Zeta Cancri: A Multiple Star System in Cancer

Zeta Cancri is a multiple star system in the constellation Cancer. It’s best viewed in the spring.

For many years, astronomers believed Zeta Cancri to be a triple star system, though now it has been found to be a quadruple system. According to Jim Kaller, the close binary pair (known as Zeta Cancri A and B) orbit each other every 59.6 years, and they are a mean distance of 22.1 Astronomical Units apart. The star system itself is 84 light year away for us. ¹

Historically speaking, Zeta Cancri has a rather early recorded date. The astronomer John Flamsteed wrote about the brighter star on March 22, 1680. Nearly a hundred years later, William Hershel observed the double star on November 21, 1781, at 4 a.m. stating: “If I do not see extremely ill this morning, the large star consists of two.” ²

My Observations

This evening with my 8mm eyepiece, I’m barely able to split the close binary Zeta Cancri A — also known as Tegmine. As shown in the sketch below, I do see Zeta 2 Cancri — also known as Tegmeni. The sky transparency is not great tonight, so I look forward to returning to this fascinating system.

Key Stats

Constellation	Cancer
Best Viewing	Spring
Visual Magnitude	+5.6 | +6.2
Absolute Visual Magnitude	+3.6 | +3.5
Distance from Earth	81 ly
Separation	5″ | 150 AU
Milky Way Location	Orion Spur
My Viewing Grade	B
Designations	Tegmine, Tegmen, Tegmeni, Zeta1, Zeta2, Struve / STF 1196

Sources and Notes

The double star banner on this page is an artist’s impression showing the double star OGLE-LMC-CEP0227 in our neighboring galaxy the Large Magellanic Cloud. This media was created by the European Southern Observatory (ESO). This file is licensed under the Creative Commons Attribution 4.0 International license.

¹ Iota cnc. (n.d.). Retrieved April 30, 2022, from http://stars.astro.illinois.edu/sow/iotacnc.html

² Argyle, R. W., Swan, M., & James, A. (2019). An anthology of visual double stars. Cambridge University Press.

Sketch by Wayne McGraw