Sigma Ceti is a triple star system in the equatorial constellation of Cetus. Its name is a Bayer designation that is Latinized from àCeti, and abbreviated Sigma Cet or àCet. With an apparent visual magnitude of 4.78, it can be seen with the naked eye on a dark night. Based upon an annual parallax shift of , it lies at an estimated distance of 87.1 light years from the Sun. It is drifting closer with a line of sight velocity component of âÂÂ29 km/s.
This is a hierarchical triple system. The inner pair contains the primary member, component Aa, and the secondary, component Ab. They form a spectroscopic and astrometric binary system, but have been resolved with the SOAR telescope. The inner system has an orbital period of 21 years and an eccentric orbit. As of 2025, the most recent periastron (closest distance between stars) was in 2015.
The primary, component Aa, appears to be a normal F-type main sequence star with a stellar classification of F5 V. However, Malaroda (1975) assigned it a classification of F4 IV, which would suggest it is a more evolved subgiant star. It is estimated to have 145% of the Sun's mass and around 211% of the radius of the Sun. With an age of about 2.1 billion years, it is radiating seven times the solar luminosity from its outer atmosphere at an effective temperature of 6,527 K.
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.
The outer component is the proper motion companion HD 15767, also called Sigma Ceti B. It has a wide separation of 345 arcseconds, or 9,300 AU at its distance. The orbital period is estimated at 500 thousand years. It is a K-dwarf star, with a faint apparent magnitude of 8.74. This star has 77% the mass of the Sun 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.
This star, along with àCet, õ Cet and àCet, was Al Sufi's Al Sadr al Ḳaiá¹Âos, the Whale's Breast.
According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, Al Sadr al Ḳaiá¹Âos were the title for four stars :àCet as Al Sadr al Ḳaiá¹Âos I, this star (àCet) as Al Sadr al Ḳaiá¹Âos II, õ Cet as Al Sadr al Ḳaiá¹Âos III and àCet as Al Sadr al Ḳaiá¹Âos IV