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.
| Date | March 30, 2021 |
| Time | 9:30 p.m. |
| Location | Seattle, WA |
| Magnification | 254x |
| Scope | Meade 8″ SCT |
| Eyepiece | 8mm |
| Seeing | Good |
| Transparency | Average |
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
The Garden Astronomer 
I would like to see a graphic of this system. Wikipedia text is a bit confusing but seems to indicate we have a 5 star system. Zeta Cancri A (1.28 m), B (1.18 m), Ca (1.15 m), Cb1 (0.5 m), Cb2 (0.5 m). Please clarify and illustrate this system. Thank you!
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You raise a good point, Joe. Your comment made me go back and check with Stelle Doppie: https://www.stelledoppie.it/index2.php?iddoppia=38015
They are indicating it is a nine-object system. They provide a graphic, but not of the entire system it appears. Let me dig a little deeper. Plus, your comment reminds me that I wanted to return to Zeta Cancri under better sky conditions to check out what more I could see—if possible. I haven’t done so since 2021. I will do with hopefully an update.
Thank you for taking the time to stop by and comment. I appreciate it. Clear skies to you.
Wayne
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