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2 Ceti

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2 Ceti
Location of 2 Ceti (circled in red)
Observation data
Epoch J2000      Equinox J2000
Constellation Cetus
Right ascension 00h 03m 44.38784s[1]
Declination −17° 20′ 09.5719″[1]
Apparent magnitude (V) 4.483[2]
Characteristics
Evolutionary stage main sequence[3][4]
Spectral type B9 IVn[5]
U−B color index −0.12[6]
B−V color index −0.047±0.003[7]
Astrometry
Radial velocity (Rv)+8.0±4.6[8] km/s
Proper motion (μ) RA: +25.17[1] mas/yr
Dec.: −9.16[1] mas/yr
Parallax (π)11.98±0.26 mas[1]
Distance272 ± 6 ly
(83 ± 2 pc)
Absolute magnitude (MV)−0.06[7]
Details
Mass2.58[9] M☉
Radius3.92[9] R☉
Luminosity119+6
−5[4] L☉
Surface gravity (log g)3.66[9] cgs
Temperature11,419±388[10] K
Metallicity [Fe/H]+0.00±0.24[5] dex
Rotational velocity (v sin i)116[4] or 237[11] km/s
Age217[10] Myr
Other designations
Hydor, 2 Cet, BD−18°6417, FK5 905, HD 225132, HIP 301, HR 9098, SAO 147059[12]
Database references
SIMBADdata

2 Ceti, also named Hydor,[13] is a single[14] star in the equatorial constellation of Cetus, near the border with Aquarius. It is visible to the naked eye with an apparent visual magnitude of 4.483.[2] The distance to 2 Ceti can be estimated from its annual parallax shift of 12.0 mas,[1] which yields a value of around 272 light years. It appears to be moving further from the Earth with a heliocentric radial velocity of about +8 km/s.[8]

The stellar classification for this star is B9 IVn,[5] matching a B-type subgiant star with "nebulous" absorption lines due to rapid rotation.[15] Estimates of the rotation rate range from 116[4] to 237[11] km/s, and this high rate of spin is giving the star an equatorial bulge that is 12% larger than the polar radius.[15] 2 Ceti is about 217 million years old[10] with 2.6 times the mass of the Sun and 3.9 times the Sun's radius.[9] It is radiating 119 times the Sun's luminosity[4] from its photosphere at an effective temperature of 11,419 K.[10] An infrared excess has been detected around this star by the Akari satellite at a wavelength of 18μm, suggesting there is an orbiting debris disk.[16]

Naming

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The ancient Greek term Hydor (ὕδωρ), meaning water, originally referred to a constellation of faint stars in the region of Aquarius and Cetus.[17] The IAU Working Group on Star Names approved the name Hydor for 2 Ceti, on 25 August 2025 and it is now so entered in the IAU Catalog of Star Names;[13] it had previously been used for λ Aquarii, for which the IAU adopted the Indian name Shatabhisha.[17]

References

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  1. ^ a b c d e f van Leeuwen, F. (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.
  2. ^ a b Høg, E.; et al. (2000). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics. 355: L27–L30. Bibcode:2000A&A...355L..27H.
  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. ^ a b c d e Zorec, J.; et al. (2012). "Rotational velocities of A-type stars. IV. Evolution of rotational velocities". Astronomy and Astrophysics. 537: A120. arXiv:1201.2052. Bibcode:2012A&A...537A.120Z. doi:10.1051/0004-6361/201117691. S2CID 55586789.
  5. ^ a b c Wu, Yue; et al. (2010). "Coudé-feed stellar spectral library – atmospheric parameters". Astronomy & Astrophysics. 525: A71. arXiv:1009.1491. Bibcode:2011A&A...525A..71W. doi:10.1051/0004-6361/201015014. S2CID 53480665.
  6. ^ Nicolet, B. (1964). "Catalogue of homogeneous data in the UBV photoelectric photometric system". Astronomy and Astrophysics Supplement Series. 34: 1–49. Bibcode:1978A&AS...34....1N.
  7. ^ a b 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.
  8. ^ a b Gontcharov, G. A. (2006). "Pulkovo Compilation of Radial Velocities for 35 495 Hipparcos stars in a common system". Astronomy Letters. 32 (11): 759–771. arXiv:1606.08053. Bibcode:2006AstL...32..759G. doi:10.1134/S1063773706110065. S2CID 119231169.
  9. ^ a b c d Stassun, Keivan G.; et al. (2019). "The Revised TESS Input Catalog and Candidate Target List". The Astronomical Journal. 158 (4): 138. arXiv:1905.10694. Bibcode:2019AJ....158..138S. doi:10.3847/1538-3881/ab3467.
  10. ^ a b c d David, Trevor J.; Hillenbrand, Lynne A. (2015). "The Ages of Early-Type Stars: Strömgren Photometric Methods Calibrated, Validated, Tested, and Applied to Hosts and Prospective Hosts of Directly Imaged Exoplanets". The Astrophysical Journal. 804 (2): 146. arXiv:1501.03154. Bibcode:2015ApJ...804..146D. doi:10.1088/0004-637X/804/2/146. S2CID 33401607.
  11. ^ a b Royer, F.; et al. (2007). "Rotational velocities of A-type stars". Astronomy and Astrophysics. 463 (2): 671. arXiv:astro-ph/0610785. Bibcode:2007A&A...463..671R. doi:10.1051/0004-6361:20065224. S2CID 18475298.
  12. ^ "2 Cet". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved December 23, 2018.
  13. ^ a b "IAU Catalog of Star Names". Retrieved 26 August 2025.
  14. ^ Chini, R.; et al. (2012). "A spectroscopic survey on the multiplicity of high-mass stars". Monthly Notices of the Royal Astronomical Society. 424 (3): 1925–1929. arXiv:1205.5238. Bibcode:2012MNRAS.424.1925C. doi:10.1111/j.1365-2966.2012.21317.x. S2CID 119120749.
  15. ^ a b van Belle, Gerard T. (March 2012). "Interferometric observations of rapidly rotating stars". The Astronomy and Astrophysics Review. 20 (1): 51. arXiv:1204.2572. Bibcode:2012A&ARv..20...51V. doi:10.1007/s00159-012-0051-2. S2CID 119273474.
  16. ^ Ishihara, Daisuke; et al. (May 2017). "Faint warm debris disks around nearby bright stars explored by AKARI and IRSF". Astronomy & Astrophysics. 601: 18. arXiv:1608.04480. Bibcode:2017A&A...601A..72I. doi:10.1051/0004-6361/201526215. S2CID 55234482. A72.
  17. ^ a b "Hydor". All Skies Encyclopaedia. IAU Working Group on Star Names. Retrieved 26 August 2025.
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