Alpha Cephei

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α Cephei
Cepheus constellation map.svg
Red circle.svg
Location of α Cephei (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Cepheus
Right ascension 21h 18m 34.7715s[1]
Declination +62° 35′ 08.061″[1]
Apparent magnitude (V) 2.5141[2]
Spectral type A8Vn[3]
U−B color index +0.12[4]
B−V color index +0.21[4]
Variable type suspected δ Sct[5]
Radial velocity (Rv)−10[1] km/s
Proper motion (μ) RA: +150.55[2] mas/yr
Dec.: 49.09[2] mas/yr
Parallax (π)66.50 ± 0.11[2] mas
Distance49.05 ± 0.08 ly
(15.04 ± 0.02 pc)
Absolute magnitude (MV)1.57[6]
Mass1.74[6] M
Radius2.3[7] R
Diameter2.3[7] D
Luminosity17[7] L
Surface gravity (log g)3.99[7] cgs
Temperature7,740 ± 170[7] K
Rotational velocity (v sin i)246[8] km/s
Other designations
Alderamin, 5 Cep, 2MASS J21183475+6235081, AG+62° 1226, BD +61°2111, CCDM J21186+6236A, FK5 803, GCTP 5139.00, Gl 826, HD 203280, HIP 105199, HR 8162, SAO 19302.
Database references

Alpha Cephei (α Cephei, abbreviated Alpha Cep, α Cep), also named Alderamin,[9] is a second magnitude star in the constellation of Cepheus near the northern pole. The star is relatively close to Earth at 49 light years (ly).


α Cephei (Latinised to Alpha Cephei) is the star's Bayer designation. It has a Flamsteed designation of 5 Cephei.

It bore the traditional name Alderamin, a contraction of the Arabic phrase الذراع اليمين al-dhirā‘ al-yamīn, meaning "the right arm". In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[10] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[11] included a table of the first two batches of names approved by the WGSN; which included Alderamin for this star.


With a declination in excess of 62 degrees north, Alderamin is mostly visible to observers in the northern hemisphere, though the star is still visible to latitudes as far south as −27°, albeit just above the horizon. The star is circumpolar throughout all of Europe, northern Asia, Canada, and American cities as far south as San Diego. Since Alderamin has an apparent magnitude of about 2.5, the star is easily observable to the naked eye, even in light-polluted cities.

Pole star[edit]

Alderamin is located near the precessional path traced across the celestial sphere by the Earth's North pole. That means that it periodically comes within 3° of being a pole star,[12] a title currently held by Polaris. Aldermain will next be the North Star in about the year 7500 AD.[13]

Preceded by Pole Star Succeeded by
Iota Cephei circa 6,800 BC Deneb


Alderamin is a white class A star, evolving off the main sequence into a subgiant, probably on its way to becoming a red giant as its hydrogen supply runs low. In 2007, the star's apparent magnitude was recalibrated at 2.5141 along with an updated parallax of 66.50 ± 0.11 mas yielding a distance of 15 parsecs or approximately 49 light years from Earth.[2]

Given a surface temperature of 7,740 Kelvin, stellar models yield a total luminosity for the star of about 17 times the luminosity of the Sun. Alderamin has a radius of 2.3 times the Sun's radius and boasting a mass that is 1.74 that of the Sun.[7] Like other stars in its class, it is slightly variable with a range in brightness of 0.06 magnitude, and is listed as a Delta Scuti variable.

Alderamin has a very high rotation speed of at least 246 km/s, completing one complete revolution in less than 12 hours, with such a rapid turnover appearing to inhibit the differentiation of chemical elements usually seen in such stars.[8] By comparison, the Sun takes almost a month to turn on its axis. Alpha Cephei is also known to emit an amount of X radiation similar to the Sun, which along with other indicators suggests the existence of considerable magnetic activity—something unexpected (though not at all unusual) for a fast rotator.

Etymology and cultural significance[edit]

This star, along with Beta Cephei (Alfirk) and Eta Cephei (Alkidr) were al-Kawākib al-Firq (الكواكب الفرق), meaning "the Stars of the Flock" by Ulug Beg.[14][15]

In Chinese, 天鈎 (Tiān Gōu), meaning Celestial Hook, refers to an asterism consisting of α Cephei, 4 Cephei, HD 194298, Eta Cephei, Theta Cephei, Xi Cephei, 26 Cephei, Iota Cephei and Omicron Cephei.[16] Consequently, Alpha Cephei itself is known as 天鈎五 (Tiān Gōu wu, English: the Fifth Star of the Celestial Hook.).[17]


USS Alderamin (AK-116) was a United States Navy Crater class cargo ship named after the star.

See also[edit]


  1. 1.0 1.1 1.2 Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  2. 2.0 2.1 2.2 2.3 2.4 van Leeuwen, F (November 2007). "Hipparcos, the New Reduction". Astronomy and Astrophysics (Centre de Données astronomiques de Strasbourg) 474 (2): 653–664. arXiv:0708.1752. Bibcode 2007A&A...474..653V. doi:10.1051/0004-6361:20078357. Retrieved 2010-12-19.
  3. Gray, R. O.; Corbally, C. J.; Garrison, R. F.; McFadden, M. T.; Robinson, P. E. (2003). "Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 Parsecs: The Northern Sample. I". The Astronomical Journal 126 (4): 2048. arXiv:astro-ph/0308182. Bibcode 2003AJ....126.2048G. doi:10.1086/378365.
  4. 4.0 4.1 Mermilliod, J.-C. (1986). "Compilation of Eggen's UBV data, transformed to UBV (unpublished)". Catalogue of Eggen's UBV data. Bibcode 1986EgUBV........0M.
  5. Samus, N. N.Expression error: Unrecognized word "etal". (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007–2013)". VizieR On-line Data Catalog: B/gcvs. Originally published in: 2009yCat....102025S 1. Bibcode 2009yCat....102025S.
  6. 6.0 6.1 Pizzolato, N.; Maggio, A.; Sciortino, S. (September 2000), "Evolution of X-ray activity of 1–3 Msun late-type stars in early post-main-sequence phases", Astronomy and Astrophysics 361: 614–628, Bibcode 2000A&A...361..614P
  7. 7.0 7.1 7.2 7.3 7.4 Malagnini, M. L.; Morossi, C. (November 1990), "Accurate absolute luminosities, effective temperatures, radii, masses and surface gravities for a selected sample of field stars", Astronomy and Astrophysics Supplement Series 85 (3): 1015–1019, Bibcode 1990A&AS...85.1015M
  8. 8.0 8.1 Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  9. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  10. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  11. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  12. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  13. Our Monthly, 4, Presbyterian Magazine Company, 1871, p. 53,
  14. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  15. Davis Jr., G. A., "The Pronunciations, Derivations, and Meanings of a Selected List of Star Names,"Popular Astronomy, Vol. LII, No. 3, Oct. 1944, p. 16.
  16. (in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
  17. (in Chinese) 香港太空館 – 研究資源 – 亮星中英對照表 Archived 2008-10-25 at the Wayback Machine., Hong Kong Space Museum. Accessed on line November 23, 2010.

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