Zeta Cancri

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ζ Cancri A/B/C
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Cancer
Right ascension 08h 12m 12.7s
Declination +17° 38′ 52″
Apparent magnitude (V) +5.58/+5.99/+6.12
Distance83.4 ± 2.9 ly
(25.6 ± 0.9 pc)
Spectral typeF7V + F9V + G0V
Other designations
Tegmen, Tegmine, 16 Cancri, HR 3208/3209/3210, HD 68257/68255/68256, BD+18°1867, HIP 40167, SAO 97645/97646, GC 11142/11141, ADS 6650, CCDM J08123+1738, WDS 08122+1739

Zeta Cancri (ζ Cancri, abbreviated Zeta Cnc, ζ Cnc) is a multiple star system in the constellation of Cancer. It is approximately 83.4 light-years from Earth, and has a combined apparent magnitude of +4.67. Since it is near the ecliptic, it can be occulted by the Moon and, very rarely, by planets.

The system is constituted as follows:[1]

  • A binary pair designated Zeta¹ Cancri or alternatively Zeta Cancri AB, the two components of which are themselves designated Zeta¹ Cancri A or simply Zeta Cancri A (formally also named Tegmine /ˈtɛɡmɪn/, the traditional name of the Zeta Cancri system)[2] and Zeta¹/Zeta Cancri B.
  • A triple star system designated Zeta² Cancri or alternatively Zeta Cancri C, consisting of a single star primary, designated Zeta²/Zeta Cancri Ca, together with a secondary binary pair, designated Zeta²/Zeta Cancri Cb. The binary pair's two components are themselves designated Zeta²/Zeta Cancri Cb1 and Cb2.


ζ Cancri (Latinised to Zeta Cancri) is the system's Bayer designation; ζ¹ Cancri and ζ² Cancri those of its two constituents. The designations of the two constituents as ζ Cancri AB and C, and those of their components - ζ Cancri A, B, Ca, Cb, Cb1 and Cb2 - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[3]

Considerable confusion had developed concerning the catalogue identities of the three bright stars; correct correspondences were worked out by Griffin:[4]

Component HR HD SAO HIP
ζ Cancri A 3208 68257 97645 40167
ζ Cancri B 3209 68255
ζ Cancri C 3210 68256 97646

Zeta Cancri bore the traditional name Tegmine (Tegmen) "the shell (of the crab)".[citation needed] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[5] to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[6] It approved the name Tegmine for the component Zeta¹ Cancri A on 12 September 2016 and it is now so included in the List of IAU-approved Star Names.[2]

In Chinese, 水位 (Shuǐ Wèi), meaning Water Level, refers to an asterism consisting of Zeta Cancri, 6 Canis Minoris, 11 Canis Minoris and 8 Cancri.[7] Consequently, Zeta Cancri itself is known as 水位四 (Shuǐ Wèi sì, English: the Fourth Star of Water Level).[8]


Zeta Cancri can be resolved as a binary star in small telescopes. Its binary nature was discovered in 1756 by Tobias Mayer. William Herschel resolved the two components that make up Zeta¹ Cancri in 1781. As early as 1831, John Herschel noticed perturbations in Zeta² Cancri's orbit around Zeta¹; this led Otto Wilhelm von Struve, in 1871, to postulate a fourth, unseen, component which orbited closely the visible member of Zeta².[4] Later observations have resolved this fourth component and have indicated that there may be one or two more unobserved components.[9][10]

Zeta¹ and Zeta² Cancri are 5.06 arcseconds apart. These two star systems orbit around their common centre of mass once every 1100 years. The radius is about 6.80.[clarification needed]

Zeta¹ Cancri[edit]

The two components are both yellow-white main sequence dwarfs of spectral class F. The apparent magnitudes of A and B are +5.58 and +5.99, respectively. They are separated, as of 2008, by 1 arcsecond, requiring a large telescope to resolve them, but this separation will increase until the year 2020. They complete one orbit every 59.6 years.[11] The estimated masses for the pair are 1.28 and 1.18 solar masses, respectively.

Zeta² Cancri[edit]

Zeta Cancri Ca is the brightest of the three components, having an apparent magnitude of +6.12. It appears to be a yellow G-type star, often reported as G5V, but now thought to be earlier, probably G0V. This star has around 1.15 solar masses.[12] The tenth magnitude Zeta Cancri Cb is a close pair of red dwarfs. The separation between Ca and Cb is approximately 0.3 arcseconds, and their orbital period is 17 years.


  1. ^ "Displaying next number in catalog HIP => 40167". Multiple Star Catalog. Retrieved 2018-02-18.
  2. ^ a b "Naming Stars". IAU.org. Retrieved 16 December 2017.
  3. ^ Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707 [astro-ph.SR].
  4. ^ a b Griffin, R. F. (2000). "Spectroscopic Binary Orbits from Photoelectrical Radial Velocities: Paper 150: ζ Cancri C". The Observatory. 120: 1–47. Bibcode:2000Obs...120....1G.
  5. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
  6. ^ "WG Triennial Report (2015-2018) - Star Names" (PDF). p. 5. Retrieved 2018-07-14.
  7. ^ (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.
  8. ^ (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived August 19, 2010, at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.
  9. ^ Hutchings, J. B.; Griffin, R. F.; Menard, F. (2000). "Direct observation of the fourth star in the Zeta Cancri system". Publications of the Astronomical Society of the Pacific (abstract). 112 (772): 833–836. arXiv:astro-ph/0004284. Bibcode:2000PASP..112..833H. doi:10.1086/316587.
  10. ^ Richichi, A. (2000). "An Investigation of the multiple star Zet Cnc by a lunar occultation". Astronomy & Astrophysics. 364: 225–231. Bibcode:2000A&A...364..225R.
  11. ^ Mason; Hartkopf, William I.; Wycoff, Gary L.; Holdenried, Ellis R. (2006). "Speckle Interferometry at the US Naval Observatory. XII". The Astronomical Journal. 132 (5): 2219–2230. Bibcode:2006AJ....132.2219M. doi:10.1086/508231.
  12. ^ Fuhrmann, Klaus (February 2008), "Nearby stars of the Galactic disc and halo - IV", Monthly Notices of the Royal Astronomical Society, 384 (1): 173–224, Bibcode:2008MNRAS.384..173F, doi:10.1111/j.1365-2966.2007.12671.x