Zeta Cancri


Zeta Cancri is a multiple star system in the constellation of Cancer. Its name is a Bayer designation that is Latinized from ζ Cancri, and abbreviated Zeta Cnc or ζ Cnc. This group is located approximately 82 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.
The system is constituted as follows:
  • 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 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.

    Nomenclature

ζ 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 for multiple star systems, and adopted by the International Astronomical Union.
Considerable confusion had developed concerning the catalogue identities of the three bright stars; correct correspondences were worked out by R. F. Griffin:
ComponentHRHDSAOHIP
ζ Cancri A3208682579764540167
ζ Cancri B3209682559764540167
ζ Cancri C3210682569764640167

Zeta Cancri bore the traditional name Tegmine "the shell ". In 2016, the International Astronomical Union organized a Working Group on Star Names to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems. 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.
In Chinese, 水位, meaning Water Level, refers to an asterism consisting of Zeta Cancri, 6 Canis Minoris, 11 Canis Minoris and 8 Cancri. Consequently, Zeta Cancri itself is known as 水位四.

Properties

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. Later observations have resolved this fourth component and have indicated that there may be one or two more unobserved components.
Zeta1 and Zeta2 Cancri are 5.06 arcseconds apart. These two star systems orbit around their common centre of mass once every 1,115 years.

Zeta1 Cancri

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

Zeta2 Cancri

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 and 1.27 solar radii. The tenth magnitude Zeta Cancri Cb is a close pair of red dwarfs. The separation between Ca and Cb is approximately 0.3 arcsecond, and their orbital period is 17 years.