Epsilon Cassiopeiae

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Epsilon Cassiopeiae
Cassiopeia constellation map.png
Segin is the leftmost star in the "W"
Observation data
Epoch J2000      Equinox J2000
Constellation Cassiopeia
Right ascension  01h 54m 23.72567s[1]
Declination +63° 40′ 12.3628″[1]
Apparent magnitude (V) 3.37[2]
Spectral type B3 V[3]
U−B color index -0.62[2]
B−V color index -0.15[2]
Variable type Periodic[4]
Radial velocity (Rv)−8.1[5] km/s
Proper motion (μ) RA: +32.09[1] mas/yr
Dec.: −18.94[1] mas/yr
Parallax (π)7.92 ± 0.43[1] mas
Distance410 ± 20 ly
(126 ± 7 pc)
Absolute magnitude (MV)−2.19[6]
Mass9.2 ± 0.2[7] M
Radius6[8] R
Luminosity2,500[9] L
Surface gravity (log g)3.5[10] cgs
Temperature15,174[10] K
Rotational velocity (v sin i)30[11] km/s
Age15.4 ± 3.0[7] Myr
Other designations
Segin, 45 Cassiopeiae, BD+62 320, FK5 63, HD 11415, HIP 8886, HR 542, SAO 12031
Database references

Epsilon Cassiopeiae (ε Cassiopeiae, abbreviated Eps Cas, ε Cas), officially named Segin /ˈsɛɡɪn/,[12][13] is a star in the constellation of Cassiopeia. With an apparent visual magnitude of 3.4,[2] this is one of the brightest stars in the constellation. The distance to this star has been determined directly using parallax measurements, yielding a value of around 390–430 light-years (120–130 parsecs) from the Sun.[1]


ε Cassiopeiae (Latinised to Epsilon Cassiopeiae) is the star's Bayer designation.

The star bore the traditional name Segin, which probably originates from an erroneous transcription of Seginus, the traditional name for Gamma Boötis, which itself is of uncertain origin.[14] Different sources report varying pronunciations, with SEG-in the most common but the variants SAY-gin and seg-EEN also appearing.[14][15] In 2016, the IAU organized a Working Group on Star Names (WGSN)[16] to catalog and standardize proper names for stars. The WGSN approved the name Segin for this star on 5 September 2017 and it is now so included in the List of IAU-approved Star Names.[13]

While some published reports incorrectly claim that this star was designated by NASA as Navi ('Ivan', backwards), in honor of astronaut Virgil Ivan "Gus" Grissom, one of the three astronauts who died in the Apollo 1 accident,[17] the actual star so designated is Gamma Cassiopeiae in the center of the constellation.[18][19]

In Chinese, 閣道 (Gé Dào), meaning Flying Corridor, refers to an asterism consisting of Epsilon Cassiopeiae, Iota Cassiopeiae, Delta Cassiopeiae, Theta Cassiopeiae, Nu Cassiopeiae and Omicron Cassiopeiae.[20] Consequently, the Chinese name for Epsilon Cassiopeiae itself is 閣道二 (Gé Dào èr, English: the Second Star of Flying Corridor.)[21]


Epsilon Cassiopeiae has a stellar classification of B3 V, indicating that it is a main sequence star fusing hydrogen in its core. Cote et al. (2003) indicate that it displays the spectral properties of a Be star, even though it is not categorized as such.[22] The presence of emission lines in the spectrum indicates the presence of a circumstellar shell of gas that has been thrown off by the star.[23] The outer atmosphere has an effective temperature of 15,174 K (14,901 °C; 26,854 °F),[10] giving it the blue-white hue of a B-type star.[24]

The interferometer-measured angular diameter is 0.43 milliarcseconds.[25] At the estimated distance of this star, this yields a physical size of roughly 6 times the radius of the Sun.[8]

Observation during the Hipparcos mission suggest that the star may undergo weak periodic variability. The amplitude of this variation is 0.0025 in magnitude with a frequency of 11.17797 times per day, or one cycle every 2.15 hours. The signal-to-noise ratio for this measured variation is 4.978.[4] Hipparcos measurements of the space velocity components for this star suggest that it is a member of the Cas-Tau group of co-moving stars, with a 93% likelihood. This group may be kinematically associated with the alpha Persei Cluster, indicating that the Cas-Tau group, including Epsilon Cassiopeiae, may have been separated from the cluster through tidal interactions.[26]


  1. ^ a b c d e f van Leeuwen, Floor (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752v1, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357 Note: see VizieR catalogue I/311.
  2. ^ a b c d Johnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4 (99): 99, Bibcode:1966CoLPL...4...99J
  3. ^ Slettebak, A. (1982). "Spectral types and rotational velocities of the brighter Be stars and A-F type shell stars". Astrophysical Journal Supplement Series. 50: 55. Bibcode:1982ApJS...50...55S. doi:10.1086/190820.
  4. ^ a b Koen, Chris; Eyer, Laurent (March 2002), "New periodic variables from the Hipparcos epoch photometry", Monthly Notices of the Royal Astronomical Society, 331 (1): 45–59, arXiv:astro-ph/0112194, Bibcode:2002MNRAS.331...45K, doi:10.1046/j.1365-8711.2002.05150.x
  5. ^ Wielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veröff. Astron. Rechen-Inst. Heidelb, Astronomisches Rechen-Institut Heidelberg, 35 (35), Bibcode:1999VeARI..35....1W
  6. ^ Huang, W.; et al. (2012), "A catalogue of Paschen-line profiles in standard stars", Astronomy & Astrophysics, 547: A62, arXiv:1210.7893, Bibcode:2012A&A...547A..62H, doi:10.1051/0004-6361/201219804.
  7. ^ a b Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (January 2011), "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun", Monthly Notices of the Royal Astronomical Society, 410 (1): 190–200, arXiv:1007.4883, Bibcode:2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x
  8. ^ a b Lang, Kenneth R. (2006), Astrophysical formulae, Astronomy and astrophysics library, 1 (3 ed.), Birkhäuser, ISBN 3-540-29692-1. The radius (R*) is given by:
  9. ^ Kaler, James B. "SEGIN (Epsilon Cassiopeiae)". University of Illinois. Retrieved 2010-02-21.
  10. ^ a b c Takeda, Yoichi; et al. (October 2010), "Oxygen and Neon Abundances of B-Type Stars in Comparison with the Sun", Publications of the Astronomical Society of Japan, 62 (5): 1239–1248, arXiv:1008.1220, Bibcode:2010PASJ...62.1239T, doi:10.1093/pasj/62.5.1239
  11. ^ Abt, Helmut A.; Levato, Hugo; Grosso, Monica (July 2002), "Rotational Velocities of B Stars", The Astrophysical Journal, 573 (1): 359–365, Bibcode:2002ApJ...573..359A, doi:10.1086/340590
  12. ^ starrynighteducation.com, Audio Pronunciation Guide
  13. ^ a b "Naming Stars". IAU.org. Retrieved 16 December 2017.
  14. ^ a b Simpson, Phil (2012). "3". Guidebook to the constellations. New York: Springer. ISBN 9781441969408.
  15. ^ "Audio Pronunciation Guide". Starry Night Education. Retrieved 7 August 2014.
  16. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  17. ^ Rao, Joe (August 29, 2003). "NightSky Friday: Rotanev, Derf, Navi, and other Backward Star Names". Space.com.
  18. ^ "Post-landing Activities". Apollo 15 Lunar Surface Journal. NASA. commentary at 105:11:33
  19. ^ "Apollo 10 Flown CSM Star Chart Directly from the Personal Collection of Mission Command Module Pilot John Young". Heritage Auction Galleries. Retrieved March 11, 2010.
  20. ^ (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.
  21. ^ (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived August 18, 2010, at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.
  22. ^ Cote, J.; van Kerkwijk, M. H. (July 1993), "New Bright Be-Stars and the Be-Star Frequency", Astronomy and Astrophysics, 274 (3): 870, Bibcode:1993A&A...274..870C
  23. ^ Andrillat, A.; Jaschek, M.; Jaschek, C. (July 1990), "A survey of Be stars in the infrared. II - Determination of envelope dimensions", Astronomy and Astrophysics Supplement Series, 84 (1): 11–28, Bibcode:1990A&AS...84...11A
  24. ^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on 2012-03-10, retrieved 2012-01-16
  25. ^ Nordgren, Tyler E.; et al. (December 1999), "Stellar Angular Diameters of Late-Type Giants and Supergiants Measured with the Navy Prototype Optical Interferometer", The Astronomical Journal, 118 (6): 3032–3038, Bibcode:1999AJ....118.3032N, doi:10.1086/301114
  26. ^ de Zeeuw, P. T.; et al. (January 1999), "A HIPPARCOS Census of the Nearby OB Associations", The Astronomical Journal, 117 (1): 354–399, arXiv:astro-ph/9809227, Bibcode:1999AJ....117..354D, doi:10.1086/300682