Tau4 Serpentis
Star in the constellation Serpens
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Tau4 Serpentis, Latinized from τ4 Serpentis, is a variable M-type giant star in the constellation of Serpens, approximately 710 light-years from the Earth.[5] Its brightness varies from magnitude 5.89 to 7.07, making it occasionally bright enough to be faintly visible to the naked eye under very good observing conditions.
| Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) | |
|---|---|
| Constellation | Serpens |
| Right ascension | 15h 36m 28.1827s[2] |
| Declination | +15° 06′ 05.240″[2] |
| Apparent magnitude (V) | 5.89 to 7.07[3] |
| Characteristics | |
| Spectral type | M5IIIa[4] |
| B−V color index | 1.2[5] |
| Variable type | SRB[3] |
| Astrometry | |
| Radial velocity (Rv) | −26±5[5] km/s |
| Proper motion (μ) | RA: −3.623[2] mas/yr Dec.: +4.476[2] mas/yr |
| Parallax (π) | 4.5628±0.2728 mas[2] |
| Distance | 710 ± 40 ly (220 ± 10 pc) |
| Absolute magnitude (MV) | −0.03[6] |
| Details | |
| Mass | 3.9[7] M☉ |
| Radius | 239[8] R☉ |
| Luminosity | 4,969[8] L☉ |
| Surface gravity (log g) | −0.14[7] cgs |
| Temperature | 3,178[7] K |
| Metallicity [Fe/H] | +0.21[7] dex |
| Other designations | |
| τ4 Ser, 17 Serpentis, BD+15°2890, GC 20983, HD 139216, HIP 76423, SAO 101641, PPM 131543[5] | |
| Database references | |
| SIMBAD | data |
With a spectral classification M5IIIa, Tau4 Serpentis is a cool red giant star. The spectrum varies,[4] and some sources classify it between M4IIIe and M6IIIe.[9] Some of its spectral lines show an inverse P Cygni profile,[10] where cold infalling gas on to the star creates redshifted hydrogen absorption lines next to the normal emission lines.[11] Sometime between the years 1868 and 1877, John Ellard Gore discovered that the star's brightness varies.[12] It is classified as a semiregular late-type variable, and its magnitude varies between +5.89 and +7.07 with a period of approximately 100 days.[3]
τ4 is unique among the stars with the Bayer designation τ Serpentis as being the only one with no HR catalog number.