Perseus (constellation)

Perseus is a constellation in the northern sky, named after the Greek mythological hero Perseus. It is one of the 48 ancient constellations listed by the 2nd-century astronomer Ptolemy,^[1] and among the 88 modern constellations defined by the International Astronomical Union (IAU).^[2] It is located near several other constellations named after ancient Greek legends surrounding Perseus, including Andromeda to the west and Cassiopeia to the north. Perseus is also bordered by Aries and Taurus to the south, Auriga to the east, Camelopardalis to the north, and Triangulum to the west. Some star atlases during the early 19th century also depicted Perseus holding the disembodied head of Medusa,^[3] whose asterism was named together as Perseus et Caput Medusae;^[4] however, this never came into popular usage.

The

In Greek mythology, Perseus was the son of

In Neo-Assyrian Babylonia (911–605 BC), the constellation of Perseus was known as the Old Man constellation (SU.GI), then associated with East in the MUL.APIN, an astronomical text from the 7th century.^[8]

In non-Western astronomy

Four

In Polynesia, Perseus was not commonly recognized as a separate constellation; the only people that named it were those of the Society Islands, who called it Faa-iti, meaning "Little Valley".^[12] Algol may have been named Matohi by the Māori people, but the evidence for this identification is disputed. Matohi ("Split") occasionally came into conflict with Tangaroa-whakapau over which of them should appear in the sky, the outcome affecting the tides. It matches the Maori description of a blue-white star near Aldebaran but does not disappear as the myth would indicate.^[13]

Characteristics

Perseus is bordered by Aries and Taurus to the south, Auriga to the east, Camelopardalis and Cassiopeia to the north, and Andromeda and Triangulum to the west. Covering 615 square degrees, it ranks twenty-fourth of the 88 constellations in size. It appears prominently in the northern sky during the Northern Hemisphere's spring. Its main asterism consists of 19 stars. The constellation's boundaries, as set by Belgian astronomer Eugène Delporte in 1930, are defined by a 26-sided polygon. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 01^h 29.1^m and 04^h 51.2^m , while the declination coordinates are between 30.92° and 59.11°.^[14] The International Astronomical Union (IAU) adopted the three-letter abbreviation "Per" for the constellation in 1922.^[15]

Features

Stars

Algol (from the Arabic رأس الغول Ra's al-Ghul, which means The Demon's Head), also known by its Bayer designation Beta Persei, is the best-known star in Perseus. Representing the head of the Gorgon Medusa in Greek mythology, it was called Horus in Egyptian mythology^[16] and Rosh ha Satan ("Satan's Head") in Hebrew. Located 92.8 light-years from Earth, it varies in apparent magnitude from a minimum of 3.5 to a maximum of 2.3 over a period of 2.867 days.^[17] The star system is the prototype of a group of eclipsing binary stars named Algol variables, though it has a third member to make up what is actually a triple star system.^[18] The brightest component is a blue-white main-sequence star of spectral type B8V,^[19] which is 3.5 times as massive and 180 times as luminous as the Sun.^[18] The secondary component is an orange subgiant star of type K0IV that has begun cooling and expanding to 3.5 times the radius of the Sun,^[20] and has 4.5 times the luminosity and 80% of its mass.^[18] These two are separated by only 0.05 astronomical units (AU)—five percent of the distance between the Earth and Sun; the main dip in brightness arises when the larger fainter companion passes in front of the hotter brighter primary.^[18] The tertiary component is a main sequence star of type A7,^[21] which is located on average 2.69 AU from the other two stars. AG Persei is another Algol variable in Perseus, whose primary component is a B-type main sequence star with an apparent magnitude of 6.69.^[22] Phi Persei is a double star, although the two components do not eclipse each other. The primary star is a Be star of spectral type B0.5, possibly a giant star, and the secondary companion is likely a stellar remnant. The secondary has a similar spectral type to O-type subdwarfs.^[23]

With the historical name Mirfak (Arabic for elbow) or Algenib, Alpha Persei is the brightest star of this constellation with an apparent magnitude of 1.79. A

GRO J0422+32 (V518 Persei) is another X-ray binary in Perseus. One component is a red dwarf star of spectral type M4.5V,^[42] which orbits a mysterious dense and heavy object—possibly a black hole—every 5.1 hours.^[43] The system is an X-ray nova, meaning that it experiences periodic outbursts in the X-ray band of the electromagnetic spectrum.^[44] If the system does indeed contain a black hole, it would be the smallest one ever recorded.^[45] Further analysis in 2012 calculated a mass of 2.1 solar masses, which raises questions as to what the object actually is as it appears to be too small to be a black hole.^[46]

GK Persei, also known as Nova Persei 1901, is a bright nova that appeared halfway between Algol and Delta Persei. Discovered on 21 February 1901 by Scottish amateur astronomer Thomas David Anderson, it peaked at magnitude 0.2—almost as bright as Capella and Vega. It faded to magnitude 13 around 30 years after its peak brightness.^[47] Xi Persei, traditionally known as Menkhib,^[48] a blue giant of spectral type O7III, is one of the hottest bright stars in the sky,^[49] with a surface temperature of 37,500 K. It is one of the more massive stars, being between 26 and 32 solar masses,^[49][50] and is 330,000 times as luminous as the Sun.^[51]

Named Gorgonea Tertia,

Seven stars in Perseus have been found to have planetary systems. V718 Persei is a star in the young open cluster IC 348 that appears to be periodically eclipsed by a giant planet every 4.7 years.^[63] This has been inferred to be an object with a maximum mass of 6 times that of Jupiter and an orbital radius of 3.3 AU.^[64]

Deep-sky objects

The

Within the Perseus Arm lie two open clusters (NGC 869 and NGC 884) known as the Double Cluster.^[67] Sometimes known as h and Chi (χ) Persei, respectively, they are easily visible through binoculars and small telescopes.^[68][69] Both lie more than 7,000 light-years from Earth and are several hundred light-years apart. Both clusters are of approximately magnitude 4 and 0.5 degrees in diameter. The two are Trumpler class I 3 r clusters, though NGC 869 is a Shapley class f and NGC 884 is a Shapley class e cluster. These classifications indicate that they are both quite rich (dense); NGC 869 is the richer of the pair.^[70] The clusters are both distinct from the surrounding star field and are clearly concentrated at their centers. The constituent stars, numbering over 100 in each cluster, range widely in brightness.^[71]

M34 is an open cluster that appears at magnitude 5.5,^[72] and is approximately 1,500 light-years from Earth.^[73] It contains about 100 stars scattered over a field of view larger than that of the full moon.^[72] M34 can be resolved with good eyesight but is best viewed using a telescope at low magnifications. IC 348 is a somewhat young open cluster that is still contained within the nebula from which its stars formed. It is located about 1,027 light-years from Earth, is about 2 million years old,^[74] and contains many stars with circumstellar disks.^[75] Many brown dwarfs have been discovered in this cluster due to its age; since brown dwarfs cool as they age, it is easier to find them in younger clusters.^[76]

There are many nebulae in Perseus.

The Perseids are a prominent annual meteor shower that appear to radiate from Perseus from mid-July, peaking in activity between 9 and 14 August each year. Associated with Comet Swift–Tuttle, they have been observed for about 2,000 years.^[89] The September Epsilon Perseids, discovered in 2012, are a meteor shower with an unknown parent body in the Oort cloud.^[90]

See also

• Perseus (Chinese astronomy)

References

Cited texts

• Levy, David H. (2005). Deep Sky Objects. Amherst, New York: Prometheus Books.
  ISBN 978-1-59102-361-6
  .
• Makemson, M. W. (1941). The Morning Star Rises: An Account of Polynesian Astronomy. New Haven, Connecticut: Yale University Press.
  Bibcode:1941msra.book.....M
  .
• Motz, L.; Nathanson, C. (1991). The Constellations: An Enthusiast's Guide to the Night Sky. London, United Kingdom: Aurum Press.
  ISBN 978-1-85410-088-7
  .
• Schlegel, Gustaaf (1967) [1875]. Uranographie Chinoise (in French). Taipei, Republic of China: Ch'eng Wen Publishing Company.
• Staal, Julius D. W. (1984). Stars of Jade: Astronomy and Star Lore of Very Ancient Imperial China. Decatur, Georgia: Writ Press.
  ISBN 978-0-914653-00-4
  .

External links

Wikimedia Commons has media related to:
Perseus (constellation) (category)

• The Deep Photographic Guide to the Constellations: Perseus
• The clickable Perseus
• Warburg Institute Iconographic Database (medieval and early modern images of Perseus)
• "Perseus, in astronomy" . Encyclopædia Britannica (11th ed.). 1911.