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[11] with higher-than-normal abundance of cyanogen and iron relative to other stars of its class.)[12] It is a red clump star that is generating energy through the nuclear fusion of helium at its core.[3]
Eta Ceti has 1.8 times more mass than the Sun[7] and its surface has expanded to 13 times the Sun's radius.[8] It is radiating 74[9] times as much luminosity as the Sun from its photosphere at an effective temperature of 4,356 K.[9] This heat gives the star the orange-hued glow of a K-type star.[13]
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 CalendariumofAl 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.[14] 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.[15]
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.[18]
Eta Ceti b has a minimum mass of 2.55MJ and an orbital period of 403.5 days (about 1.1 years), while Eta Ceti c has a minimum mass of 3.32MJ 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.[19]
^Keenan, P.; McNeil, R. (October 1989), "The Perkins catalog of revised MK types for the cooler stars", Astrophysical Journal Supplement Series, 71: 245–266, Bibcode:1989ApJS...71..245K, doi:10.1086/191373.
^ abBaines, Ellyn K.; Thomas Armstrong, J.; Clark, James H.; Gorney, Jim; Hutter, Donald J.; Jorgensen, Anders M.; Kyte, Casey; Mozurkewich, David; Nisley, Ishara; Sanborn, Jason; Schmitt, Henrique R.; Van Belle, Gerard T. (2021), "Angular Diameters and Fundamental Parameters of Forty-four Stars from the Navy Precision Optical Interferometer", The Astronomical Journal, 162 (5): 198, arXiv:2211.09030, Bibcode:2021AJ....162..198B, doi:10.3847/1538-3881/ac2431
^Keenan, Philip C.; et al. (July 1987), "Recognition and classification of strong-CN giants", Publications of the Astronomical Society of the Pacific, 99: 629–636, Bibcode:1987PASP...99..629K, doi:10.1086/132025.
^Knobel, E. B. (June 1895), "Al Achsasi Al Mouakket, on a catalogue of stars in the Calendarium of Mohammad Al Achsasi Al Mouakket", Monthly Notices of the Royal Astronomical Society, 55: 429–438, Bibcode:1895MNRAS..55..429K, doi:10.1093/mnras/55.8.429
