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
Distance 83.4 ± 2.9 ly
(25.6 ± 0.9 pc)
Spectral type F7V + 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 contains at least four stars in two binary pairs, designated Zeta¹ and Zeta² Cancri. The two components of Zeta¹ CnC are themselves designated Zeta Cancri A (also named Tegmine[1]) and Zeta Cancri B, whilst those of Zeta² Cnc are designated Zeta Cancri C and Zeta Cancri D.

Zeta Cancri 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.


ζ Cancri (Latinised to Zeta Cancri) is the system's Bayer designation and ζ¹ Cancri and ζ² Cancri those of its two binary components. Considerable confusion had developed concerning the catalogue identities of the three bright stars; correct correspondences were worked out by Griffin:[2]

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

Zeta Cancri bore the traditional name Tegmine (Tegmen) "the shell (of the crab)". In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[3] to catalogue and standardize proper names for stars. The WGSN approved the name Tegmine for Zeta Cancri A on 12 September 2016 and it is now so entered in the IAU Catalog of Star Names.[1]

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.[4] Consequently, Zeta Cancri itself is known as 水位四 (Shuǐ Wèi sì, English: the Fourth Star of Water Level.)[5]


Zeta Cancri can be resolved as a binary star in small telescopes. Its binary nature was discovered in 1756 by Johann 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.[2] Later observations have resolved this fourth component and have indicated that there may be one or two more unobserved components.[6][7]

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

Zeta¹ Cancri[edit]

The 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.[8] The estimated masses for the pair are 1.28 and 1.18 solar masses, respectively.

Zeta² Cancri[edit]

Zeta Cancri C is the brighter of the two 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.[9] The tenth magnitude Zeta Cancri D has the color of a red dwarf, and may in fact be a close pair of two red dwarfs. The separation between C and D is approximately 0.3 arcseconds, and their orbital period is 17 years.


  1. ^ a b "IAU Catalog of Star Names". Retrieved 28 July 2016. 
  2. ^ 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. 
  3. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016. 
  4. ^ (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.
  5. ^ (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.
  6. ^ 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. Bibcode:2000PASP..112..833H. arXiv:astro-ph/0004284Freely accessible. doi:10.1086/316587. 
  7. ^ 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. 
  8. ^ 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. 
  9. ^ 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