2 Ceti
Star in the constellation Cetus
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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]
 | |
| 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] |
| Distance | 272 ± 6 ly (83 ± 2 pc) |
| Absolute magnitude (MV) | −0.06[7] |
| Details | |
| Mass | 2.58[9] M☉ |
| Radius | 3.92[9] R☉ |
| Luminosity | 119+6 −5[4] L☉ |
| Surface gravity (log g) | 3.66[9] cgs |
| Temperature | 11,419±388[10] K |
| Metallicity [Fe/H] | +0.00±0.24[5] dex |
| Rotational velocity (v sin i) | 116[4] or 237[11] km/s |
| Age | 217[10] Myr |
| Other designations | |
| Hydor, 2 Cet, BD−18°6417, FK5 905, HD 225132, HIP 301, HR 9098, SAO 147059[12] | |
| Database references | |
| SIMBAD | data |
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
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]