ Andromedae (Upsilon Andromedae)

Upsilon Andromedae is another binary star system in the Andromeda constellation, composed of a yellow-white dwarf and a fainter red dwarf. The primary star, Upsilon Andromedae A, has four planets in orbit, presumed to be jovian planets (similar to Jupiter). It is approximately 3.1 billion years old, which is to say younger than the Sun, as well as more massive and more luminous. Upsilon Andromedae B is a red dwarf that lies 750 AU from the primary star. It is both less massive and less luminous than the Sun.

Upsilon Andromedae is 44 light years distant. It is ranked 21st on the list of the top 100 target stars for the NASA Terrestrial Planet Finder mission.

Adhil – ξ Andromedae (Xi Andromedae)

Adhil, or Xi Andromedae, is also a double star. Its name derives from the Arabic word al-dhayl, which means “train” (or literally “tail”). Adhil is approximately 196 light years distant and has an apparent visual magnitude of +4.875. Its spectral class is G9.

Nembus – 51 Andromedae

With an apparent magnitude of 3.57, Nembus, or 51 Andromedae, is the fifth brightest star in the constellation. It is an orange K-type giant 177 light years distant. Even though Ptolemy originally included the star in the Andromeda constellation, Johann Bayer later moved it to Perseus as Upsilon Persei. English astronomer John Flamsteed moved it back to Andromeda, where it stays to this day.

μ Andromedae (Mu Andromedae)

Mu Andromedae is a white A-type dwarf that lies approximately 136 light years from Earth. It has an apparent magnitude of +3.86.

Other notable stars:

Pi Andromedae is another binary star, approximately 660 light years distant. The primary component has an apparent visual magnitude of 4.3, while the fainter companion is only 9th magnitude.

R Andromedae is a Mira type variable star lying about four degrees southwest of the Andromeda Galaxy. Its brightness varies between 5.8 and 14.9 over 409 days.

RX Andromedae is a Z Camelopardalis type variable star, with its brightness ranging between 10.3 and 14 over 14 day periods.

56 Andromedae is a binary star consisting of two magnitude six components.

Groombridge 34 is also a binary star, consisting of two red dwarfs. Only 11.7 light years distant, it is one of the closest double stars to the Sun. The two components are designated GX And (Groombridge 34 A) and GQ And (Groombridge 34 B).


Andromeda Galaxy (Messier 31, M31, NGC 224)

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Messier 31: Andromeda Galaxy, photo by John Lanoue

Messier 31 (NGC 224) is better known as the Andromeda Galaxy. It is a spiral galaxy approximately 2.5 million light years distant. In the past, it was also referred to as the Great Andromeda Nebula.

Andromeda Galaxy is the closest spiral galaxy to the Milky Way, and also the farthest object in the night sky visible to the naked eye. It has an apparent visual magnitude of 3.4, which makes it one of the brightest Messier objects. The galaxy contains a trillion stars. This is significantly more than the Milky Way Galaxy, which has approximately 200-400 billion stars.

Only the brightest central region of the Andromeda Galaxy is visible through a small telescope but, when observed through a larger telescope, the Andromeda Galaxy is about six times as wide as the full Moon.

Andromeda belongs to the Local Group of galaxies, along with the Milky Way, Triangulum Galaxy (Messier 33), and 30 or so smaller galaxies. Andromeda is the largest of the group. Its mass is estimated to be roughly equal to that of the Milky Way Galaxy, with which it is expected to collide in about 4.5 billion years.

The Andromeda Galaxy contains a notable deep sky object in itself: NGC 206, a bright giant star cloud. It also has 14 known dwarf galaxies as its satellites. The ones that are easiest to find in the night sky are Messier 32 and Messier 110.

Messier 32 (Le Gentil, NGC 221)

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Messier 32, photo by NASA

Messier 32, a dwarf elliptical galaxy approximately 2.65 million light years distant, was originally discovered by the French astronomer Guillaume Le Gentil in 1749. The galaxy is still occasionally known by the astronomer’s last name, Le Gentil.

Messier 32 was the first elliptical galaxy ever discovered. It is a satellite galaxy to the Andromeda Galaxy. M32 is small, but quite bright, and can easily be seen in a small telescope.

It is located 22 arc minutes south of Andromeda’s central region. It appears spread over Andromeda’s spiral arms and is believed to be on the side that is closer to us. M32 does not contain any globular clusters.

The galaxy is suspected to have been much larger at one point, but then lost its outer stars and globular clusters when it collided with the Andromeda Galaxy.

M32 is about 6.5 kly in diameter and mostly contains old stars, with no star formation going on inside the galaxy.

It has a supermassive black hole in its centre: The mass of the black hole is estimated to be between 1.5 and 5 million solar masses.

Along with Messier 110, Messier 32 is the closest bright elliptical galaxy to our solar system.

Messier 110 (NGC 205)

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Messier 110, Atlas Image courtesy of 2MASS/UMass/IPAC-Caltech/NASA/NSF

Messier 110 is a dwarf elliptical galaxy. It is often classified as a dwarf spheroidal galaxy. It contains eight globular clusters in the halo that surrounds it.

Atypical for a dwarf elliptical galaxy, M110 is showing signs of recent star formation. It does not appear to have a supermassive black hole, or at least there is no evidence that one exists at the centre of the galaxy.

Messier 110 was observed and described by Charles Messier in 1773 together with the Andromeda Galaxy and other objects he discovered, but M110 was not included on his original list of objects.

The galaxy was independently discovered by Caroline Herschel a decade later. Her discovery was then noted by her brother William Herschel in 1785, but it was not given a Messier number until 1967, when Kenneth Glyn Jones finally gave it one.

M110 is about 2.9 million light years distant.

Other notable deep sky objects:

NGC 752 is an open cluster in Andromeda. It is also sometimes known as Caldwell 28. The cluster is 1,300 light years distant.

NGC 891 is an edge-on spiral galaxy which can be seen four degrees east of Almach (Gamma Andromedae). It is visible in 4.5-inch telescopes.

NGC 7662 (The Blue Snowball Nebula or Caldwell 22) is a planetary nebula with a faint variable star, a bluish dwarf, at its centre and a blue disk surrounding it. It is located a degree west of Kappa Andromedae, a magnitude 4 star. The Blue Snowball Nebula is estimated to be about 1,800 light years away. It can be seen in a small refractor telescope, but only appears as a star-like object with some nebulosity.

NGC 7686 is an open cluster with an apparent magnitude of 5.6. It is 900 light years distant.