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Andromeda is a constellation of the northern hemisphere, named after the princess Andromeda, a character of the Greek mythology. It was one of the 48 constellations of Ptolemy.

Origin of the name

Andromeda's mother, Cassiopeia, kept bragging about her, saying to whoever wanted to hear it how Andromeda was the most beautiful in the world. When Cassiopeia went as far as saying Andromeda was even more beautiful than the Nymphs of the Sea, the latter asked their father Poseidon to avenge them. He sent a monster, Cetus, to rampage the coasts of the kingdom; the only way to calm the monster would be to sacrifice Andromeda.

Cepheus eventually accepted the condition and had his daughter chained to a rock so Cetus could eat her. Thankfully, hero Perseus happened to be nearby at the moment. He petrified Cetus using Medusa's head and freed Andromeda, whom he took away on his horse, Pegasus.

Shape and location

The constellation can be found using Algol (Beta Persei) and the Great square of Pegasus. From that square can be figured a "dipper" about thrice as big as the Big Dipper fromed by the square (Alpha Pegasi, Beta Pegasi, Lambda Pegasi and Alpha Andromedae (Alpheratz or Sirrah)) and the constellation Andromeda, beginning with Alpha Andromedae and continuing with Beta Andromedae (Mirach) and Gamma Andromedae (Almach); Algol can be found by extending the alignement.

The constellation's shape, depending on the books, will either be the aforementionned alignement of stars, or the same with a second alignement just above starting with Alpha Andromedae and connecting Mu Andromedae and 51 Andromedae.

It is bordered by : Perseus, Cassiopeia, Lacerta, Pegasus, Pisces and Triangulum.

Notable stars

Alpheratz marks Andromeda's head. Formerly, Alpheratz was considered part of Pegasus (its name means shoulder of the horse in Arab); this earned it the Bayer designation of Delta Pegasi. Along with α, β, and γ Pegasi, it forms an asterism called the Great Square of Pegasus. It's a very hot star, 110 times as bright as the Sun. It has a companion, a faint star of apparent magnitude 11.3.

Mirach ("the belt" in Arab), known as Beta Andromedae, is a red giant 30 times as large as the Sun. About 200 light-years away, it's apparent magnitude is 2.1.

Almach (or Alamak, "the lynx of the desert" in Arab), or Gamma Andromedae, is a multiple star with beautiful colour contrasts located at the tip of the southern "leg" of Andromeda. The main star, γ1 And, is an orange giant 80 times the size of the Sun and 2000 times as luminous as it. γ2 And is itself a double star, with both members blue. γ2-A And is of magnitude 5.0, and γ2-B And of magnitude 5.5; they take 61 years to turn around each other, in a strongly elliptic orbit. Finally, γ2-A And is itself a double star, its companion needing only 2.7 days to complete a revolution around it.

Upsilon Andromedae has three known exoplanets, 0,71 times, 2,11 times and 4,61 times as massive as Jupiter.

Finally, R Andromedae is a Mira variable which magnitude goes from to 5.8 to 14.9 on a period of 409 days.

Notable Deep sky objets

The main object of the deep sky to be observed in Andromeda is of course the Andromeda galaxy, (M31) one of the farthest object that can be seen with naked eyes. It's a large spiral galaxy, very much like the Milky Way, that can be found by drawing a line between Beta and Mu Andromedae, and extending the line about the same lenght past Mu Andromedae.

NGC 7640 is a SBb-type barred spiral galaxy, rather easy to locate.

NGC 7662 is a planetary nebula that can be seen easily with an amateur telescope, with which it reveals to be an elliptic blue-green disc.

NGC 752 is an open cluster occupying a rather large area near 56 Andromedae. Its made up of about a hundred magnitude 9 and 10 stars.

List of the stars found in the constellation

B F Name(s) and other designations Mag. Dist. (ly) Comments
α 21 Alpheratz, Alpherat, Al'faret, Al'ferats, Sirrah, Sirah, Sirrakh, Alpha Andromedae 2,06 97,1 From Arab سرة الفرس (surrat[u] al-faras), "horse's navel" (سرة seul est surrah, "navel")
Variable star de type α² CVn
β 43 Mirach, Merach, Mirac, Mirak, Mirakh, Merak, Mirar, Mirath, Mirax, El Mizar, Al Mizar, Beta Andromedae 2,07 199 De l'arabe al-maraqq, "the belt"
γ¹,² 57 Almach, Almaak, Alamak, Almak, Almaach, Alamac, Al'mak, Gamma Andromedae 2,26 355 From Arab الارض (al-canāq al-arđ̧) "the lynx of the desert"
Binary star ; components of magnitude 2,26 and 4,84
δ 31 Delta Andromedae 3,27 101
51 51 Andromedae 3,59 174
ο 1 Omicron Andromedae 3,62 692 Quadruple star
λ 16 Lambda Andromedae 3,81 84,2 Variable star de type RS CVn
μ 37 Mu Andromedae 3,86 136
ζ 34 Zeta Andromedae 4,08 181 Variable star de type RS CVn
υ 50 Upsilon Andromedae 4,09 43,9 Has three known exoplanets
κ 19 Kappa Andromedae 4,15 170
φ 42 Phi Andromedae, Keun Nan Mun 4,26 740 From Mandarin 軍南門 (jūnnánmén) "the camp's southern gate"
The name Keun Nan Mun is shared with χ And
ι 17 Iota Andromedae 4,29 503
ε 30 Epsilon Andromedae 4,34 169
π 29 Pi Andromedae 4,34 660
η 34 Eta Andromedae 4,40 243
σ 25 Sigma Andromedae 4,51 141
ν 35 Nu Andromedae 4,53 680
7 7 Andromedae 4,53
θ 24 Theta Andromedae 4,61 253
3 3 Andromedae 4,64
65 65 Andromedae 4,73
58 58 Andromedae 4,78
8 8 Andromedae 4,82
ω 48 Omega Andromedae 4,83 92,3
b 60 60 Andromedae 4,84
ξ 46 Adhil, Xi Andromedae 4,88 196 From Arab الذيل (að-ðayl), « la queue »
τ 53 Tau Andromedae 4,96 680
ψ 20 Psi Andromedae 4,97 1 310
χ 52 Chi Andromedae, Keun Nan Mun 5,01 242 From Mandarin 軍南門 (jūnnánmén) "the camp's southern gate"
The name Keun Nan Mun is shared with φ And
22 22 Andromedae 5,01
41 41 Andromedae 5,04
2 2 Andromedae 5,09
ρ 27 Rho Andromedae 5,16 160
64 64 Andromedae 5,19
28 28 Andromedae 5,20
14 14 Andromedae 5,22
A 49 49 Andromedae 5,27
4 4 Andromedae 5,30
32 32 Andromedae 5,30
c 62 62 Andromedae 5,31
18 18 Andromedae 5,35
55 55 Andromedae 5,42
11 11 Andromedae 5,44
36 36 Andromedae 5,46
15 15 Andromedae 5,55
63 63 Andromedae 5,57
47 47 Andromedae 5,60
44 44 Andromedae 5,67
5 5 Andromedae 5,68
56 56 Andromedae 5,69
23 23 Andromedae 5,71
13 13 Andromedae 5,75
12 12 Andromedae 5,77
45 45 Andromedae 5,80
10 10 Andromedae 5,81
6 6 Andromedae 5,91
39 39 Andromedae 5,95
9 9 Andromedae 5,98
59 59 Andromedae 6,09
26 26 Andromedae 6,10
66 66 Andromedae 6,16
HR 465, GY Andromedae 6,36 455 Its electromagnetic spectrum has rays of Promethium
Variable star of α² CVn type
Groombridge 34, GQ Andromedae, GX Andromedae 8.01 11.62 Near
Binary star ; components of magnitudes 8.08 et 11.06
Ross 248 12.29 10.32 Near

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