Andromeda Constellation – The Astronomer’s Guide to the Chained Princess

In this article, we’ll be sharing with you everything you need to know about the constellation of Andromeda, including how to find it, deep space objects contained within it, and how to use it to find other night sky objects more easily.

The Constellation of Andromeda

Andromeda is one of the constellations that Greek astronomer Ptolemy cataloged more than 2000 years ago.

In Greek mythology, Andromeda was the daughter of Egyptian King Cepheus and Queen Cassiopeia. Cassiopeia boasted that she and Andromeda were more beautiful than any of Poseidon’s nymphs so Poseidon sent a monster to destroy Cepheus’s kingdom.

Cepheus consulted with an Oracle and was told that he could stop the monster by sacrificing Andromeda.

Andromeda was chained to a rock near the shore and would have died if Perseus had not saved her. Andromeda and Perseus married and after her death, Andromeda was placed in the heavens with her parents and husband.

To help you spot Andromeda, here’s what SkySafari 6 shows. Note the bright stars of Cassiopeia in the lower-left corner, including Caph, Shedar, and Navi.

The mythical figure of Andromeda the chained princess overlaid against the constellation's stars
The Chained Lady, Andromeda. Click for full-screen

Andromeda has an area of 722 square degrees making it the 19th largest of the 88 recognized constellations.

Andromeda is often depicted as a woman whose arms are outstretched above her head. Alternatively, imagine the major stars in the constellation represent Andromeda’s flowing hair. In both interpretations, Alpheratz represents Andromeda’s head.

In the sky chart below, you’ll see that Alpheratz forms one corner of the Great Square of Pegasus. You can also see the ‘W’ shape of Cassiopeia more clearly.

The most recognizable element of Andromeda is the two ‘legs’ of bright stars that seem to emerge from Alpheratz.

Andromeda's outline, close to the Great Square of Pegasus and Cassiopeia.
Andromeda’s outline, close to the Great Square of Pegasus and Cassiopeia. Click for full-screen.

In the next section discover how to find Andromeda.

How To Find Andromeda In The Night Sky

Andromeda is part of the Perseus family of constellations and is visible to observers at latitudes between +90° and -40°.

Northern Hemisphere observers can see it from August to February. The best months to see it in the evening are November and December when it is directly overhead. Southern Hemisphere observers can see it from October to December.

The most southeasterly parts of the constellation are circumpolar in mid-northern latitudes. Further north, such as in Alaska, Canada, and Scotland, practically the whole constellation is circumpolar, i.e. it never sets below the horizon.

The constellation of Andromeda is bordered by the constellations Cassiopeia, Lacerta, Pegasus, Perseus, Pisces, and Triangulum.

Showing the six constellations that border Andromeda
Six constellations border Andromeda. Click for full-screen

To find Andromeda, do a naked-eye search for the bright ‘W’ shape of Cassiopeia. Draw an imaginary line Caph to Shedar, then extend this line ~20° past Upsilon Persei to Almach.

You can measure night sky distances with your hand at arm’s length.

Alternatively, do a naked-eye search for The Great Square of Pegasus. Identify the northeast corner star, Alpheratz, that actually belongs to Andromeda. The constellation we’re seeking is the two streams of stars extending from Alpheratz in an approximate northeast direction.

How to find Andromeda constellation from Cassiopeia or Pegasus
Two methods for finding Andromeda. Stars are shown to magnitude 5.0. Click for full-screen

To find Andromeda’s exact position for your location on any night, use astronomy software such as Stellarium (free) or SkySafari 6.

The Brightest Stars in Andromeda

Andromeda is a large and bright constellation with around 30 stars brighter than magnitude 5.0. The star chart below, from SkySafari 6 shows these for reference. The more interesting ones are listed below that with some information about them.

The bright stars of Andromeda
Andromeda’s stars brighter than magnitude 5.0. Click for full-screen.

Alpha Andromedae (Alpheratz) – This variable double star is the brightest in Andromeda and marks both the northeast corner of the Great Square of Pegasus and Andromeda’s head.

The magnitude 2.05 blue-white subgiant primary and magnitude 11.11 secondary components are 91.2 arcseconds apart.

Alpheratz is a rotating variable star ranging in magnitude from 2.06 to 2.09 over 23 hours and it’s 97 light-years away.

Alpheratz means ‘the horse’s shoulder’ in Arabic (as in Pegasus, the horse).

Beta Andromedae (Mirach) – This double star is the second brightest star in Andromeda and marks Andromeda’s hips. The magnitude 2.07 orange-red giant primary and magnitude 14.40 main-sequence secondary components are 30.6 arcseconds apart.

Mirach is 200 light-years away from our solar system and is used by astronomers as the first stepping stone on the way to finding M31, Andromeda Galaxy.

Gamma Andromedae (Almach) – is the third brightest in the constellation of Andromeda and is widely regarded as one of the most beautiful triple stars in the night sky.

Almach marks Andromeda’s chained foot and is comprised of a magnitude 2.17 orange giant primary and a blue-green secondary component shining at magnitude 5.02 that are 9.4 arcseconds apart.

The secondary component is also a double star whose secondary is a magnitude 6.3 main-sequence. These two are only half an arcsecond apart, too close to be resolved by a regular astronomy telescope.

The Almach star system is 355 light-years away from us.

Delta Andromedae – is another double star and the fourth brightest in Andromeda. It is 106 light-years away and consists of a magnitude 3.26 orange giant, main-sequence primary, and a magnitude 12.44 secondary component that are 29.4 arcseconds apart.

This is the nearest bright star to Alpheratz in the southeastern ‘leg’ of Andromeda.

Epsilon Andromedae – is 1.5° south of Delta And. and shines at magnitude 4.34. It is a yellow-orange giant 169 light-years from Earth and moving away from us at 52 miles per second (54 km/s).

Its mass is 2.4 Solar masses and its radius is 9.8 Solar radii.

Zeta Andromedae – Keep traveling south from Epsilon and you end up at this variable double star.

Zeta And. is 189 light-years away from us and comprises a magnitude 4.09 orange giant primary and a magnitude 15.30 secondary that are 36.7 arcseconds apart.

This is a rotating variable ranging in magnitude from 3.92 to 4.14 over 17.8 days.

Eta Andromedae – This double star is 230 light-years away and 2.5° east of Zeta Andromedae. The primary component is a yellow-orange giant shining at magnitude 4.40. Its partner is much dimmer, at magnitude 12.34, and is 129.3 arcseconds away.

Theta Andromedae – Is to the northwest of the northwestern ‘leg’. It is a double star 309 light-years away from Earth. The two parties are so close together that they can’t be split in a backyard scope.

The two stars orbit each other every three years and are only four astronomical units (AU) apart. The combined magnitude of this binary star is 4.61.

Iota Andromedae – This magnitude 4.28 blue-white main-sequence star is 499 light-years away.

It’s a bright star, outshining our sun by a factor of 755 even though it is less than four times as massive.

Kappa Andromedae – is located 1° north of Iota And. This double star is 168 light-years away and is comprised of a magnitude 4.15 blue-white subgiant primary and a magnitude 11.30 secondary. The two components are 47.4 arcseconds apart.

Lambda Andromedae – This variable double star is 84 light-years away. The magnitude 3.86 yellow-orange giant primary and its magnitude 13.40 secondary component are 32.3 arcseconds apart.

This is an eruptive variable ranging in magnitude from 3.65 to 4.05 over a period of 54 days.

Additionally, three dwarf stars orbit the primary component making this is a quintuple system.

Mu Andromedae – We move back onto the main northwestern ‘leg’ with this double star.

Its magnitude 3.86 white main-sequence primary and magnitude 12.90 secondary components are 51.8 arcseconds apart and 130 light-years away.

Nu Andromedae – This star may only be 3° northwest of Mu And. but, at 620 light-years, it is almost five times further away from us.

Nu And. is a magnitude 4.5 blue-white main-sequence star. It’s a close double star with a tidally synchronized rotation. The secondary component is so close that the two orbit each other every 4.3 days.

This star is the second stepping stone to seeing Andromeda Galaxy after setting off from Mirach.

Xi Andromedae – This orange giant shines at magnitude 4.86 and is 214 light-years away. Its mass is 1.1 Solar masses, its diameter is 12.1 Solar diameters and it’s 19% cooler than the Sun.

Omicron Andromedae – Is found on the constellation’s western boundary with Lacerta. This double star is 687 light-years away and comprised of a magnitude 3.64 blue-white giant primary and a magnitude 6.03 secondary component that are 0.2 arcseconds apart.

Pi Andromedae – Moving back to the northwestern ‘leg’ we find this double star that is 598 light-years away.

The primary component is a magnitude 4.34 blue-white main-sequence star, and its secondary shines at magnitude 7.08 magnitude, 35.7 arcseconds away.

Sigma Andromedae – This variable double star is 135 light-years away. The magnitude 4.51 blue-white main-sequence primary and its secondary component are 6.6 arcseconds apart. This is an eruptive variable with a magnitude range of 4.48 to 4.55.

Tau Andromedae – The two stars in this binary shine at magnitudes 4.96 and 11.50. They are 53 arcseconds apart and 712 light-years away from Earth.

Upsilon Andromedae (Titawin) – This double star is 44 light-years away. The magnitude 4.09 yellow main-sequence primary and the magnitude 12.60 red dwarf secondary components are 110.9 arcseconds apart.

Phi Andromedae – This double star is 735 light-years away. The 4.26 magnitude blue-white main-sequence primary and magnitude 5.61 secondary components are, at 0.5″ apart, too close to be split with a regular telescope.

Chi Andromedae – This magnitude 5.01 yellow-orange giant sits between Almach and Upsilon Per. at the end of the two ‘legs’. It’s 250 light-years away from us, has a mass similar to our sun but it is 17% cooler and has a diameter 12 times larger.

Psi Andromedae – This double star is 1004 light-years away. The magnitude 4.98 yellow-orange supergiant primary and magnitude 14.60 magnitude secondary components are 24.8 arcseconds apart.

Omega Andromedae – This is another double, this time located 93 light-years away. The primary is a yellow-white subgiant shining at magnitude 4.82, while the secondary shines at magnitude 11.70. The two stars in this system are 0.7 arcseconds apart.

51 Andromedae (Upsilon Persei) – This star is unusual in that it’s named for the Perseus constellation even though it is today part of Andromeda.

The star shines at magnitude 3.58 at the end of the northwestern ‘leg’ of the constellation. It is an orange giant located 177 light-years away. Its mass is 1.2 times that of our sun but it has a 24 times larger diameter and is 26% cooler.

Star Hopping From Andromeda

Andromeda is the starting point for two well-known star hops. Star hopping is the process of finding dimmer, fainter objects by hopping from brighter, easy-to-find ones to begin with. You can read more about how to do this here, which opens a new tab.

M31 – Identify Alpheratz. Hop ~7° east to Delta Andromedae, ~8° northeast to Mirach, ~4° northwest to Mu Andromedae, and then ~4° northwest to M31.

See the guide below, which uses a star chart from SkySafari 6.

Finding the Andromeda Galaxy (M31) starting from Alpheratz
Finding the Andromeda Galaxy (M31) starting from Alpheratz. Click for full-screen

Read our detailed guide to finding Andromeda, including all the star charts you need to get there.

NGC 404 (Mirach’s Ghost Galaxy) – This magnitude 10.31 galaxy is ~0.1° northeast of the star Mirach. See above for finding it.

Objects To See Within Andromeda

Andromeda contains three Messier objects and some other deep sky objects suitable for small telescope users.

M31 (NGC 224, Andromeda Galaxy) – This spiral galaxy has a magnitude of 3.28 and an apparent size of 177.8 x 69.7 arcminutes. It’s 2.54 million light-years away and is at right ascension 00h 42m 44s and declination 41° 16’ 08” (J2000.0).

M32 (NGC 221) – This elliptical galaxy is the baby sibling of M31, both can be seen in the same eyepiece view, along with M110 (see next).

M32 has a magnitude of 7.92 and an apparent size of 7.7 x 4.9 arcminutes. It’s 2.5 million light-years away and is at right ascension 00h 42m 42s and declination 40° 51’ 55”.

M110 (NGC 205) – This elliptical galaxy is the third which can be seen alongside M31 and M32. It has a magnitude of 8.09 and an apparent size of 16.2 x 9.6 arcminutes. This galaxy is 2.7 million light-years away and is located at right ascension 00h 40m 22s and declination 41° 41’ 07”.

NGC 752 – This open cluster has a magnitude of 5.69 and an apparent size of 75.0 arcminutes. It is 1,300 light-years away and contains ~60 stars, all of which are magnitude 9.0 and dimmer. This cluster is located at right ascension 01h 57m 41s and declination 37° 47’ 00”.

NGC 891 – This spiral galaxy has a magnitude of 9.80 and an apparent size of 13.0 x 3.0 arcminutes, meaning it can be glimpsed even in smaller scopes under dark skies. This Caldwell object (C23) is 32 million light-years away and at right ascension 02h 22m 34s and declination 42° 20’ 51”.

NGC 7662 (Blue Snowball Nebula) – This planetary nebula has a magnitude of 8.30 and an apparent size of 0.5 x 0.5 arcminutes. It’s 3,600 light-years away and is at right ascension 23h 25m 54s and declination 42° 32’ 06”.

NGC 7686 – This open cluster has a magnitude of 5.59 and an apparent size of 16.0 arcminutes. It’s 5,003 light-years away, making it ~23 light-years from edge to edge. Find this cluster at right ascension 23h 30m 07s and declination 49° 08’ 00”.

Summary

Andromeda is the 19th largest constellation and, while famous for its eponymous galaxy – the most distant object visible to the naked eye – it also contains several notable stars and plenty of deep sky objects for small telescope users.

Look for it between Cassiopeia and the Great Square of Pegasus on a winter evening and be prepared to be awestruck by what the chained princess has to show you.


Written by Tanya C. Forde