Eta Ceti

η Ceti

Template:Location mark Location of η Ceti (circled)
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Cetus
Right ascension	Template:RA
Declination	Template:DEC
Apparent magnitude (V)	+3.446
Characteristics
Evolutionary stage	Red-giant branch
Spectral type	K2−IIIb
U−B Template:Engvar	+1.194
B−V Template:Engvar	+1.161
Astrometry
Radial velocity (Rv)	km/s
Proper motion (μ)	RA: +215.922 mas/yr Dec.: −139.029 mas/yr
Parallax (π)	27.0603±0.1799 mas
Distance	Template:Rnd ± Template:Rnd ly (Template:Rnd ± Template:Rnd pc)
Absolute magnitude (MV)	+0.68
Details
Mass	Template:Solar mass
Radius	Template:Solar radius
Luminosity	Template:Solar luminosity
Surface gravity (log g)	2.5 cgs
Temperature	K
Metallicity [Fe/H]	−0.03 dex
Rotational velocity (v sin i)	4.8 km/s
Age	1.80 Gyr
Metallicity [Fe/H]	{{{metal_fe2}}} dex
Other designations
Deneb Algenubi, Algenudi, 31 Ceti, BD−10 240, FK5 40, HD 6805, HIP 5364, HR 334, SAO 147632, 2MASS J01083539-1010560
Database references
SIMBAD	data

Eta Ceti (η Cet, η Ceti) is a star in the equatorial constellation of Cetus. It has the traditional name Deneb Algenubi or Algenudi. The apparent visual magnitude of this star is +3.4, making it the fourth-brightest star in this otherwise relatively faint constellation. The distance to this star can be measured directly using the parallax technique, yielding a value of .

This is a giant star that has been chosen a standard for the stellar classification of K2−IIIb. It has exhausted the hydrogen at its core and evolved away from the main sequence of stars like the Sun. (The classification is sometimes listed as K1.5 IIICN1Fe0.5, indicating a strong CN star with higher-than-normal abundance of cyanogen and iron relative to other stars of its class.) It is probably on the red giant branch fusing hydrogen in a shell, although there is a possibility that it is a red clump star that is generating energy through the nuclear fusion of helium at its core.

Eta Ceti has 1.7 times more mass than the Sun and its surface has expanded to 13 times the Sun's radius. It is radiating 74 times as much luminosity as the Sun from its photosphere at an effective temperature of 4,356 K. This heat gives the star the orange-hued glow of a K-type star.

In culture

The name Deneb Algenubi was from Arabic ذنب القيطس الجنوبي – al-dhanab al-qayṭas al-janūbī, meaning the southern tail of the sea monster. In the catalogue of stars in the Calendarium of Al Achsasi al Mouakket, this star was designated Aoul al Naamat (أول النعامات – awwil al naʽāmāt), which was translated into Latin as Prima Struthionum, meaning the first ostrich. This star, along with θ Cet (Thanih al Naamat), τ Cet (Thalath Al Naamat), ζ Cet (Baten Kaitos) and υ Cet, were Al Naʽāmāt (النعامات), the Hen Ostriches.

In Chinese, (), meaning Square Celestial Granary, refers to an asterism consisting of η Ceti, ι Ceti, θ Ceti, ζ Ceti, τ Ceti and 57 Ceti. Consequently, the Chinese name for η Ceti itself is (, Template:Langx).

Planetary system

In 2014, two exoplanets around the star were discovered using the radial velocity method. Planets discovered by radial velocity have poorly known masses because if the orbit of the planets were inclined away from the line of sight, a much larger mass would have to compensate for the angle.

Eta Ceti b has a minimum mass of Template:Jupiter mass and an orbital period of 403.5 days (about 1.1 years), while Eta Ceti c has a minimum mass of Template:Jupiter mass and an orbital period of 751.9 days (2.06 years). Assuming the orbits of the two are coplanar, then the two planets must be locked in a 2:1 orbital resonance, otherwise the system would become dynamically unstable. Although the inclinations from the line of sight are unknown, the value is constrained to be 70° or less: if any higher, the higher masses would render the system dynamically unstable, with no stable solutions. Template:OrbitboxPlanet begin Template:OrbitboxPlanet Template:OrbitboxPlanet

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