# Epsilon Cassiopeiae

Observation data Characteristics Epoch J2000      Equinox J2000 Segin is the leftmost star in the "W" Constellation Cassiopeia Right ascension 01h 54m 23.72567s[1] Declination +63° 40′ 12.3628″[1] Apparent magnitude (V) 3.37[2] Spectral type B3 III[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 Distance 410 ± 20 ly (126 ± 7 pc) Mass 9.2 ± 0.2[6] M☉ Radius 6[7] R☉ Luminosity 2,500[8] L☉ Surface gravity (log g) 3.5[9] cgs Temperature 15,174[9] K Rotational velocity (v sin i) 30[10] km/s Age 15.4 ± 3.0[6] Myr Segin, 45 Cassiopeiae, BD+62 320, FK5 63, HD 11415, HIP 8886, HR 542, SAO 12031.[3] SIMBAD data

Epsilon Cassiopeiae (ε Cas, ε Cassiopeiae) is a star system in the constellation Cassiopeia. It has the traditional name Segin. 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).[1]

## Properties

This is a giant star with a stellar classification of B3 III,[3] indicating that it has exhausted the hydrogen at its core and entered a later evolutionary stages of its lifetime. Cote et al. (2003) indicate that it displays the spectral properties of a Be star, even though it is not categorized as such.[11] 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.[12] The outer atmosphere of Epsilon Cassiopeiae has an effective temperature of 15,174 K,[9] giving it the blue-white hue of a B-type star.[13] The interferometer-measured angular diameter is 0.43 milliarcseconds.[14] At the estimated distance of this star, this yields a physical size of roughly 6 times the radius of the Sun.[7]

Observation of this star during the Hipparcos mission suggest that it 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.[15]

## Etymology

In Chinese, 閣道 (Gé Dào), meaning Flying Corridor, refers to an asterism consisting of ε Cassiopeiae, ι Cassiopeiae, δ Cassiopeiae, θ Cassiopeiae, ν Cassiopeiae and ο Cassiopeiae.[16] Consequently, ε Cassiopeiae itself is known as 閣道二 (Gé Dào èr, English: the Second Star of Flying Corridor.)[17]

While some published reports incorrectly claim that this star was designated by NASA as Navi, in honor of astronaut Virgil Ivan "Gus" Grissom, one of the three astronauts who died in the Apollo 1 accident,[18] the actual star so designated is Gamma Cassiopeiae in the center of the constellation.[19][20]

## References

1. 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), Bibcode:1966CoLPL...4...99J
3. ^ a b c "SIMBAD query result: Eps Cas -- Be Star". Centre de Données astronomiques de Strasbourg. Retrieved 2010-02-21.
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 (35), Astronomisches Rechen-Institut Heidelberg, Bibcode:1999VeARI..35....1W
6. ^ 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
7. ^ 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:
\begin{align} 2\cdot R_* & = \frac{(10^{-3}\cdot 126\cdot 0.43)\ \text{AU}}{0.0046491\ \text{AU}/R_{\bigodot}} \\ & \approx 12\cdot R_{\bigodot} \end{align}
8. ^ Kaler, James B. "SEGIN (Epsilon Cassiopeiae)". University of Illinois. Retrieved 2010-02-21.
9. ^ 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
10. ^ 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
11. ^ 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
12. ^ 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
13. ^ "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, retrieved 2012-01-16
14. ^ 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
15. ^ 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
16. ^ (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. ^ (Chinese) 香港太空館 - 研究資源 - 亮星中英對照表, Hong Kong Space Museum. Accessed on line November 23, 2010.
18. ^ Rao, Joe (August 29, 2003). "NightSky Friday: Rotanev, Derf, Navi, and other Backward Star Names". Space.com.
19. ^ "Post-landing Activities". Apollo 15 Lunar Surface Journal. NASA. commentary at 105:11:33
20. ^ "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.