Epoch J2000.0 Equinox J2000.0
|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|
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. Consequently, ζ Cancri itself is known as 水位四 (Shuǐ Wèi sì, English: the Fourth Star of Water Level.)
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. Later observations have resolved this fourth component and have indicated that there may be one or two more unobserved components.
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. 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. 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:
|ζ Cancri A||3208||68257||97645||40167|
|ζ Cancri B||3209||68256|
|ζ Cancri C||3210||68255||97646|
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