  | |
| Observation data EpochJ2000.0 EquinoxJ2000.0 | |
|---|---|
| Constellation | Pisces |
| Right ascension | 23h 46m 23.51645s[1] |
| Declination | +03° 29′ 12.5190″[1] |
| Apparent magnitude (V) | 4.79 - 5.20[2] |
| Characteristics | |
| Evolutionary stage | AGB[3] |
| Spectral type | C7,2[4] |
| U−Bcolor index | +3.33[5] |
| B−Vcolor index | +2.60[5] |
| Variable type | Lb[2] |
| Astrometry | |
| Radial velocity (Rv) | +11.5±1.5[6] km/s |
| Proper motion (μ) | RA: −32.086[1]mas/yr Dec.: −24.545[1]mas/yr |
| Parallax (π) | 4.0894±0.2124 mas[1] |
| Distance | 800 ± 40 ly (240 ± 10 pc) |
| Absolute magnitude (MV) | −2.22[6] |
| Details | |
| Mass | 1 – 3[7] M☉ |
| Radius | 295[7] R☉ |
| Luminosity | 7,019 – 7,876[7] L☉ |
| Surface gravity (log g) | −0.2 – −0.5[7] cgs |
| Temperature | 3,080 – 3,170[7] K |
| Metallicity[Fe/H] | −0.39[3] dex |
| Other designations | |
| 19 Psc,TX Psc,BD+02°4709,FK5 3908,HD 223075,HIP 117245,HR 9004,SAO 128374 | |
| Database references | |
| SIMBAD | data |
TX Piscium (19 Piscium) is a variablered giant star in theconstellationPisces. It is amongst the reddestnaked eye stars, with a significant reddish hue when seen inbinoculars. It is approximately 800light years fromEarth. It is close to—and sometimes considered part of—the asterism on the western end of the constellation called thecirclet of Pisces.
TX Piscium is a very red star, 2.6magnitudes fainter at blue wavelengths than in the middle of the visual range, and another 3.3 magnitudes fainter in theultraviolet.[5]
It has been given aspectral class C7,2, indicating a relatively cool carbon star with only modestC2 band strength.[4] It has alternately been classified as C-N5 C24, suggesting a warmer star with stronger C2 bands.[8] Spectral features have been observed to vary.[9]
Theapparent magnitude of TX Piscium varies between +4.9 and +5.5 and it is classified as aslow irregular variable.[2] Photometry has shown some periodicity in the brightness of 224 days, and some spectral variation over 450 days, suggesting the star is not entirely irregular.[9]
The star apparently lies on theperiod-luminosity relation corresponding tofundamental mode pulsations, unusual for a low-amplitude semi-regular or irregular variable which usually pulsate in anovertone.[11] The angular diameter has been measured at around 10 mas, although this varies depending on the observedwavelength and the atmosphere appears to be highly asymmetric. There may be one or more "blobs" of ejected material near the star.[11][9] Some observations show the angular diameter to change in synchrony with thevisual brightness.[11]
TX Piscium is a thermally-pulsingasymptotic giant branch (TP-AGB) red giant star, which means that it is alternatelyfusing helium in a shell around its core andfusing hydrogen in a shell closer to its surface. Stars on the TP-AGB are generally unstable, with high mass loss and pulsations.
The periodicflashes of the helium shell in a red giant star are known as thermal pulses and cause the hydrogen shell to be extinguished. This creates strong convection and the thirddredge-up (TDU) which brings helium fusion products such ascarbon to the surface. After several TDUs, the abundance of carbon in the atmosphere begins to exceed that ofoxygen and then the star is known as a carbon star. In TX Piscium, the carbon/oxygen ratio is calculated to be 1.03, which is at the low end of the scale of carbon stars.[3] It is thought to be a relatively new carbon-rich red giant star.[9]
TX Piscium has a mass of approximately 2 M☉. Modelling of its observed properties suggest a mass between 1 and 2 M☉, while evolutionary models suggest a mass between 2 and 3 M☉. In particular, stars with a mass below 2 M☉ are not expected to become carbon stars.[7]
The temperature of TX Piscium is thought to vary between 3,080 K at minimum visual brightness and 3,170 K at visual maximum. Similarly, its luminosity varies between 7,019 and 7,876 L☉. Many of its physical properties are uncertain due to uncertainty in its distance. TheHipparcos parallax corresponds to a distance of 275 parsecs, but other estimates give distances as high as 315 parsecs.[7] These properties imply a radius of 295 R☉; if TX Piscium took the place of the Sun, it would extend beyond the orbit of theEarth, but not reachMars.
