Gamma Cassiopeiae

Last updated
γ Cassiopeiae
Cassiopeia constellation map.svg
Red circle.svg
Location of γ Cassiopeiae (circled)
Observation data
Epoch J2000       Equinox J2000
Constellation Cassiopeia [1]
Right ascension 00h 56m 42.50108s [2]
Declination +60° 43′ 00.2984″ [2]
Apparent magnitude  (V)2.47 [3]
Characteristics
Evolutionary stage main sequence [4]
Spectral type B0.5IVe [5]
U−B color index −1.08 [3]
B−V color index −0.15 [3]
Variable type γ Cas [6]
Astrometry
Radial velocity (Rv)−6.8 [7] km/s
Proper motion (μ)RA: +25.17 mas/yr [2]
Dec.: −3.92 mas/yr [2]
Parallax (π)5.94±0.12  mas [2]
Distance 550 ± 10  ly
(168 ± 3  pc)
Absolute magnitude  (MV)−3.98 [1]
Orbit [5]
PrimaryAa
CompanionAb
Period (P)203.523±0.076 d
Semi-major axis (a)1.632+0.002
−0.001  AU [8]
Eccentricity (e)0
Inclination (i)45°
Semi-amplitude (K1)
(primary)		4.297±0.090 km/s
Orbit [9]
PrimaryAab
CompanionAc
Period (P)60.0  yr
Semi-major axis (a)0.15″
Details
Aa
Mass 15±2 [10]   M☉
Radius 10.9+0.8
−0.6 (equatorial)
7.9±0.4 (polar) [10]   R☉
Luminosity 19,000±500 [11]   L☉
Surface gravity (log g)3.50 [12]   cgs
Temperature 26,500 (polar)
17,300 (equatorial) [10]   K
Rotational velocity (v sin i)389±20 [10]  km/s
Age 8.0±0.4 [13]   Myr
Ab
Mass 0.93+0.04
−0.01 [8]   M☉
Radius 6,000+65
−320 [8]   km
Other designations
Tiansi, Tsih [14] , Navi, γ Cas, 27 Cassiopeiae, AAVSO 0050+60, BD+59°144, FK5 32, HD 5394, HIP 4427, HR 264, SAO 11482, ADS 782, WDS 00567+6043 [15]
Database references
SIMBAD data

Gamma Cassiopeiae, officially named Tiansi, [16] is a multiple star system at the center of the distinctive "W" asterism in the northern circumpolar constellation of Cassiopeia. It was observed in 1866 by Angelo Secchi, the first star ever observed with emission lines. [17] [18] It is now considered a Be star.

Contents

Gamma Cassiopeiae is a variable star system. Based upon parallax measurements made by the Hipparcos satellite, it is located at a distance of roughly 550 light-years from Earth. Together with its common-proper-motion companion, HD 5408, the system could contain a total of eight stars. [19] It is one of the highest multiplicity systems known. [18]

Names

γ Cassiopeiae (Latinized to Gamma Cassiopeiae, abbreviated Gamma Cas or γ Cas) is the object's Bayer designation, and it has the Flamsteed designation 27 Cassiopeiae. Although it is a fairly bright star with a combined apparent visual magnitude of 2.47, it had no traditional Arabic or Latin name.

The Chinese name Tsih, "the whip" (Chinese :策; pinyin :Cè), is commonly associated with this star. [20] [21] The name however originally referred to κ Cassiopeiae, [22] [23] and γ Cassiopeiae was just one of four horses (Chinese :天駟; pinyin :Tiān Sì; "Heavenly Quadriga") pulling the chariot of legendary charioteer Wangliang (see β Cassiopeiae). [22] This representation was later changed to make Gamma the whip. [22] The IAU Working Group on Star Names approved the name Tiansi for γ Cassiopeiae Aa on 13 November 2025, after the older Chinese name, and it is now so entered in the IAU Catalog of Star Names. [16]

The star was used as an easily identifiable navigational reference point during space missions and American astronaut Virgil Ivan "Gus" Grissom nicknamed the star Navi after his own middle name spelled backwards. [24] [25]

Physical properties

A light curve for Gamma Cassiopeiae, plotted from data published by Labadie-Bartz et al. (2021) GammaCasLightCurve.png
A light curve for Gamma Cassiopeiae, plotted from data published by Labadie-Bartz et al. (2021)
Amateur image of g Cassiopeiae and the associated nebulae IC63 and IC59. The bright star due south of Gamma Cassiopeiae is HD 5408, a common proper motion companion. (Neil Michael Wyatt) Gamma Cassiopeiae and its associated nebulosity.jpg
Amateur image of γ Cassiopeiae and the associated nebulae IC63 and IC59. The bright star due south of Gamma Cassiopeiae is HD 5408, a common proper motion companion. (Neil Michael Wyatt)

Gamma Cassiopeiae is an eruptive variable star. It is the prototype of the class of Gamma Cassiopeiae variable stars. In the late 1930s it underwent what is described as a shell episode and the brightness increased to above magnitude 2.0, then dropped rapidly to 3.4. [27] It has since been gradually brightening back to around 2.2. [28] At maximum intensity, γ Cassiopeiae outshines both Schedar (α Cas; magnitude 2.25) and Caph (β Cas; 2.3).

Gamma Cassiopeiae is a rapidly spinning star with a projected rotational velocity of 389 km/s, giving it a pronounced equatorial bulge. When accounted for its axial inclination, the true value is found to be 450±20 km/s. The equatorial radius is 10.9  R☉ and the polar radius is 7.9 R☉. [10] When combined with the star's high luminosity, the result is the ejection of matter that forms a hot circumstellar disk of gas. The emissions and brightness variations are apparently caused by this "decretion disk".

The spectrum of this massive star matches a stellar classification of B0.5 IVe. A luminosity class of IV identifies it as a subgiant star that has reached a stage of its evolution where it is exhausting the supply of hydrogen in its core region and transforming into a giant star. The 'e' suffix is used for stars that show emission lines of hydrogen in the spectrum, caused in this case by the circumstellar disk. This places it among a category known as Be stars; in fact, the first such star ever to be so designated. [29] It has 17 times the Sun's mass and is radiating as much energy as 19,000 Suns. At this rate of emission, the star is at least a third of the way through its main-sequence life [4] after a relatively brief 8 million years. [13] The outer atmosphere has an intense effective temperature of 25,000 K, which is causing it to glow with a blue-white hue.

X-ray emission

Gamma Cassiopeiae is the prototype of a small group of stellar sources of X-ray radiation that is about 10 times stronger than emitted from other B or Be stars. The character of the X-ray spectrum is Be thermal, possibly emitted from plasmas of temperatures up to least ten million kelvins, and shows very short term and long-term cycles. Historically, it has been held that these X-rays might be excited by matter originating from the star, from a hot wind or a disk around the star, accreting onto the surface of a degenerate companion, such as a white dwarf or neutron star. However, there are difficulties with either of these hypotheses. For example, it is not clear that enough matter can be accreted by a white dwarf, at the distance of the purported secondary star implied by the orbital period, sufficient to power an X-ray emission of nearly 1033 erg/s or 100 YW. A neutron star could easily power this X-ray flux, but X-ray emission from neutron stars is known to be non-thermal, and thus in apparent variance with the spectral properties.

Evidence suggests that the X-rays may be associated with the Be star itself or caused by some complex interaction between the star and surrounding decretion disk. One line of evidence is that the X-ray production is known to vary on both short and long time scales with respect to various UV line and continuum changes associated with a B star or with circumstellar matter close to the star. [30] [31] Moreover, the X-ray emissions exhibit long-term cycles that correlate with the light curves in the visible wavelengths. [32]

Gamma Cassiopeiae exhibits characteristics consistent with a strong disordered magnetic field. No field can be measured directly from the Zeeman effect because of the star's rotation-broadened spectral lines. Instead, the presence of this field is inferred from a robust periodic signal of 1.21 days that suggests a magnetic field rooted on the rotating star's surface. The star's UV and optical spectral lines show ripples moving from blue to red over several hours, which indicates clouds of matter being held frozen over the star's surface by strong magnetic fields. This evidence suggests that a magnetic field from the star is interacting with the decretion disk, resulting in the X-ray emission. A disk dynamo has been advanced as a mechanism to explain this modulation of the X-rays. However, difficulties remain with this mechanism, among which is that there are no disk dynamos known to exist in other stars, rendering this behavior more difficult to analyze. [33]

Companions

Gamma Cassiopeiae has three faint companions, listed in double star catalogues as components B, C, and D. [34] [35] [19] Star B is about 2 arc-seconds distant and magnitude 11, and has a similar space velocity to the bright primary, making it likely to be physically associated. Component C is magnitude 13, nearly an arc-minute distant, [36] [37] and is listed in Gaia Early Data Release 3 as having a very different proper motion and being much more distant than Gamma Cassiopeiae. [38] Finally, component D, about 21 arc-minutes distant, is the naked-eye star HR 266 (HD 5408), itself a quadruple system. [19]

Gamma Cassiopeiae A, the bright primary, itself contains a spectroscopic binary with an orbital period of about 203.5 days and an eccentricity alternately reported as 0.26 and "near zero." The companion is a white dwarf with a mass 0.93 times that of the Sun, and a size just smaller than Earth. [8] It is likely to be more evolved than the primary and to have transferred mass to it during an earlier stage of evolution. [5] [39] Additionally, Hipparcos data show a "wobble" with an amplitude of about 150 mas, that may correspond to the orbit of a third star. This star would have an orbital period of at least 60 years. [40]

See also

References

  1. 1 2 Anderson, E.; Francis, Ch. (2012). "XHIP: An extended hipparcos compilation". Astronomy Letters . 38 (5): 331. arXiv: 1108.4971 . Bibcode:2012AstL...38..331A. doi:10.1134/S1063773712050015. XHIP record for this object at VizieR.
  2. 1 2 3 4 5 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.
  3. 1 2 3 Nicolet, B. (1978). "Photoelectric photometric Catalogue of homogeneous measurements in the UBV System". Astronomy and Astrophysics Supplement Series. 34: 1–49. Bibcode:1978A&AS...34....1N.
  4. 1 2 Zorec, J.; et al. (2005). "On the evolutionary status of Be stars. I. Field Be stars near the Sun". Astronomy and Astrophysics. 441 (1): 235–248. arXiv: astro-ph/0509119 . Bibcode:2005A&A...441..235Z. doi:10.1051/0004-6361:20053051. S2CID   17592657.
  5. 1 2 3 Nemravová, J.; et al. (2012). "Properties and nature of Be stars. 29. Orbital and long-term spectral variations of γ Cassiopeiae". Astronomy & Astrophysics. 537: A59. arXiv: 1111.3761 . Bibcode:2012A&A...537A..59N. doi:10.1051/0004-6361/201117922. S2CID   34272401.
  6. ↑ Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007–2013)". VizieR On-line Data Catalog: B/GCVS. 1: 02025. Bibcode:2009yCat....102025S.
  7. ↑ Wilson, Ralph Elmer (1953). "General Catalogue of Stellar Radial Velocities". Carnegie Institute Washington D.C. Publication. Bibcode:1953GCRV..C......0W.
  8. 1 2 3 4 Gunderson, Sean J.; et al. (2024-12-27). "A Time-dependent Spectral Analysis of γ Cassiopeiae". The Astrophysical Journal. 978 (1): 105. arXiv: 2411.11825 . doi: 10.3847/1538-4357/ad944e . ISSN   0004-637X.
  9. ↑ Tokovinin, Andrei (March 2018). "The Updated Multiple Star Catalog". The Astrophysical Journal Supplement Series . 235 (1): 6. arXiv: 1712.04750 . Bibcode:2018ApJS..235....6T. doi: 10.3847/1538-4365/aaa1a5 . ISSN   0067-0049. S2CID   119047709.
  10. 1 2 3 4 5 Archer, A.; et al. (2025-06-18). "Measurement of the photosphere oblateness of γ Cassiopeiae via Stellar Intensity Interferometry with the VERITAS Observatory". arXiv: 2506.15027 [astro-ph.SR].
  11. ↑ Klement, R.; et al. (2017). "Revealing the structure of the outer disks of be stars". Astronomy and Astrophysics. 601: A74. arXiv: 1703.07321 . Bibcode:2017A&A...601A..74K. doi:10.1051/0004-6361/201629932. S2CID   56439879.
  12. ↑ Sigut, T. A. A.; Jones, C. E. (October 2007). "The Thermal Structure of the Circumstellar Disk Surrounding the Classical Be Star γ Cassiopeiae". Astrophysical Journal. 668 (1): 481–491. arXiv: 0706.4036 . Bibcode:2007ApJ...668..481S. doi:10.1086/521209. S2CID   14362961.
  13. 1 2 Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (2011). "A catalogue of young runaway Hipparcos stars within 3 KPC from the Sun". Monthly Notices of the Royal Astronomical Society. 410 (1): 190. arXiv: 1007.4883 . Bibcode:2011MNRAS.410..190T. doi: 10.1111/j.1365-2966.2010.17434.x .
  14. ↑ Falkner, David E. (2011). "The Autumn Constellations". The Mythology of the Night Sky . Patrick Moore's Practical Astronomy Series. pp.  139–162. doi:10.1007/978-1-4614-0137-7_8. ISBN   978-1-4614-0136-0. S2CID   127279699.
  15. ↑ "gam Cas -- High Mass X-ray Binary". SIMBAD. Strasbourg astronomical Data Center. Retrieved 20 July 2025.
  16. 1 2 "IAU Catalog of Star Names" . Retrieved 13 November 2025.
  17. ↑ Secchi, A. (1867). "Schreiben des Herrn Prof.Secchi, Dir. Der Sternwarte des Collegio Romano, an den Herausgeber". Astronomische Nachrichten. 68 (4): 63–64. Bibcode:1866AN.....68...63S. doi:10.1002/asna.18670680405.
  18. 1 2 Mamajek, Eric (April 2017). "Gamma Cassiopeiae and HR 266: A Massive Septuplet Illuminating the IC 59 and IC 63 Nebulae at d = 168 pc". Journal of Double Star Observations. 13 (2): 264–267. Bibcode:2017JDSO...13..264M.
  19. 1 2 3 Hutter, D. J.; et al. (2021). "Surveying the Bright Stars by Optical Interferometry. III. A Magnitude-limited Multiplicity Survey of Classical Be Stars". The Astrophysical Journal Supplement Series. 257 (2): 69. arXiv: 2109.06839 . Bibcode:2021ApJS..257...69H. doi: 10.3847/1538-4365/ac23cb . S2CID   237503492.
  20. ↑ 唐山 (Tángshān) (1967). 天文學太空航空學辭典 [Dictionary of astronomy and astronautics] (in Chinese). 廣文書局. OCLC   22797568.
  21. ↑ Allen, Richard Hinckley (1963) [1899]. Star-names and their meanings. New York, NY: Dover Publications. p. 146. ISBN   978-1-931559-44-7.{{cite book}}: CS1 maint: ignored ISBN errors (link)
  22. 1 2 3 Ridpath, Ian (1988). "Cassiopeia". Star Tales. James Clarke & Co. ISBN   9780718826956.
  23. ↑ Sun, Xiaochun; Kistemaker, Jacob (1997). The Chinese Sky During the Han: Constellating Stars and Society. Leiden, New York, Köln: Koninklijke Brill. pp. 150, 168. ISBN   9789004107373.
  24. ↑ "Post-landing Activities". Apollo 15 Lunar Surface Journal. NASA. commentary at 105:11:33
  25. ↑ "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.
  26. ↑ Labadie-Bartz, Jonathan; et al. (March 2021). "Short-term variability and mass loss in Be stars – VI. Frequency groups in γ Cas detected by TESS". Monthly Notices of the Royal Astronomical Society. 502 (1): 242–259. arXiv: 2012.06454 . doi: 10.1093/mnras/staa3913 .
  27. ↑ Baldwin, Ralph B.; Torp-Smith, Robert (1941). "Light Curves of Gamma Cassiopeiae". Popular Astronomy. 49: 127. Bibcode:1941PA.....49..127B.
  28. ↑ Pollmann, E.; et al. (2014). "Long-term monitoring of Halpha emission strength and photometric V magnitude of gamma Cas". Information Bulletin on Variable Stars. 6109: 1. Bibcode:2014IBVS.6109....1P.
  29. ↑ Merrill, P. W.; et al. (1925). "Discovery and Observations of Stars of Class Be". Astrophysical Journal. 61: 389. Bibcode:1925ApJ....61..389M. doi: 10.1086/142899 .
  30. ↑ Smith, M. A.; Robinson, R. D. (1999). "A multiwavelength campaign on γ Cassiopeiae. III. The case for magnetically controlled circumstellar kinematics". Astrophysical Journal. 517 (2): 866–882. Bibcode:1999ApJ...517..866S. doi:10.1086/307216. S2CID   122521231.
  31. ↑ Cranmer, S.; et al. (2000). "A Multiwavelength Campaign on γ Cassiopeiae. IV. The Case for Illuminated Disk-enhanced Wind Streams". Astrophysical Journal. 537 (1): 433–447. Bibcode:2000ApJ...537..433C. doi: 10.1086/309008 .
  32. ↑ Smith, Myron A.; et al. (2004). "High-Resolution Chandra Spectroscopy of γ Cassiopeiae (B0.5e)". Astrophysical Journal. 600 (2): 972–985. arXiv: astro-ph/0309293 . Bibcode:2004ApJ...600..972S. doi:10.1086/379873. S2CID   166002.
  33. ↑ Robinson, R. D.; et al. (2002). "X-Ray and Optical Variations in the Classical Be Star γ Cassiopeia: The Discovery of a Possible Magnetic Dynamo". Astrophysical Journal. 575 (1): 435–448. arXiv: astro-ph/0205278 . Bibcode:2002ApJ...575..435R. doi:10.1086/341141. S2CID   119495841.
  34. ↑ Mason, Brian D.; et al. (2001). "The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog". The Astronomical Journal. 122 (6): 3466. Bibcode:2001AJ....122.3466M. doi: 10.1086/323920 .
  35. ↑ Aitken, Robert Grant; Doolittle, Eric (1932). New general catalogue of double stars within 120° of the North pole. Carnegie Institution of Washington. Bibcode:1932ngcd.book.....A.
  36. ↑ "Hipparcos catalogue record". VizieR . Retrieved 2009-04-13.
  37. ↑ Dommanget, J.; Nys, O. (1994). "Catalogue des composantes d'etoiles doubles et multiples (CCDM) premiere edition - Catalogue of the components of double and multiple stars (CCDM) first edition". Communications de l'Observatoire Royal de Belgique. 115: 1. Bibcode:1994CoORB.115....1D.
  38. ↑ Brown, A. G. A.; et al. (Gaia collaboration) (2021). "Gaia Early Data Release 3: Summary of the contents and survey properties". Astronomy & Astrophysics . 649: A1. arXiv: 2012.01533 . Bibcode:2021A&A...649A...1G. doi: 10.1051/0004-6361/202039657 . S2CID   227254300. (Erratum:  doi:10.1051/0004-6361/202039657e). Gaia EDR3 record for this source at VizieR.
  39. ↑ Miroschnichenko, A. S.; et al. (2002). "Binary nature and long term nature of Gamma Cassiopeiae". Publications of the Astronomical Society of the Pacific. 114 (801): 1226. Bibcode:2002PASP..114.1226M. doi: 10.1086/342766 .
  40. ↑ Gontcharov, G. A.; et al. (2000). "New astrometric binaries among HIPPARCOS stars". Astronomy and Astrophysics. 355: 1164. Bibcode:2000A&A...355.1164G.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.