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Deneb Algenubi, Eta Ceti, 31 Ceti

Proper NamesDheneb, Deneb Algenubi
Bayer DesignationEta Ceti
Flamsteed Number31 Ceti
HR (BSC)334
Right Ascension1h 8m 35s
Declination-10° 10' 56"
Distance121 light years
37 parsecs
MagnitudeApparent: +3.46
Absolute: +0.62
Spectral ClassK2-IIIb orange giant
Planets in this systemDheneb b (Eta Ceti b), gas giant
Dheneb c (Eta Ceti c), gas giant
Optimum VisibilityOctober

This star in Cetus takes its name from the Arabic for 'tail', a common element in the names of stars across the sky, though more usually spelt Deneb. Most famously, Deneb in Cygnus represents the tail of the Swan, whereas Dheneb in Cetus represents the tail, or part of the tail, of the Whale or sea monster. Dheneb is also known as Deneb Algenubi, the 'southern tail', to distinguish it from Deneb Kaitos Shemali, the 'northern tail of the whale', which lies westward and a little northward from Dheneb. A third star, formally known as Diphda, also carries the name Deneb Kaitos, the 'tail of the whale'.

Together, these three tail stars of Cetus form a near equilateral triangle, with Dheneb lying at the northeastern corner. While all three stars are clearly visible to the naked eye (Dheneb itself shines with an apparent magnitude of +3.46), none of them are particularly bright, and the shape of the Whale's tail can be difficult to distinguish in the night sky.

Imagery provided by Aladin sky atlas

Dheneb is an evolved orange star some 121 light years from the Solar System. The star has burned through its hydrogen fuel and is now consuming helium in its core, a process that has caused it to expand to some fifteen times the Sun's diameter. Giant stars like this are, however, much less dense than main sequence stars, and despite its size, the mass of Dheneb is only a little greater than that of the Sun. Dheneb is a metal-rich star, noted for showing unusually hight levels of cyanogen and iron in its make-up.

Dheneb is known to possess at least two planets, both of which are enormous gas giants. The inner planet, Eta Ceti b, has some 2.6 times the mass of Jupiter, and follows an orbit some 1.3 AU from its star. The outer giant planet, Eta Ceti c, has a mass at least 3.3 times greater than Jupiter, and orbits at 1.9 AU. To put these dimensions into perspective, if the two planets existed within the Solar System, then Eta Ceti b would follow an orbit just within that of Mars, while the orbit of Eta Ceti c would approach the inner edge of the Asteroid Belt.


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