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Constellations: Perseus 'the Hero'

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The constellation of Perseus 'the Hero'.
Perseus seek for by her feet
Whichever at his shoulder are revolving.
Tallest of all his compeers at the North he towers.
His right hand stretches toward the chair of his bride's mother1.

- Phenomena of Aratos by Nathaniel Langdon Frothingham (1793 - 1870).
Name:Perseus
Genitive:Persei
Short form:Per
Area:615 sq deg
Co-ordinates2:Right Ascension 03h, Declination +45°
Origin:Ancient

Perseus is a typical constellation of the autumn and winter sky. However from European latitudes (or more northern) the constellation is at least partly circumpolar. It is very easy to find, since it is surrounded by very well-recognised constellations such as the 'W' of Cassiopeia, Auriga, Andromeda, Triangulum and Taurus. The 'Perseus Arm' is one of four major spiral arms of our galaxy, the Milky Way. We are in the minor 'Orion Arm'.

The constellation is 28 degrees in length, one of the largest in the heavens. Its principal stars form a curved line that bears the name 'the Segment of Perseus' - a figure that is almost as much of a stellar landmark as the Great Square of Pegasus, in mythology the winged horse. Perseus is directly above the Pleiades (M45), the unmistakable naked-eye star cluster in the constellation Taurus, and rises just before Andromeda.

Mythology

Perseus was a hero in Greek mythology. Unfortunately his story has been told in various versions. All sources agree that he was the son of Zeus and the mortal Danaë. Perseus and his mother had to flee to an island where King Polydectes ruled. The King married Danaë and after Perseus had grown up he was sent out in the world for adventure; he was supposed to cut off the head of the terrible monster Medusa. Other sources claim that Polydectes wanted to have Danaë's love, but she rejected his approaches. Perseus had to go and kill Medusa to save his mother from the intrusive king.

Medusa used to be a beautiful servant of Pallas Athena, the Greek goddess of Wisdom. But in hubris she compared herself with Athena, and so Athena turned her into a Gorgon, a terrible creature with snakes instead of hair, and boar's teeth. She was so frightening that anybody who looked at her face turned into stone. Another source says that Poseidon, the Greek god of the sea, raped Medusa, and Athena bewitched her because of the lost virginity. (Surely this is a questionable punishment, however it was typical of Athena: Arachne - who had also regarded herself to be better looking than the goddess, continued life as a spider).

In any case, Perseus was a favourite of Athena and Hermes the messenger of the gods, so he got valuable items from them: a magic hood for attaining invisibility, winged sandals so that he could fly, a sickle, a very well-polished shield, and a bag. With this equipment he approached Medusa while she and her Gorgon sisters3 were sleeping. He watched them indirectly via the shield which acted as a mirror, to avoid being turned into a rock. He cut off Medusa's head with the sickle and put it in the bag. Perseus sometimes used this head as a powerful weapon by just showing it to the enemy.

His second heroic deed was the rescue of Andromeda, daughter of King Cepheus and Queen Cassiopeia, from the sea monster Cetus. Perseus married Andromeda and became the ancestor of a whole race of people. All of the main figures of this legend have been set in the sky as constellations by the gods. Even Medusa found its place: the star Algol represents her eye.

Principal Stars

The scientific star names are simple to understand (if you know your Greek alphabet). The brightest star in the constellation is alpha, the next brightest is beta, and so on. Combined with the genitive name, this is known as the 'Bayer designation'. Some are named stars, like alpha Persei is known as Mirphak. Other stars are known by their catalogue number.

The brightest star in Perseus is Mirphak (alpha Persei), a white supergiant. Beta Persei, Algol, is a particularly interesting binary star system. It belongs to a class of variables that bears Algol's name. Stars of that classification actually consist of two stars that are in orbit around each other with a rather high frequency. So, within days, the smaller component is visible next to, or in front of, the greater companion, or hidden behind it. Consequently we see a change in brightness.

As already mentioned, Algol represented the terrible eye of Medusa for the ancient Greeks; in ancient Arabia it was a devilish demon called Gul. For the Hebrew people Algol was Lilith, Adam's first wife, who had turned into a vampire.

Star Table

StarDesignationNameMagnitudeDistance
(light years4)
Remarks
α Peralpha PerseiMirfak (elbow)1.8592Yellow supergiant
β Perbeta PerseiAlgol (the Ghoul5)2.1–3.495Eclipsing binary
γ Pergamma PerseiAl Fakhir
(the Excellent One)
2.9256Multiple star system
δ Perdelta PerseiAdid Borealis2.9527Blue giant
ε Perepsilon PerseiAdid Australis3.0538Multiple star system
ζ Perzeta PerseiMenkhib2.9980Blue supergiant (19 x solar mass)
η Pereta PerseiMiram3.761,330Orange supergiant
θ Pertheta Persei13 Persei3.936Yellow dwarf
ι Periota PerseiSAO 385974.0534Yellow dwarf
κ Perkappa PerseiMisam3.8112Orange giant:
Triple star system
ξ Perxi PerseiMenchib4.01,600Blue giant illuminates NGC 1499
ο Peromicron PerseiAl Atik3.81,480Binary star system
HD 16175HIP 12191unnamed7.3195Yellow G-type.
Has a planet HD 16175 b
(discovered Sept 2007)
HD 17092SAO 38313unnamed7.7355Orange giant.
Has a planet HD 17092 b
(discovered 2007)
HD 23596SAO 39110unnamed7.25170Yellow main sequence.
Has a planet HD 23596 b
(discovered 2002)
GK PerNova Persei 1901GK Persei0.2 max Nova remnant discovered 1901.
Brightest modern-time nova until 1918.

New General Catalogue (NGC)

The NGC was compiled by John Louis Emil Dreyer, the director of the Armagh Observatory from 1882 to 1916. Perseus is rich in open stellar clusters. M34 is particularly interesting for the not-so-well equipped stargazer. NGC 884 and NGC 869, better known as 'h Persei' and 'chi Persei', are a very famous cluster pair; they are a marvellous sight in a small telescope - don't magnify too much.

Catalogue NoNameTypeMagnitudeDistance
(light years)
Notes
NGC 1039M34open cluster61,500+100 stars
NGC 650/651Little Dumbbell Nebula
(M76)
Planetary
nebula
103,400-15,000aka Barbell Nebula
NGC 869Melotte 13open cluster4.56,800Double cluster with NGC 884
NGC 884Melotte 14open cluster67,600First recorded by Hipparchus
NGC 1499California NebulaEmission nebulaunknown1,000Illuminated by xi Persei
NGC 1275Perseus ASeyfert galaxy12.6250 millionMerging with
Perseus Cluster (Abell 426)
NGC 1333UnnamedReflection
nebula
101,000Less than a million years old

Meteor Showers

Meteor showers occur when the Earth passes through the debris path left by the tail of a comet. There are two annual meteor showers called the Perseids - so named because they appear to originate from the constellation of Perseus. The earlier Perseids are probably the most famous meteor shower. You should watch out for them from 20 July to 19 August, with the maximum around 11 August. Their origin is Comet 1862 III.

Extrasolar Planets in Perseus

There are several stars with planets - that we know of - in Perseus. HD 23596 b is at least eight times as massive as Jupiter, our Solar System's largest planet, which is the usual marker extrasolar planets are given, now known as the 'Jovian scale'. HD 17092 b has an eccentric orbit and is 1.3AU6 from its parent star, that is around a third further out than Earth's distance from our Sun. HD 16175 b is 4.5 times the mass of Jupiter. HD 16760 b orbits a binary star similar to our Sun. Its orbit of 465 days puts it within the system's habitable zone, but its mass, at 13× that of Jupiter, leads astronomers to suspect that it is a brown dwarf (failed star). The catchily-named WASP-11/HAT-P-10 b was discovered in 2008 in an extremely tight orbit around an orange dwarf star.

New Look in 2007

Perseus does not look 'Perseus' familiar to us due to the bright stellar object now.
- Comet 17P/Holmes observer reporting from Japan.

The constellation Perseus took on a new 'look' in late 2007. An insignificant visitor to the inner Solar System, Comet 17P/Holmes takes just under seven years to orbit the Sun once. Barely worth a mention in newsprint, Comet 17P/Holmes suddenly became a media megastar in October 2007. Instead of being around 17th magnitude - only visible to powerful telescopes - it had an eruption, brightening the comet by a factor of a million, and became 3rd magnitude. The eruption could have been caused by a build-up of gas below the surface suddenly spewing out, or another space body could have impacted with it. Comet 17P/Holmes was passing through a particularly volatile place at the time, the Asteroid Belt between the orbits of Mars and Jupiter, where there are lots of potential collisions. The eruption on the comet made it easy to spot with the unaided eye, even in cities with heavy light pollution.

Ancient cultures (the Chinese, in particular, thought comets were 'celestial ambassadors') would have had a field day with this comet's eruption, as comets have long been thought portents of momentous events. Comet 17P/Holmes was featured as Astronomy Picture of the Day's star-of-the-day for 3 November, 2007. The celestial firework was honoured again on 5 November when it displayed a green coma and blue tail. On 9 November Astronomy Picture of the Day published an image of the comet and the surrounding skyscape, showing the comet forming a triangle with alpha and delta Persei. On 19 November the comet passed directly in front of alpha Persei, Mirfak, changing the constellation's appearance once again.

1Cassiopeia.2Current IAU guidelines use a plus sign (+) for northern constellations and a minus sign (−) for southern ones.3Curiously, Medusa was mortal while her sisters were not.4A light year is the distance light travels in one year, roughly 5.88 trillion miles or 9.46 trillion km.5Arabian demon.6Astronomical Unit - 1AU is the average distance between Earth and the Sun, around 93 million miles (150 million km).

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