Sigma Ceti

Sigma Ceti (σ Ceti) is a triple star system^[2] in the equatorial constellation of Cetus. With an apparent visual magnitude of 4.78,^[6] it can be seen with the naked eye on a dark night. Based upon an annual parallax shift of 37.46 mas,^[1] it lies at an estimated distance of 87.1 light years from the Sun.

Sigma Ceti

Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Cetus
σ Cet A
Right ascension	02h 32m 05.22884s[1]
Declination	−15° 14′ 40.8278″[1]
Apparent magnitude (V)	4.75[2]
σ Cet B
Right ascension	02h 31m 42.47292s[3]
Declination	−15° 16′ 24.4275″[3]
Apparent magnitude (V)	8.74[2]
Characteristics
σ Cet A
Spectral type	F5 V[4] or F4 IV[5]
U−B color index	−0.03[6]
B−V color index	+0.45[6]
σ Cet B
Spectral type	K2.5V[2]
Astrometry
σ Cet A
Radial velocity (Rv)	−28.85±0.12[2] km/s
Proper motion (μ)	RA: −80.21 mas/yr[1] Dec.: −146.29 mas/yr[1]
Parallax (π)	37.46±0.25 mas[1]
Distance	87.1 ± 0.6 ly (26.7 ± 0.2 pc)
Absolute magnitude (MV)	+2.68[4]
σ Cet B
Proper motion (μ)	RA: −73.371 mas/yr[3] Dec.: −117.842 mas/yr[3]
Parallax (π)	37.1885±0.0188 mas[3]
Distance	87.70 ± 0.04 ly (26.89 ± 0.01 pc)
Absolute magnitude (MV)	+6.48[7]
Orbit[2]
Primary	σ Cet Aa
Companion	σ Cet Ab
Period (P)	20.68±0.67 years
Semi-major axis (a)	0.3616″±0.0049″ (9.65 AU)[a]
Eccentricity (e)	0.854±0.009
Inclination (i)	120.3±1.3°
Longitude of the node (Ω)	23.1±1.3°
Argument of periastron (ω) (primary)	23.4±2.6°
Semi-amplitude (K1) (primary)	7.56±0.25 km/s
Position (relative to σ Cet A)[2]
Component	σ Cet B
Angular distance	345″
Projected separation	9,800[b] AU
Details
σ Cet Aa
Mass	1.45[2] M☉
Radius	2.108[8] R☉
Luminosity	6.95[8] L☉
Surface gravity (log g)	4.07±0.07[9] cgs
Temperature	6,527±59[9] K
Metallicity [Fe/H]	−0.12±0.04[9] dex
Rotational velocity (v sin i)	20[4] km/s
Age	2.135[10] Gyr
σ Cet Ab
Mass	0.70[2] M☉
σ Cet B
Mass	0.77[2] M☉
Radius	0.698±0.048[11] R☉
Luminosity	0.262±0.008[11] L☉
Temperature	4,935±145[11] K
Other designations
A: σ Cet, 76 Cet, BD−15°449, FK5 1071, HD 15798, HIP 11783, HR 740, SAO 148445[12]
B: HIP 11759, HD 15767, BD−15°447, TIC 66574369, SAO 148443[13]
Database references
SIMBAD	σ Cet A
	σ Cet B

Characteristics

This is a hierarchical triple system. The inner pair contains the primary, component Aa, and the secondary, component Ab. They form a spectroscopic and astrometric binary system, and also 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.^[2]

The primary, component Aa, appears to be a normal F-type main sequence star with a stellar classification of F5 V.^[4] However, Malaroda (1975) assigned it a classification of F4 IV,^[5] which would suggest it is a more evolved subgiant star. It is estimated to have 145% of the Sun's mass^[2] and around 211% of the radius of the Sun.^[8] With an age of about 2.1 billion years,^[10] it is radiating seven times the solar luminosity from its outer atmosphere^[8] at an effective temperature of 6,527 K.^[9]

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 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. 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]

Name

This star, along with π Cet, ε Cet and ρ Cet, was Al Sufi's Al Sadr al Ḳaiṭos, the Whale's Breast.^[14]

According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, Al Sadr al Ḳaiṭos were the title for four stars :ρ Cet as Al Sadr al Ḳaiṭos I, this star (σ Cet) as Al Sadr al Ḳaiṭos II, ε Cet as Al Sadr al Ḳaiṭos III and π Cet as Al Sadr al Ḳaiṭos IV^[15]

Notes

1. Calculated using the parallax of 0.03746 arcseconds and the separation of 0.3616 arcseconds. 0.3616" / 0.03719" = 9.65 AU.
2. Calculated using the parallax of 0.03719 arcseconds and the separation of 345 arcseconds. 345" / 0.03719" = 9,300 AU.