Alpha Hydri
 Location of α Hydri (circled) |
|
| Observation data Epoch J2000.0 Equinox J2000.0 |
|
|---|---|
| Constellation | Hydrus |
| Right ascension | 01h 58m 46.19467s |
| Declination | −61° 34′ 11.4948″ |
| Apparent magnitude (V) | +2.90 |
| Characteristics | |
| Spectral type | F0 IV |
| U−B color index | +0.189 |
| B−V color index | +0.290 |
| Astrometry | |
| Radial velocity (Rv) | +7 km/s |
| Proper motion (μ) |
RA: +263.66 mas/yr Dec.: +26.77 mas/yr |
| Parallax (π) | 45.43 ± 0.44mas |
| Distance | 71.8 ± 0.7 ly (22.0 ± 0.2 pc) |
| Absolute magnitude (MV) | +1.153 |
| Details | |
| Mass | 2.0 M☉ |
| Radius | 1.8 R☉ |
| Luminosity | 32 L☉ |
| Surface gravity (log g) | 3.73 cgs |
| Temperature | 7,077 K |
| Metallicity [Fe/H] | 0.11 dex |
| Rotational velocity (v sin i) | 118 km/s |
| Age | 0.81 Gyr |
| Other designations | |
Alpha Hydri (α Hyi, α Hydri) is the second brightest star in the southern circumpolar constellation of Hydrus. It is readily visible to the naked eye with an apparent visual magnitude of +2.9. It is sometimes informally known as the Head of Hydrus. This should not be confused with Alpha Hydrae (Alphard) in the constellation Hydra. Alpha Hydri is one of only three stars in the constellation Hydrus that are above the fourth visual magnitude. This star can be readily located as it lies to the south and west of the prominent star Achernar in the constellation Eridanus.
Based upon parallax measurements from the Hipparcos mission, Alpha Hydri is located at a distance of about 71.8 light-years (22.0 parsecs) from Earth. This subgiant star is 80% larger and twice as massive as the Sun, with a stellar classification of F0 IV. It is about 810 million years old and is radiating 32 times the Sun's luminosity from its outer atmosphere at an effective temperature of 7,077 K. Alpha Hydri emits X-rays similar to Altair. The space velocity components of this star are [U, V, W] = [−14, −14, -2] km/s.
In Chinese caused by adaptation of the European southern hemisphere constellations into the Chinese system, 蛇首 (Shé Shǒu), meaning Snake's Head, refers to an asterism consisting of α Hydri and β Reticuli. Consequently, α Hydri itself is known as 蛇首一 (Shé Shǒu yī, English: the First Star of Snake's Head.)
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