Star in the constellation Hydra
Eta Hydrae (η Hydrae ) is a
star in the equatorial
constellation of
Hydra . With an
apparent visual magnitude of 4.3,
[2] it is visible to the naked eye. However, it is the faintest of the five stars that form the "head" of the hydra.
[11] Based upon an annual
parallax shift of 5.56
mas ,
[1] it is located roughly 590
light years from the
Sun .
A
light curve for Eta Hydrae, plotted from
TESS data
[12]
This is a
B-type main sequence star with a
stellar classification of B3V.
[3] It has been classified as a candidate
Beta Cephei variable with a period of 2.2 days, although this designation was rejected by Stankov and Handler (2005).
[4] The
spectrum shows a slight underabundance of carbon, compared to the Sun.
[13] The star is around 32
[6] million years old and is spinning with a
projected rotational velocity of 101
[9] km/s. It has an estimated seven
[6] times the
mass of the Sun and nearly four
[7] times the
Sun's radius . Eta Hydrae radiates 2,680
[8] times the
solar luminosity from its
outer atmosphere at an
effective temperature of 18,630
[9] K.
Name and etymology
This star, along with
δ Hya (Lisan al Sudja),
ε Hya ,
ζ Hya ,
ρ Hya and
σ Hya (Minchir), were Ulug Beg's Min al Azʽal , "Belonging to the Uninhabited Spot".
[14] According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars , Min al Azʽal or Minazal were the title for five stars:
δ Hya as Minazal I , η Hya as Minazal II ,
ε Hya as Minazal III ,
ρ Hya as Minazal IV and
ζ Hya as Minazal V (exclude
σ Hya ).
[15]
In
Chinese , 柳宿 (Liǔ Sù ), meaning
Willow , refers to an
asterism consisting of η Hydrae,
δ Hydra ,
σ Hydrae ,
ρ Hydrae ,
ε Hydrae ,
ζ Hydrae ,
ω Hydrae and
θ Hydrae
[16] Consequently, η Hydrae itself is known as 柳宿三 (Liǔ Sù sān , English: the Third Star of Willow ).
[17]
The people of
Groote Eylandt called Unwala , "The Crab", for the star cluster including this star,
δ Hya (Lisan al Sudja),
ε Hya ,
ζ Hya ,
ρ Hya and
σ Hya (Minchir).
[18]
References
^
a
b
c
d
e
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^
a
b
c
d Oja, T. (August 1986), "UBV photometry of stars whose positions are accurately known. III", Astronomy and Astrophysics Supplement Series , 65 (2): 405–409,
Bibcode :
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^
a
b Levenhagen, R. S.; Leister, N. V. (2006), "Spectroscopic Analysis of Southern B and Be Stars", Monthly Notices of the Royal Astronomical Society , 371 (1): 252–62,
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^
a
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^ Ammler-von Eiff, Matthias; Reiners, Ansgar (June 2012), "New measurements of rotation and differential rotation in A-F stars: are there two populations of differentially rotating stars?", Astronomy & Astrophysics , 542 : A116,
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a
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S2CID
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