Beta Cassiopeiae

Beta Cassiopeiae (β Cassiopeiae, abbreviated Beta Cas or β Cas), officially named Caph /ˈkæf/,[12][13] is a Delta Scuti variable star in the constellation of Cassiopeia. It is a giant star belonging to the spectral class F2. The white star of second magnitude (+2.28 mag, variable) has an absolute magnitude of +1.3 mag.

Caph, Beta Cassiopeiae
Location of β Cassiopeiae (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Cassiopeia
Right ascension 00h 09m 10.68518s[1]
Declination +59° 08′ 59.2120″[1]
Apparent magnitude (V) +2.28 (2.25–2.31)[2]
Characteristics
Spectral type F2 III[3]
U−B color index 0.11[4]
B−V color index 0.34[4]
Variable type δ Sct[5]
Astrometry
Radial velocity (Rv)11.3[6] km/s
Proper motion (μ) RA: +523.50[1] mas/yr
Dec.: −179.77[1] mas/yr
Parallax (π)59.58 ± 0.38 mas[1]
Distance54.7 ± 0.3 ly
(16.8 ± 0.1 pc)
Absolute magnitude (MV)+1.3[7]
Details
Mass1.91 ± 0.02[8] M☉
Radius3.43–3.69[8] R☉
Luminosity27.3[8] L☉
Surface gravity (log g)3.40[5] cgs
Temperature7,079[5] K
Metallicity [Fe/H]0.03[8] dex
Rotational velocity (v sin i)71.0[9] km/s
Age1.09–1.18[8] Gyr
Other designations
Caph, Chaph, Kaff, Al Sanam al Nakah,[10] 11 Cas, Gl 8, HR 21, BD+58°3, HD 432, LHS 1027, GCTP 16.00, SAO 21133, FK5 2, HIP 746, GC 147, ADS 107, CCDM J00092+5909[11]
Database references
SIMBADdata

Nomenclature

Beta Cassiopeiae is the star's Bayer designation. It also bore the traditional names Caph (from the Arabic word كف kaf, "palm" – i.e. reaching from the Pleiades), Chaph and Kaff, as well as al-Sanam al-Nakah "the Camel's Hump".[10] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[14] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[15] included a table of the first two batches of names approved by the WGSN; which included Caph for this star.

Originally, the pre-Islamic Arabic term al-Kaff al-Khadib "the stained hand" referred to the five stars comprising the 'W' of the constellation Cassiopeia, and depicted a hand stained with henna. The term was abbreviated and somehow came to signify β Cassiopeiae alone. The old "stained hand" was part of an asterism called Thuraya stretching from the Pleiades, which signified the "head" through Taurus and Perseus and into Cassiopeia, while the other "hand" was in Cetus.[16]

In Chinese, 王良 (Wáng Liáng), in the Legs (Chinese constellation),[17] refers to an asterism consisting of β Cassiopeiae, κ Cassiopeiae, η Cassiopeiae, α Cassiopeiae and λ Cassiopeiae.[18] Consequently, the Chinese name for β Cassiopeiae itself is 王良一 (Wáng Liáng yī, English: the First Star of Wang Liang.)[19]

Together with Alpha Andromedae (Alpheratz) and Gamma Pegasi (Algenib), Beta Cassiopeiae was one of three bright stars known as the "Three Guides" marking the equinoctial colure.[10] This is an imaginary line running due south from Beta Cassiopeiae through Alpha Andromedae to the celestial equator, at a point where the Sun's path (the ecliptic) crosses it each autumn and spring equinox.[20]

Visibility

With a mean apparent magnitude (V-band) of +2.28, it is one of the five stars which make up the 'W' of Cassiopeia, adjacent to the just brighter Schedar (Alpha Cassiopeiae). SN 1572, traditionally known as Tycho's Star, appeared about 5 degrees to the northwest of Caph in 1572.[10]

As a star in the deep northern hemisphere of the sky, Beta Cassiopeiae is prominent to viewers in the northern hemisphere but not often seen by those in the southern hemisphere. The constellation of Cassiopeia does not rise above the horizon to viewers in Tasmania, and only low if one were in Cairns.[21]

System
A light curve for Beta Cassiopeiae, plotted from TESS data[22]

Beta Cassiopeiae is a yellow-white hued giant of stellar class F2 III with a surface temperature around 7,000 K. More than three times the size of and 28 times brighter than the Sun, Caph has an absolute magnitude of +1.3. It was once an A-type star with about double the Sun's mass.[8] It is now in the process of cooling and expanding to become a red giant. Its core is likely to have used up its hydrogen and is shrinking and heating, while its outer envelope of hydrogen is expanding and cooling. Stars do not spend much time in this state and are relatively uncommon. Caph's corona is unusually weak.[20]

Beta Cassiopeiae is a variable star of the Delta Scuti type; in fact, the fourth brightest of such stars in the sky after Denebola, Vega and Altair.[23] It is a monoperiodic pulsator,[24] with a brightness that ranges from magnitude +2.25 to +2.31 with a period of 2.5 hours.[20] This type of variable includes subgiant or main-sequence stars of spectral classes F5–A0, having masses between 1.5–2.5 solar masses and nearing the end of their core hydrogen fusion lifetime.[24] Their pulsations are related to the same helium instability strip on the Hertzsprung–Russell diagram as that of classical Cepheids. Delta Scuti stars are located at the intersection of the strip with the main sequence.[25]

This star is rotating at about 92% of its critical velocity, completing 1.12 rotations every day. This is giving the star an oblate spheroid shape with an equatorial bulge that is 24% larger than the polar radius. This shape is causing the polar region to have a higher temperature than the equator: the temperature difference is about 1,000 K. The axis of rotation is inclined about 20 degrees to the line of sight from the Earth.[8]

Beta Cassiopeiae was once considered to be a spectroscopic binary with a faint companion in a 27-day orbit, but it is now thought to be a single star.[26]

References
  1. van Leeuwen, F. (November 2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600.
  2. Kukarkin, B. V.; et al. (1971). "General Catalogue of Variable Stars containing information on 20437 variable stars discovered and designated till 1968". General Catalogue of Variable Stars (3rd ed.). Bibcode:1971GCVS3.C......0K.
  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. S2CID 119417105.
  4. 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.
  5. Daszyńska, J.; Cugier, H. (2003). "Far-Ultraviolet light curves of the δ Scuti variable: β Cassiopeiae". Advances in Space Research. 31 (2): 381–386. Bibcode:2003AdSpR..31..381D. doi:10.1016/S0273-1177(02)00630-0.
  6. Evans, D. S. (June 20–24, 1966). "The Revision of the General Catalogue of Radial Velocities". In Batten, Alan Henry; Heard, John Frederick (eds.). Determination of Radial Velocities and their Applications, Proceedings from IAU Symposium no. 30. Vol. 30. University of Toronto: International Astronomical Union. p. 57. Bibcode:1967IAUS...30...57E.
  7. Jaschek, C.; Gomez, A. E. (1998). "The absolute magnitude of the early type MK standards from HIPPARCOS parallaxes". Astronomy and Astrophysics. 330: 619. Bibcode:1998A&A...330..619J.
  8. Che, X.; et al. (May 2011). "Colder and Hotter: Interferometric Imaging of β Cassiopeiae and α Leonis". The Astrophysical Journal. 732 (2): 68. arXiv:1105.0740. Bibcode:2011ApJ...732...68C. doi:10.1088/0004-637X/732/2/68. S2CID 14330106.
  9. Schröder, C.; Reiners, Ansgar; Schmitt, Jürgen H. M. M. (January 2009), "Ca II HK emission in rapidly rotating stars. Evidence for an onset of the solar-type dynamo" (PDF), Astronomy and Astrophysics, 493 (3): 1099–1107, Bibcode:2009A&A...493.1099S, doi:10.1051/0004-6361:200810377
  10. Allen, Richard Hinckley (1963) [1899]. Star-names and their meanings. New York, NY: Dover Publications. p. 146. ISBN 1-931559-44-9.
  11. "V* bet Cas—Variable Star of delta Sct type". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2011-12-16.
  12. Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN 978-1-931559-44-7.
  13. "IAU Catalog of Star Names". Retrieved 9 August 2023.
  14. "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  15. "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 9 August 2023.
  16. Kunitzsch, Paul; Smart, Tim (2006). Dictionary of Modern Star Names. Cambridge, MA: Sky Publishing. p. 26. ISBN 1-931559-44-9.
  17. Wang Liang was a famous charioteer during the Spring and Autumn period
  18. (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.
  19. (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived 2009-09-29 at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.
  20. Kaler, James B. (2002). The Hundred Greatest Stars. New York: Copernicus Books. p. 27. ISBN 0-387-95436-8.
  21. Newell, W. J. (1965). The Australian Sky. Jacaranda Press. p. 84. ISBN 0-7016-0037-3.
  22. "MAST: Barbara A. Mikulski Archive for Space Telescopes". Space Telescope Science Institute. Retrieved 8 December 2021.
  23. Buzasi, D. L.; et al. (2005). "Altair: The Brightest δ Scuti Star". The Astrophysical Journal. 619 (2): 1072–76. arXiv:astro-ph/0405127. Bibcode:2005ApJ...619.1072B. doi:10.1086/426704. S2CID 16524681.
  24. Poretti, E. (2000). "The Frequency Content of δ Scuti Stars as Determined by Photometry". In Breger, Michel; Montgomery, Michael (eds.). Delta Scuti and Related Stars, Reference Handbook and Proceedings of the 6th Vienna Workshop in Astrophysics, held in Vienna, Austria, 4–7 August 1999. ASP Conference Series. Vol. 210. p. 45. arXiv:astro-ph/0002304. Bibcode:2000ASPC..210...45P.
  25. Percy, John R. (2007). Understanding variable stars. Cambridge University Press. pp. 139–144, 182–187. ISBN 978-0-521-23253-1.
  26. Teays, Terry J.; Schmidt, Edward G.; Pasinetti Fracassini, Laura E.; Fracassini, Massimo (15 August 1989). "The chromosphere of Beta Cassiopeiae". Astrophysical Journal. 343: 916. Bibcode:1989ApJ...343..916T. doi:10.1086/167761. S2CID 121071246.{{cite journal}}: CS1 maint: date and year (link)
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