Pi Hydrae
Pi Hydrae, Latinized from π Hydrae, is a star in the constellation Hydra with an apparent visual magnitude of 3.3, making it visible to the naked eye. Parallax measurements put this star at a distance of about 101 light-years (31 parsecs) from the Earth.
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Hydra |
| Right ascension | 14h 06m 22.29749s |
| Declination | –26° 40′ 56.5024″ |
| Apparent magnitude (V) | 3.25 |
| Characteristics | |
| Spectral type | K1 III–IV or K2-III Fe-0.5 |
| U−B color index | +1.040 |
| B−V color index | +1.120 |
| Astrometry | |
| Radial velocity (Rv) | +26.7 km/s |
| Proper motion (μ) | RA: +43.70 mas/yr Dec.: −141.18 mas/yr |
| Parallax (π) | 32.30 ± 0.16 mas |
| Distance | 101.0 ± 0.5 ly (31.0 ± 0.2 pc) |
| Absolute magnitude (MV) | 0.79 |
| Details | |
| Mass | 1.76 to 2.45 M☉ |
| Radius | 12–13 R☉ |
| Surface gravity (log g) | 2.65 cgs |
| Temperature | 4,670 K |
| Metallicity [Fe/H] | 0.04 dex |
| Rotational velocity (v sin i) | 2.25 km/s |
| Other designations | |
| Database references | |
| SIMBAD | data |
The spectrum of this star shows it to have a stellar classification of K1 III-IV, with the luminosity class of 'III-IV' suggesting it is in an evolutionary transition stage somewhere between a subgiant and a giant star. It has a low projected rotational velocity of 2.25 km s−1. Pi Hydrae is radiating energy from its outer envelope with an effective temperature of 4,670 K, giving it the orange hue of a K-type star.
Pi Hydrae is a type of giant known as a cyanogen-weak star, which means that its spectrum displays weak absorption lines of CN− relative to the metallicity. (The last is a term astronomers use when describing the abundance of elements other than hydrogen and helium.) Otherwise, it appears to be a normal star of its evolutionary class, having undergone first dredge-up of nuclear fusion by-products onto its surface layers. The measured angular diameter of this star, after correction for limb darkening, is 3.76 ± 0.04 mas. At its estimated distance, this yields a physical size of about 12–13 times the radius of the Sun. It has an estimated mass of 2.45 times the mass of the Sun.