Article · Wikipedia archive · Last revised Jul 29, 2026

Phi2 Hydrae

Phi2 Hydrae, Latinized from φ2 Hydrae, is a star in the constellation Hydra. It originally received the Flamsteed designation of 1 Crateris before being placed in the Hydra constellation. Based upon an annual parallax shift of 2.9 mas as seen from Earth, it is located roughly 1,110 light years from the Sun. The star is faintly visible to the naked eye with an apparent visual magnitude of about six. It forms a triangle with the fainter φ1 Hydrae and the brighter φ3 Hydrae, between μ Hydrae and ν Hydrae.

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Phi2 Hydrae

A light curve for Phi2 Hydrae, plotted from Hipparcos data1
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Hydra2
Right ascension 10h 36m 16.65890s3
Declination −16° 20′ 39.5764″3
Apparent magnitude (V) 5.97 to 6.044
Characteristics
Evolutionary stage AGB5
Spectral type M1 III6
U−B color index +1.957
B−V color index +1.647
Variable type SR8
Astrometry
Radial velocity (Rv)+15.7±2.99 km/s
Proper motion (μ) RA: −30.6513 mas/yr
Dec.: +4.2123 mas/yr
Parallax (π)2.929±0.1349 mas3
Distance1,110 ± 50 ly
(340 ± 20 pc)
Absolute magnitude (MV)−0.802
Details
Mass6.43 M☉
Radius8510 R☉
Luminosity1,47010 L☉
Surface gravity (log g)1.253 cgs
Temperature3,87310 K
Other designations
φ2 Hya, BD−15°3087, HD 91880, HIP 51905, HR 4156, SAO 15609311
Database references
SIMBADdata

Phi2 Hydrae, Latinized from φ2 Hydrae, is a star in the constellation Hydra. It originally received the Flamsteed designation of 1 Crateris before being placed in the Hydra constellation.12 Based upon an annual parallax shift of 2.9 mas as seen from Earth, it is located roughly 1,110 light years from the Sun. The star is faintly visible to the naked eye with an apparent visual magnitude of about six. It forms a triangle with the fainter φ1 Hydrae and the brighter φ3 Hydrae, between μ Hydrae and ν Hydrae.

Pulsations8
Period
(days) 		Amplitude
(magnitude)
11.0 0.008
110.3 0.012
153.6 0.015

This is an evolved red giant star with a stellar classification of M1 III.5 It is currently on the asymptotic giant branch,5 and is a semiregular variable that undergoes changes in luminosity according to three pulsation periods,8 although it is formally still only a suspected variable.13 The star is radiating an estimated 1,470 times the Sun's luminosity from its photosphere at an effective temperature of 3,873 K.

Phi2 Hydrae has a faint visual companion: a magnitude 12.20 star at an angular separation of 3.50 arc seconds along a position angle of 280°, as of 1959.14 The companion has a similar Gaia Data Release 3 parallax to Phi2 Hydrae15 and is at a distance of 329+5
−4 pc.16

References

References

  1. EAS (1997), "The HIPPARCOS and TYCHO catalogues", Astrometric and Photometric Star Catalogues Derived from the ESA Hipparcos Space Astrometry Mission, ESA SP Series, 1200, Noordwijk, Netherlands: ESA Publications Division, Bibcode:1997HIP...C......0E, ISBN 9290923997, retrieved 14 December 2024.{{citation}}: CS1 maint: work parameter with ISBN (link)
  2. Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015, S2CID 119257644.
  3. Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
  4. "phi 2 Hya". International Variable Star Index. AAVSO. Retrieved 2026-05-20.
  5. Eggen, Olin J. (July 1992), "Asymptotic giant branch stars near the sun", Astronomical Journal, 104 (1): 275–313, Bibcode:1992AJ....104..275E, doi:10.1086/116239.
  6. Houk, Nancy; Smith-Moore, M. (1978), Michigan catalogue of two-dimensional spectral types for the HD stars, vol. 4, Ann Arbor: Dept. of Astronomy, University of Michigan, Bibcode:1988mcts.book.....H.
  7. Mermilliod, J.-C. (1986), "Compilation of Eggen's UBV data, transformed to UBV (unpublished)", Catalogue of Eggen's UBV Data, SIMBAD, Bibcode:1986EgUBV........0M.
  8. Tabur, V.; et al. (2009), "Long-term photometry and periods for 261 nearby pulsating M giants", Monthly Notices of the Royal Astronomical Society, 400 (4): 1945–61, arXiv:0908.3228, Bibcode:2009MNRAS.400.1945T, doi:10.1111/j.1365-2966.2009.15588.x, S2CID 15358380.
  9. de Bruijne, J. H. J.; Eilers, A.-C. (October 2012), "Radial velocities for the HIPPARCOS-Gaia Hundred-Thousand-Proper-Motion project", Astronomy & Astrophysics, 546: 14, arXiv:1208.3048, Bibcode:2012A&A...546A..61D, doi:10.1051/0004-6361/201219219, S2CID 59451347, A61.
  10. Fetherolf, Tara; Pepper, Joshua; Simpson, Emilie; Kane, Stephen R.; Močnik, Teo; English, John Edward; Antoci, Victoria; Huber, Daniel; Jenkins, Jon M.; Stassun, Keivan; Twicken, Joseph D.; Vanderspek, Roland; Winn, Joshua N. (2023), "Variability Catalog of Stars Observed during the TESS Prime Mission", The Astrophysical Journal Supplement Series, 268 (1): 4, arXiv:2208.11721, Bibcode:2023ApJS..268....4F, doi:10.3847/1538-4365/acdee5.
  11. "* phi02 Hya", SIMBAD, Centre de données astronomiques de Strasbourg, retrieved 2017-03-17.
  12. Wagman, M. (August 1987), "Flamsteed's Missing Stars", Journal for the History of Astronomy, 18 (3): 216, Bibcode:1987JHA....18..209W, doi:10.1177/002182868701800305, S2CID 118445625.
  13. Samus, N. N.; Durlevich, O. V.; et al. (2009), "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)", VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S, 1: B/GCVS, Bibcode:2009yCat....102025S.
  14. Mason, B. D.; et al. (2014), "The Washington Visual Double Star Catalog", The Astronomical Journal, 122 (6): 3466–3471, Bibcode:2001AJ....122.3466M, doi:10.1086/323920.
  15. Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
  16. Bailer-Jones, C. A. L.; Rybizki, J.; Fouesneau, M.; Demleitner, M.; Andrae, R. (2021), "Estimating Distances from Parallaxes. V. Geometric and Photogeometric Distances to 1.47 Billion Stars in Gaia Early Data Release 3", The Astronomical Journal, 161 (3): 147, arXiv:2012.05220, Bibcode:2021AJ....161..147B, doi:10.3847/1538-3881/abd806.