Lambda Aquarii

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λ Aquarii
Aquarius constellation map.svg
Red circle.svg
Location of λ Aquarii (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Aquarius
Right ascension  22h 52m 36.87441s[1]
Declination −07° 34′ 46.5542″[1]
Apparent magnitude (V) 3.722[2]
Spectral type M2.5 IIIa Fe–1[3]
U−B color index +1.721[2]
B−V color index +1.641[2]
Variable type Lb[4]
Radial velocity (Rv)−10.46 ± 0.53[5] km/s
Proper motion (μ) RA: +17.02[1] mas/yr
Dec.: +33.03[1] mas/yr
Parallax (π)8.47 ± 0.66[1] mas
Distance390 ± 30 ly
(118 ± 9 pc)
Absolute magnitude (MV)–1.5[6]
Mass3.6[7] M
Temperature3,835[7] K
Metallicity [Fe/H]–2.25[6] dex
Other designations
73 Aquarii, BD–08°968, FK5 864, HD 216386, HIP 112961, HR 8698, SAO 146362.[8]
Database references

Lambda Aquarii (λ Aqr, λ Aquarii), informally known as Hydor /ˈhdɔːr/,[9][10] is the Bayer designation for a star in the equatorial constellation of Aquarius. The apparent visual magnitude of this star is 3.722,[2] which is bright enough to be visible with the naked eye. It is roughly 390 light-years (120 pc) from Earth.[1]


Lambda Aquarii is a red giant star with a stellar classification of M2.5 III.[3] It is classified as slow irregular variable and pulsation periods of 24.5, 32.0, and 49.5 days have been identified.[11] This star is on the asymptotic giant branch and is generating energy through the nuclear fusion of hydrogen and helium along concentric shells surrounding an inert core of carbon and oxygen.[12]


Hydor is from Greek Ὕδωρ "water", a name given by Proclus, according to Richard Hinckley Allen. Another Greek name for the star is Ekkhysis, from εκχυσις "outpouring".[10]

In Chinese, 壘壁陣 (Lěi Bì Zhèn), meaning Line of Ramparts, refers to an asterism consisting of λ Aquarii, κ Capricorni, ε Capricorni, γ Capricorni, δ Capricorni, ι Aquarii, σ Aquarii, φ Aquarii, 27 Piscium, 29 Piscium, 33 Piscium and 30 Piscium.[13] Consequently,</ref> λ Aquarii itself is 壘壁陣七 (Lěi Bì Zhèn qī, English: the Seventh Star of Line of Ramparts.)[14]

Night viewing[edit]

The star is eclipsed by the sun from about 1-4 March,[15] thus the star can be viewed the whole night, crossing the sky, in early September, in the current epoch. It lies 0.39 degrees south of the ecliptic.[16]


  1. ^ a b c d e f 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 d Gutierrez-Moreno, Adelina; et al. (1966), "A System of photometric standards", Publ. Dept. Astron. Univ. Chile, Publicaciones Universidad de Chile, Department de Astronomy, 1: 1–17, Bibcode:1966PDAUC...1....1G.
  3. ^ a b Eggleton, P. P.; Tokovinin, A. A. (September 2008), "A catalogue of multiplicity among bright stellar systems", Monthly Notices of the Royal Astronomical Society, 389 (2): 869–879, arXiv:0806.2878, Bibcode:2008MNRAS.389..869E, doi:10.1111/j.1365-2966.2008.13596.x.
  4. ^ 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. Bibcode:2009yCat....102025S.
  5. ^ Famaey, B.; et al. (May 2009), "Spectroscopic binaries among Hipparcos M giants. I. Data, orbits, and intrinsic variations", Astronomy and Astrophysics, 498 (2): 627–640, arXiv:0901.0934, Bibcode:2009A&A...498..627F, doi:10.1051/0004-6361/200810698.
  6. ^ a b Dupree, A. K.; Li, Timothy Q.; Smith, Graeme H. (October 2007), "Hubble Space Telescope Observations of Chromospheres in Metal-Deficient Field Giants", The Astronomical Journal, 134 (4): 1348–1359, arXiv:0709.1709, Bibcode:2007AJ....134.1348D, doi:10.1086/520925.
  7. ^ a b Tsuji, Takashi (May 2007). "Isotopic abundances of Carbon and Oxygen in Oxygen-rich giant stars". In Kupka, F.; Roxburgh, I.; Chan, K. (eds.). Convection in Astrophysics, Proceedings of IAU Symposium #239 held 21-25 August, 2006 in Prague, Czech Republic. Proceedings of the International Astronomical Union. 2. pp. 307–310. arXiv:astro-ph/0610180. Bibcode:2007IAUS..239..307T. doi:10.1017/S1743921307000622.
  8. ^ "* lam Aqr". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2012-07-01.
  9. ^ Kaler, Jim. "Hydor". Retrieved 2017-08-17.
  10. ^ a b Allen, Richard Hinckey. "Star Names: Their Lore and Meaning". Retrieved 2017-08-17.
  11. ^ Tabur, V.; et al. (December 2009), "Long-term photometry and periods for 261 nearby pulsating M giants", Monthly Notices of the Royal Astronomical Society, 400 (4): 1945–1961, arXiv:0908.3228, Bibcode:2009MNRAS.400.1945T, doi:10.1111/j.1365-2966.2009.15588.x.
  12. ^ Lebzelter, T.; Hron, J. (January 2008), "BRITE stars on the AGB", Communications in Asteroseismology, 152: 178–181, Bibcode:2008CoAst.152..178L, doi:10.1553/cia152s178.
  13. ^ ‹See Tfd›(in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
  14. ^ ‹See Tfd›(in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 7 日
  15. ^ In the Sky Earth astronomy reference utility showing the ecliptic and relevant date as at J2000 - present
  16. ^ John Pratt's stars re-publication by Dr J.P. Pratt (Doctor of Astronomy, University of Arizona) of sidereal coordinate data. Note: possibly a non-book published source.

External links[edit]