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 Cnc, ζ Cancri) is a star system in the constellation Cancer containing at least four stars. It has the traditional name Tegmine (Tegmen) "the shell (of the crab)". The star system is approximately 83.4 light years from Earth, and has a combined apparent magnitude of +4.67. Since ζ Cancri is near the ecliptic, it can be occulted by the Moon and, very rarely, by planets.

In Chinese, 水位 (Shuǐ Wèi), meaning Water Level, refers to an asterism consisting of ζ Cancri, 6 Canis Minoris, 11 Canis Minoris and 8 Cancri.[1] Consequently, ζ Cancri itself is known as 水位四 (Shuǐ Wèi sì, English: the Fourth Star of Water Level.)[2]


The ζ Cancri system contains two binary pairs, ζ¹ Cancri and ζ² Cancri, which 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

ζ Cancri can be resolved as a double star in small telescopes. The double nature of ζ Cancri was discovered in 1756 by Johann Tobias Mayer. It was discovered to be a triple star in 1781 by William Herschel when he resolved the two components that make up ζ¹ Cancri. As early as 1831, John Herschel noticed perturbations in ζ² orbit around ζ¹ Cancri. This led Otto Wilhelm von Struve, in 1871, to postulate a fourth, unseen, component which orbited closely the visible member of ζ² Cancri.[3] Later observations have resolved this fourth component and have indicated that there may be one or two more unobserved components.[4][5]

The components of ζ¹ Cancri are denoted ζ Cancri A and ζ Cancri B. They are both yellow-white main sequence dwarfs of spectral class F. The apparent magnitude the two stars are +5.58 and +5.99, respectively. The two stars 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.[6] The estimated masses for the pair are 1.28 and 1.18 solar masses, respectively.

The components of ζ² Cancri are denoted ζ Cancri C and ζ Cancri D. ζ Cancri C is the brighter of the pair, 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.[7] The tenth magnitude ζ 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.


Considerable confusion has developed concerning the catalogue identities of the three bright stars. Correct correspondences were worked out by Griffin:[3]

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


  1. ^ (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.
  2. ^ (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.
  3. ^ 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. 
  4. ^ 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/0004284free to read. Bibcode:2000PASP..112..833H. doi:10.1086/316587. 
  5. ^ 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. 
  6. ^ 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. 
  7. ^ 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