Perseus Constellation – A Hero For Our Skies

In this article, we’ll be sharing with you everything you need to know about the constellation of Perseus, 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 Perseus

Greek astronomer Ptolemy cataloged this constellation more than 2000 years ago.

Perseus, the hero ‘rescuer of Andromeda’ was the son of Zeus and mortal woman Danae, daughter of Acrisius. When Perseus was born, King Acrisius placed Danae and Perseus in a wooden chest and threw them into the sea. Dictys, a fisherman from Seriphos, rescued them.

A series of events leading from that point brings us to Perseus on his way home one day when he sees Andromeda chained to a rock along the shoreline. He rescued her and was to marry her but a battle ensued and a desperate Perseus used Medusa’s head to turn his enemies to stone, killing Queen Cassiopeia and King Cepheus (Andromeda’s parents), in the process.

Perseus seems less of a hero still when we learn that he returned to Seriphos to present King Polydectes with Medusa’s head but ended up using it to turn the king and his followers into stone!

Even after all that, Perseus was deemed to have earned his place among the stars. To help you spot him, here’s what SkySafari shows.

Perseus 'hero' image with Medusa
Perseus the Hero, from SkySafari 6. Click for full screen

Perseus has an area of 615 square degrees making it the 24th largest of the 88 recognized constellations. Perseus doesn’t have a clear asterism and is usually depicted holding Medusa’s head in one hand, as shown above.

In common with most other constellations, the boundaries of Perseus extend beyond its asterism. The image below shows the boundaries of the constellation, which reach in between Cassiopeia and Andromeda.

Showing the boundary of Perseus constellation
The boundaries of Perseus, click for full screen

In the next section, we’ll discover how to find Perseus.

How To Find Perseus In The Night Sky

Perseus lends its name to the Perseus family of constellations, so-called for their involvement with the legend surrounding Perseus. They include Cassiopeia, Cepheus, Andromeda, Pegasus, and Cetus.

Perseus is visible to observers at latitudes between +90° and -35° and is circumpolar for observers north of 40° latitude. That includes the northernmost states of the USA and most of Europe.

Northern Hemisphere observers can see Perseus from August to March while Southern Hemisphere observers can see it from mid-spring to early summer. Evening observation is best in January when our Hero is directly overhead at 10 pm.

The constellation of Perseus is bordered by the constellations Andromeda, Aries, Auriga, Camelopardalis, Cassiopeia, Taurus, and Triangulum. If you’ve observed the annual Perseid meteor shower (peak Aug 12-13), you may recognize Perseus as the location of its radiant.

The constellations sharing a boundary with Perseus
Seven other constellations share a boundary with Perseus

To find Perseus, first, locate the big ‘W’ shape of Cassiopeia. Draw an imaginary line from Navi to Ruchbah, which is about 4°, and extend this line four times to reach Mirfak, the brightest star in Perseus. You can use your hand at arm’s length to estimate these distances.

Using Cassiopeia to find Perseus
Finding Perseus from Cassiopeia, click for full screen

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

Perseus’s Brightest Stars

There are no super-bright stars in Perseus but it is home to several 2nd and 3rd magnitude suns that are worth studying. The image below shows all those brighter than magnitude 5 with their proper names. Each is covered in more detail below the sky chart from SkySafari 6.

Named stars of Perseus brighter than magnitude 5.0
Stars brighter than magnitude 5.0 in Perseus. Click for full screen

Alpha Persei (Mirfak, Algenib) – This yellow-white, variable, double star is the brightest in Perseus. The 1.80 magnitude primary and 12.81 magnitude secondary components are 164 arcseconds apart. The companion is too faint for all but the largest backyard telescopes.

Mirfak is the brightest member of Perseus and is 590 light-years away. Its name is derived from the Arabic phrase meaning ‘the elbow of the Pleiades’. The alternative name for this star, Algenib, is derived from the Arabic word for ‘side’.

Beta Persei (Algol, Gorgonea Prima) – This triple star system is the second brightest star in Perseus and famous for its periodic dimming. The 2.10 magnitude, blue-white main-sequence primary (Beta Persei A), and 12.70 magnitude secondary component are 58.5 arcseconds apart.

The secondary component is actually a pair of stars: orange subgiant Beta Persei B and white, main sequence Beta Persei C. Every 2.87 days Beta Persei A is eclipsed by Beta Persei B, resulting in a decrease in magnitude to 3.4.

Algol represents Medusa’s evil eye and is 93 light-years away. Gorgonea Prima comes from Greek mythology and denotes that Beta Persei is the first Gorgon sister.

Gamma Persei (Al Fakhbir, Alphecher) – This 2.94 magnitude, yellow giant, variable, double star is the fifth brightest star in Perseus. The primary and 10.80 magnitude secondary component are 56.8 arcseconds apart.

Gamma Persei is Perseus’s second brightest eclipsing binary… but it only happens every 14 years, the last time being in 2019. Gamma Persei is 260 light-years away. Both its common names are derived from the Arabic phrase for ‘the Excellent One’.

Delta Persei (Basel, Adid Borealis) – This 3.01 magnitude, blue-white, variable, double star is the sixth brightest star in Perseus. The giant primary and 11.37 magnitude secondary components are 104 arcseconds apart.

Delta Persei is 530 light-years away. Basel is derived from the Arabic phrase for ‘The Brave One’ and Adid Borealis is derived from the Arabic-Latin phrase meaning ‘Northern One of the Upper Arm’ (of the Pleiades).

Epsilon Persei (Adid Australis) – This 2.91 magnitude, blue-white, variable, double star is the fourth brightest star in Perseus. The main-sequence primary and its 8.9 magnitude secondary component are 9.1 arcseconds apart, making them a great target for backyard double star hunts.

Epsilon Persei is 540 light-years away and its name, Adid Australis, comes from the Arabic phrase ‘Southern One of the Upper Arm’ (of the Pleiades).

Zeta Persei – This 2.88 magnitude, white supergiant, variable double star is the third brightest star in Perseus. The primary and 9.16 magnitude secondary component are 12.9 arcseconds apart, making for another lovely double. Zeta Persei is between 800 and 1300 light-years away.

Eta Persei (Miram) – This is a variable double star. The 3.75 magnitude, orange-red, giant primary, and 8.5 magnitude, secondary component are 30.4 arcseconds apart. The secondary component is a gold and blue pair of stars. Eta Persei is 1300 light-years away.

Theta Persei – This is a double star system. The 4.11 magnitude, yellow, main sequence, primary and 10.00 magnitude, red dwarf, secondary component are 20.4 arcseconds apart. Theta Persei is 37 light-years away.

Iota Persei – This is a double star. The 4.05 magnitude, yellow main sequence primary, and 12.40 magnitude secondary components are 154.4 arcseconds apart. Iota Persei is 34 light-years away.

Kappa Persei (Misam) – This magnitude 3.78, yellow-orange giant is part of a double or multiple star system. The primary and 13.50 magnitude secondary component are 44.1 arcseconds apart. The secondary has an unknown orbit. Kappa Persei is 113 light-years away.

Lambda Persei – This magnitude 4.28, blue-white supergiant is 422 light-years away. Its surface temperature is 1.7 times that of the Sun, it’s mass of 2.9 times that of the Sun and its luminosity is 340 times that of the Sun. 

Mu Persei – This yellow-orange supergiant is part of a double or multiple star system. The 4.25 magnitude primary and 10.28 magnitude secondary components are 14.1 arcseconds apart. The secondary component’s orbit is unknown. Mu Persei is an eruptive variable whose magnitude varies from 4.1 to 4.2. It’s 901 light-years away.

Nu Persei – This bright yellow-white giant, is part of a double or multiple star system. The 3.77 magnitude primary and 12.10 magnitude secondary component are 31.6 arcseconds apart. The secondary component’s orbit is unknown. Nu Persei is an eruptive variable whose magnitude varies from 3.8 to 3.9. It’s 557 light-years away.

Xi Persei (Menkib) – This 4.05 magnitude, blue-white giant is a variable double star. The primary and 14.00 magnitude secondary component are 2.4 arcseconds apart.

With a surface temperature around 37,000 Kelvin, Menkib is one of the hottest and most massive stars visible with the naked eye.

Menkib is 1600 light-years away and this name is derived from the Arabic meaning ‘shoulder’ (of the Pleiades and referring to a larger constellation of which Atik – below – is the collarbone).

Omicron Persei (Atik) – This magnitude 3.85, white giant is a variable double star. The primary and 6.7 magnitude, red dwarf, secondary component are 1.1 arcseconds apart. Omicron Persei is 1,475 light-years away. Atik is Arabic for ‘the shoulder’ of the Pleiades.

Pi Persei (Gorgonea Secunda) – This magnitude 4.69, white, main-sequence star is 310 light-years away. Gorgonea Secunda comes from Greek mythology and denotes that Pi Persei is the second Gorgon sister.

Rho Persei (Gorgonea Tertia) – This magnitude 3.41, bright orange giant is a semi-regular variable star which changes in magnitude from 3.3 to 4.0 over a period of ~50 days.

Gorgonea Tertia comes from Greek mythology and denotes that Rho Persei is the third Gorgon sister. Rho Persei is 317 light-years away.

Tau Persei – This magnitude 3.94, yellow giant is part of a double or multiple star system. The primary and 12.28 magnitude secondary component are 51.0 arcseconds apart. The secondary component’s orbit is unknown.

Tau Persei is an eclipsing variable whose magnitude varies from 3.9 to 4.1 with an irregular period and it is 288 light-years away.

Phi Persei – This magnitude 4.03, blue-white, variable, double star is an eruptive variable whose magnitude varies from 4.0 to 4.1 over a period of 19.5 days. The primary and 7.80 magnitude secondary component are so close that they can’t be split in a backyard telescope. Phi Persei is 718 light-years away.

Psi Persei – This magnitude 4.32 blue-white, main sequence variable star is an eruptive variable whose magnitude varies from 4.2 to 4.3 with an irregular period. Psi Persei is 283 light-years away.

Omega Persei (Gorgonea Quatra) – This magnitude 4.61, orange giant is part of a double or multiple star system. The primary and 12.4 magnitude secondary component are 181.2 arcseconds apart.

Gorgonea Quatra comes from Greek mythology and denotes that Omega Persei is the fourth Gorgon sister. Omega Persei is 288 light-years away.

Star Hopping From Perseus

Perseus leads the way to some deep sky objects.

M34 – This is an open cluster inside Perseus, near the boundary with Andromeda. Draw an imaginary line connecting Algol and Misam. Imagine this is the base of an isosceles triangle ~5° tall pointing east towards Almach in Andromeda. M34 is at the top of this triangle and accessible with the naked eye under perfect conditions.

M45 (The Pleiades) – In the list of stars above you’ve seen a lot of references to the Pleiades. While the awesome starfield is nearby, it’s actually in the constellation of Taurus, not Perseus.

To find it, draw an imaginary line from Ada Australis to Zeta Perseus (~8°), then extend this line another 8° to M45. This is one of the most pleasing views in the night sky and easy to see with the naked eye.

Melotte 20 (Alpha Persei Association) – Locate Mirfak. The cluster of stars surrounding it is known as Melotte 20. It does not have the formal designation of ‘open cluster’ but that is what it looks like for us on the ground.

The group contains over 100 stars in a 3° circle and is a rich area of star formation.

Objects To See Within Perseus

Perseus contains two Messier objects and many other deep sky objects available to smaller telescopes.

M34 (NGC 1039) – This open cluster has a magnitude of 5.19 and an apparent size of 35.0 arcminutes. It’s 1,400 light-years away and is at right ascension 2h 43m 24s and declination +42° 50’ 10”.

M76 (NGC 650 & NGC 651, Little Dumbell Nebula) – This planetary nebula has a magnitude of 10.10 and an apparent size of 2.7 x 1.8 arcminutes. It’s between 1700 and 15000 light-years away and is at right ascension 01h 43m 39s and declination +51° 40’ 15”.

The little dumbbell nebula
The Little Dumbbell Nebula (source)

Melotte 20 (Alpha Persei Association) – This group of related stars has a magnitude of 2.29 and an apparent size of 300 arcminutes. It’s 600 million light-years away and is at right ascension 3h 25m 47s and declination +49° 55’ 17” (see more detail in the section above).

NGC 869 & NGC 884 (Double Cluster)– This is a pair of open star clusters. NGC 869 has a magnitude of 5.30 and an apparent size of 18.0 arcminutes. It’s 6,800 light-years away and is at right ascension 2h 20m 27s and declination +57° 12’ 36”.

NGC 884 has a magnitude of 6.09 and an apparent size of 18.0 arcminutes. It’s 9,600 light-years away and is at right ascension 2h 23m 50s and declination +57° 12’ 32”.

NGC 1499 (California Nebula) – This massive but faint nebula has a magnitude of 5.00 and an apparent size of 145.0 x 40.0 arcminutes. It’s 1,800 light-years away and is at right ascension 4h 2m 2s and declination +36° 40’ 23”.

NGC 1333 – This bright nebula has a magnitude of 10.0 and an apparent size of 9.0 x 7.0 arcminutes. It’s 1,000 light-years away and is at right ascension 3h 30m 33s and declination +31° 29’ 10”.

NGC 1275 (Perseus A) – This spiral galaxy has a magnitude of 11.85 and an apparent size of 2.2 x 1.4 arcminutes. It’s 235 million light-years away and is at right ascension 3h 21m 9s and declination +41° 35’ 4”.

Abell 426 (Perseus Cluster) – This cluster of galaxies is one of the most massive objects in the known universe. It’s 240 million light-years away and is at right ascension 3h 18m 36s and declination +41° 30’ 54”.

NGC 1275 (above) is near the center of this cluster.

Summary

Perseus is an easy to find constellation with many notable stars and deep sky objects. Hop over from Cassiopeia to enjoy the wonders of this constellation.


Written by Tanya C. Forde