Eta Hydrae
A candidate Beta Cephei variable in the head of Hydra.
Eta Hydrae, also designated η Hydrae, is a star in the equatorial constellation Hydra. It shines at an apparent visual magnitude of 4.3, making it visible without a telescope, yet it is the dimmest among the five stars that compose the hydra’s head. Parallax measurements place it about 590 light-years from the Sun. The star is a B-type main sequence star, classified as B3V. Though once considered a candidate Beta Cephei variable with a 2.2-day period, that classification was later ruled out by Stankov and Handler in 2005. Its spectrum reveals a slight carbon deficiency relative to the Sun. Eta Hydrae is roughly 32 million years old and rotates with a projected rotational velocity of 101 km/s. It has about seven times the Sun’s mass and nearly four times its radius. From its outer atmosphere, it radiates 2,680 times the Sun’s luminosity, with an effective temperature of 18,630 K. Historically, this star was part of Ulug Beg’s asterism Min al Azʽal, meaning “Belonging to the Uninhabited Spot.” In the star catalog Technical Memorandum 33-507, it appears as Minazal II, one of five stars in that group. In Chinese astronomy, it belongs to the Willow asterism, which includes several other Hydra stars, and is known by the designation corresponding to that asterism. The people of Groote Eylandt included this star in the cluster they called Unwala, “The Crab,” along with δ, ε, ζ, ρ, and σ Hydrae.
Quick Facts
- Apparent magnitude
- 4.3
- Distance
- 590 light years
- Spectral type
- B3V
- Mass
- 7 solar masses
- Radius
- 4 solar radii
- Luminosity
- 2680 solar luminosities
- Effective temperature
- 18,630 K
Facts from the source article.
Lore & Background
Eta Hydrae is a B-type main sequence star with a stellar classification of B3V. Its spectrum shows a slight underabundance of carbon compared to the Sun. The star is around 32 million years old and is spinning with a projected rotational velocity of 101 km/s. It radiates 2,680 times the solar luminosity from its outer atmosphere at an effective temperature of 18,630 K.
In the star catalogue of Ulug Beg, this star was part of the asterism Min al Azʽal, meaning 'Belonging to the Uninhabited Spot'. Along with δ Hya, ε Hya, ζ Hya, ρ Hya and σ Hya, it was included in that group. According to the Technical Memorandum 33-507, the title Minazal was given to 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, excluding σ Hya.
In Chinese astronomy, η Hydrae is part of the asterism Willow, which also includes δ Hydra, σ Hydrae, ρ Hydrae, ε Hydrae, ζ Hydrae, ω Hydrae and θ Hydrae. The people of Groote Eylandt called the star cluster including this star Unwala, 'The Crab'.
Reader's Guide
Eta Hydrae holds significance as a candidate Beta Cephei variable, a class of pulsating stars, though its variable status was rejected by Stankov and Handler (2005). Its position as the faintest of the five stars forming the head of Hydra makes it a notable marker in the constellation. The star's inclusion in multiple cultural asterisms—Ulug Beg's Min al Azʽal, the Chinese Willow, and the Groote Eylandt Unwala—reflects its role in historical and indigenous sky lore. The Technical Memorandum 33-507 catalogued it as Minazal II, part of a five-star sequence. Its physical properties, such as being a B3V star with a carbon underabundance and a rotation speed of 101 km/s, contribute to understanding stellar evolution in the B-type main sequence. The star's distance of roughly 590 light years and luminosity of 2,680 times that of the Sun place it among the more luminous members of its class, though it remains the dimmest in its immediate asterism.
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