This is a hierarchical triple system. The inner pair contains the primary, component Aa, and the secondary, component Ab. They form aspectroscopic andastrometric binary system, and also have been resolved with theSOAR telescope. The inner system has an orbital period of 21 years and aneccentric orbit. As of 2025, the most recentperiastron (closest distance between stars) was in 2015.[2]
The secondary, Sigma Ceti Ab, is 3.89 magnitudes fainter than the primary in the I-band. Little is known about it. Its mass is estimated at 70% the mass of the Sun.[2]
The outer component is theproper motion companion HD 15767, also called Sigma Ceti B. It has a wide separation of 345arcseconds, or 9,300 AU at its distance. The orbital period is estimated at 500 thousand years. It is aK-dwarf star, with a faint apparent magnitude of 8.74. It has 77% the mass of the Sun[2] and 70% of the Sun's radius. It radiates 0.26 solar luminosities from its photosphere at an effective temperature of about 5,000 K.[11]
This star, along withπ Cet,ε Cet andρ Cet, was Al Sufi'sAl Sadr al Ḳaiṭos, the Whale's Breast.[14]
According to the catalogue of stars in theTechnical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars,Al Sadr al Ḳaiṭos were the title for four stars :ρ Cet asAl Sadr al Ḳaiṭos I, this star (σ Cet) asAl Sadr al Ḳaiṭos II,ε Cet asAl Sadr al Ḳaiṭos III andπ Cet asAl Sadr al Ḳaiṭos IV[15]
^abcdTakeda, Yoichi; et al. (February 2005), "High-Dispersion Spectra Collection of Nearby F--K Stars at Okayama Astrophysical Observatory: A Basis for Spectroscopic Abundance Standards",Publications of the Astronomical Society of Japan,57 (1):13–25,Bibcode:2005PASJ...57...13T,doi:10.1093/pasj/57.1.13.
^abDavid, Trevor J.; Hillenbrand, Lynne A. (2015), "The Ages of Early-Type Stars: Strömgren Photometric Methods Calibrated, Validated, Tested, and Applied to Hosts and Prospective Hosts of Directly Imaged Exoplanets",The Astrophysical Journal,804 (2): 146,arXiv:1501.03154,Bibcode:2015ApJ...804..146D,doi:10.1088/0004-637X/804/2/146,S2CID33401607.
