Alpha Hydri

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±0.44 mas
Distance	71.8 ± 0.7 ly ; (22.0 ± 0.2 pc)
Absolute magnitude (MV)	+1.153
Details
Mass	2.10+0.17; −0.03 M☉
Radius	3.27+0.11; −0.04 R☉
Luminosity	21.00±0.75 L☉
Surface gravity (log g)	3.64±0.05 cgs
Temperature	7,059+21; −56 K
Metallicity [Fe/H]	0.21+0.10; −0.08 dex
Rotational velocity (v sin i)	118 km/s
Age	810 Myr
Other designations
	α Hyi, CD−62°71, GJ 83, HIP 9236, HR 591, SAO 248474
Database references
SIMBAD	data

Alpha Hydri, Latinized from α Hydri, is the second brightest star in the southern circumpolar constellation of Hydrus. It is readily visible to the naked eye in locations south of 28°N with an apparent visual magnitude of +2.9. It is sometimes informally known as the Head of Hydrus.^[8] 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 east of the prominent star Achernar in the constellation Eridanus.^[9]

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 3.3 times larger and 2.1 as massive as the Sun,^[7] with a stellar classification of F0 IV.^[3] It is about 810 million years old^[6] and is radiating 21 times the Sun's luminosity^[3] from its outer atmosphere at an effective temperature of 7,059 K.^[7] Alpha Hydri emits X-rays similar to Altair.^[10] The space velocity components of this star are [U, V, W] = [−14, −14, -2] km/s.^[11]

Naming

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.)^[12]

References

^ ^a ^b ^c ^d 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. S2CID 18759600.
^ ^a ^b "LTT 1059". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2009-10-13.
^ ^a ^b ^c ^d Rains, Adam D.; et al. (April 2020). "Precision angular diameters for 16 southern stars with VLTI/PIONIER". Monthly Notices of the Royal Astronomical Society. 493 (2): 2377–2394. arXiv:2004.02343. Bibcode:2020MNRAS.493.2377R. doi:10.1093/mnras/staa282. S2CID 214802418.
^ ^a ^b Gutierrez-Moreno, Adelina; et al. (1966). "A System of photometric standards". Publications of the Department of Astronomy University of Chile. 1. Publicaciones Universidad de Chile, Department de Astronomy: 1–17. Bibcode:1966PDAUC...1....1G.
^ Wilson, R. E. (1953). "General Catalogue of Stellar Radial Velocities". Carnegie Institute Washington D.C. Publication. Carnegie Institute of Washington D.C. Bibcode:1953GCRV..C......0W.
^ ^a ^b ^c Lachaume, R.; et al. (August 1999). "Age determinations of main-sequence stars: combining different methods". Astronomy and Astrophysics. 348: 897–909. Bibcode:1999A&A...348..897L. Retrieved 2009-10-13.
^ ^a ^b ^c Vines, Jose I.; Jenkins, James S. (June 2022). "ARIADNE: measuring accurate and precise stellar parameters through SED fitting". Monthly Notices of the Royal Astronomical Society. 513 (2): 2719–2731. arXiv:2204.03769. Bibcode:2022MNRAS.513.2719V. doi:10.1093/mnras/stac956. ISSN 0035-8711.
^ Allen, Richard Hinckley (1963). Star Names, Their Lore and Meaning. New York: Dover. p. 377. ISBN 0-486-21079-0. {{cite book}}: ISBN / Date incompatibility (help)
^ Moore, Patrick (2005). The observer's year: 366 nights of the universe (2nd ed.). Springer. p. 4. ISBN 1-85233-884-9.
^ Schmitt, J. H. M. M.; Golub, L.; Harnden, F. R. Jr.; Maxson, C. W.; Rosner, R.; Vaiana, G. S. (March 1985). "An Einstein Observatory X-ray survey of main-sequence stars with shallow convection zones". Astrophysical Journal. 290 (Part 1): 307–20. Bibcode:1985ApJ...290..307S. doi:10.1086/162986.
^ Gliese, W. (1969). Catalogue of Nearby Stars. Karlsruhe.{{cite book}}: CS1 maint: location missing publisher (link)
^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 27 日 Archived 2011-05-22 at the Wayback Machine

[aaa474_2_653-1] 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. S2CID 18759600.

[SIMBAD-2] "LTT 1059". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2009-10-13.

[Rains2020-3] Rains, Adam D.; et al. (April 2020). "Precision angular diameters for 16 southern stars with VLTI/PIONIER". Monthly Notices of the Royal Astronomical Society. 493 (2): 2377–2394. arXiv:2004.02343. Bibcode:2020MNRAS.493.2377R. doi:10.1093/mnras/staa282. S2CID 214802418.

[gutierrez-4] Gutierrez-Moreno, Adelina; et al. (1966). "A System of photometric standards". Publications of the Department of Astronomy University of Chile. 1. Publicaciones Universidad de Chile, Department de Astronomy: 1–17. Bibcode:1966PDAUC...1....1G.

[gcsrv-5] Wilson, R. E. (1953). "General Catalogue of Stellar Radial Velocities". Carnegie Institute Washington D.C. Publication. Carnegie Institute of Washington D.C. Bibcode:1953GCRV..C......0W.

[aaa348-6] Lachaume, R.; et al. (August 1999). "Age determinations of main-sequence stars: combining different methods". Astronomy and Astrophysics. 348: 897–909. Bibcode:1999A&A...348..897L. Retrieved 2009-10-13.

[Vines2022-7] Vines, Jose I.; Jenkins, James S. (June 2022). "ARIADNE: measuring accurate and precise stellar parameters through SED fitting". Monthly Notices of the Royal Astronomical Society. 513 (2): 2719–2731. arXiv:2204.03769. Bibcode:2022MNRAS.513.2719V. doi:10.1093/mnras/stac956. ISSN 0035-8711.

[allen1963-8] Allen, Richard Hinckley (1963). Star Names, Their Lore and Meaning. New York: Dover. p. 377. ISBN 0-486-21079-0. {{cite book}}: ISBN / Date incompatibility (help)

[moore2005-9] Moore, Patrick (2005). The observer's year: 366 nights of the universe (2nd ed.). Springer. p. 4. ISBN 1-85233-884-9.

[Schmitt1985-10] Schmitt, J. H. M. M.; Golub, L.; Harnden, F. R. Jr.; Maxson, C. W.; Rosner, R.; Vaiana, G. S. (March 1985). "An Einstein Observatory X-ray survey of main-sequence stars with shallow convection zones". Astrophysical Journal. 290 (Part 1): 307–20. Bibcode:1985ApJ...290..307S. doi:10.1086/162986.

[11] Gliese, W. (1969). Catalogue of Nearby Stars. Karlsruhe.{{cite book}}: CS1 maint: location missing publisher (link)

[12] (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 27 日 Archived 2011-05-22 at the Wayback Machine

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Observation data Epoch J2000.0 Equinox J2000.0
Location of α Hydri (circled)
Constellation	Hydrus
Right ascension	01^h 58^m 46.19467^s^[1]
Declination	−61° 34′ 11.4948″^[1]
Apparent magnitude (V)	+2.90^[2]
Characteristics
Spectral type	F0 IV^[3]
U−B color index	+0.189^[4]
B−V color index	+0.290^[4]
Astrometry

Radial velocity (R_v)	+7^[5] km/s
Proper motion (μ)	RA: +263.66 mas/yr^[1] Dec.: +26.77 mas/yr^[1]
Parallax (π)	45.43±0.44 mas^[1]
Distance	71.8 ± 0.7 ly (22.0 ± 0.2 pc)
Absolute magnitude (M_V)	+1.153^[6]

Details^[7]

Mass	2.10+0.17 −0.03 `M`_☉
Radius	3.27+0.11 −0.04 `R`_☉
Luminosity	21.00±0.75^[3] `L`_☉
Surface gravity (log g)	3.64±0.05 cgs
Temperature	7,059+21 −56 K
Metallicity [Fe/H]	0.21+0.10 −0.08 dex
Rotational velocity (v sin i)	118 km/s
Age	810^[6] Myr

Other designations
α Hyi, CD−62°71, GJ 83, HIP 9236, HR 591, SAO 248474^[2]
Database references
SIMBAD	data

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