# Pi Hydrae

Observation data Constellation Epoch J2000      Equinox J2000 Location of π Hydra (circled) Hydra 14h 06m 22.29749s[1] –26° 40′ 56.5024″[1] 3.25[2] K1 III–IV[3] +1.040[4] +1.120[4] Radial velocity (Rv) +26.7[2] km/s Proper motion (μ) RA: +43.70[1] mas/yr Dec.: −141.18[1] mas/yr Parallax (π) 32.30 ± 0.16[1] mas Distance 101.0 ± 0.5 ly (31.0 ± 0.2 pc) Absolute magnitude (MV) 0.79[5] Mass 1.76[5] to 2.45[6] M☉ Radius 12–13[7] R☉ Surface gravity (log g) 2.65[8] cgs Temperature 4,670[8] K Metallicity [Fe/H] 0.04[9] dex Rotational velocity (v sin i) 2.25[8] km/s Markeb, 49 Hydrae, HD 123123, HR 5287, SAO 182244, FK5 519, CPD−26 5170, HIP 68895.[10]

Pi Hydrae (π Hya, π Hydrae) is a star in the constellation Hydra with an apparent visual magnitude of 3.3,[2] 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.

The spectrum of this star shows it to have a stellar classification of K1 III-IV,[3] 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.[8] Pi Hydrae is radiating energy from its outer envelope with an effective temperature of 4,670 K,[8] giving it the orange hue of a K-type star.[11]

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.[9] The measured angular diameter of this star, after correction for limb darkening, is 3.76 ± 0.04 mas.[12] At its estimated distance, this yields a physical size of about 12–13 times the radius of the Sun.[7] It has an estimated mass of 2.45 times the mass of the Sun.[6]

## References

1. van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357
2. ^ a b c Wielen, R. et al. (1999), Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions (35), Astronomisches Rechen-Institut Heidelberg, Bibcode:1999VeARI..35....1W
3. ^ a b Gray, R. O. et al. (July 2006), "Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 pc-The Southern Sample", The Astronomical Journal 132 (1): 161–170, arXiv:astro-ph/0603770, Bibcode:2006AJ....132..161G, doi:10.1086/504637
4. ^ a b Jennens, P. A.; Helfer, H. L. (September 1975), "A new photometric metal abundance and luminosity calibration for field G and K giants", Monthly Notices of the Royal Astronomical Society 172: 667–679, Bibcode:1975MNRAS.172..667J, doi:10.1093/mnras/172.3.667
5. ^ a b Hekker, S. et al. (August 2006), "Precise radial velocities of giant stars. I. Stable stars", Astronomy and Astrophysics 454 (3): 943–949, arXiv:astro-ph/0604502, Bibcode:2006A&A...454..943H, doi:10.1051/0004-6361:20064946
6. ^ a b Edvardsson, B. (January 1988), "Spectroscopic surface gravities and chemical compositions for 8 nearby single sub-giants", Astronomy and Astrophysics 190 (1-2): 148–166, Bibcode:1988A&A...190..148E
7. ^ a b Lang, Kenneth R. (2006), Astrophysical formulae, Astronomy and astrophysics library 1 (3rd ed.), Birkhäuser, ISBN 3-540-29692-1. The radius (R*) is given by:
\begin{align} 2\cdot R_* & = \frac{(10^{-3}\cdot 31\cdot 3.76)\ \text{AU}}{0.0046491\ \text{AU}/R_{\bigodot}} \\ & \approx 25.1\cdot R_{\bigodot} \end{align}
8. Hekker, S.; Meléndez, J. (December 2007), "Precise radial velocities of giant stars. III. Spectroscopic stellar parameters", Astronomy and Astrophysics 475 (3): 1003–1009, arXiv:0709.1145, Bibcode:2007A&A...475.1003H, doi:10.1051/0004-6361:20078233
9. ^ a b Luck, R. Earle (February 1991), "Chemical abundances for cyanogen-weak giants", Astrophysical Journal Supplement Series 75: 579–610, Bibcode:1991ApJS...75..579L, doi:10.1086/191542
10. ^ "49 Hya -- Star", SIMBAD (Centre de Données astronomiques de Strasbourg), retrieved 2012-01-16
11. ^ "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, retrieved 2012-01-16
12. ^ Richichi, A.; Percheron, I.; Khristoforova, M. (February 2005), "CHARM2: An updated Catalog of High Angular Resolution Measurements", Astronomy and Astrophysics 431: 773–777, Bibcode:2005A&A...431..773R, doi:10.1051/0004-6361:20042039