Pi1 Gruis
| Observation data Epoch J2000 Equinox J2000 |
|
|---|---|
| Constellation | Grus |
| Right ascension | 22h 22m 44.20571s [1] |
| Declination | −45° 56′ 52.6115″ [1] |
| Apparent magnitude (V) | 6.14v |
| Characteristics | |
| Spectral type | S5, G0V[1] |
| Variable type | semi-regular variable[2] |
| Astrometry | |
| Parallax (π) | 6.13 ± 0.76 mas |
| Distance | approx. 530 ly (approx. 160 pc) |
| Absolute magnitude (MV) | - |
| Other designations | |
Pi1 Gruis is a star in the constellation Grus. It is a semi-regular red giant of spectral type S5, ranging from apparent magnitude 5.31 to 7.01 over a period of 191 days,[2] and is around 532 light-years from Earth.[1] It is one of the brightest stars of a class of stars known as S Stars, and has a companion star of apparent magnitude 10.9 that is sunlike in properties—a yellow main sequence star of spectral type G0V. Separated by 2.71′′, the pair make up a likely binary system. The primary star has a measured diameter of 21 milliarcseconds, corresponding to a diameter 694 times that of the Sun.[3] It is an ageing star, thought be well on its way transitioning from a red giant to a planetary nebula. A shell of material has been detected at a distance of 0.91 light-years (0.28 parsecs), which is estimated to have been ejected 21,000 years ago.[4] Closer to the star, there appears to be a cavity within 200 AU, suggesting a drop off in the ejection of material in the past 90 years. The presence of one companion makes the shape of the shell irregular (rather than spherical), and there may as yet be another undetected companion contributing to this.[3]
The star was catalogued by French explorer and astronomer Nicolas Louis de Lacaille in 1756 but not given a name. Instead, he gave the Bayer designation of "Pi Gruis" to Pi2 Gruis. It was Thomas Brisbane who designated this star as Pi1.[5] Annie Jump Cannon was the first to report its unusual spectrum, sending a plate of its spectrograph made in 1895 to Paul W. Merrill and noting its similarity to R Andromedae. Merrill selected these two stars along with R Cygni to be the three prototypes of the S Star class.[6] Pi1 Gruis was one of the first 17 stars defined as S-stars by Merrill in 1922; the only star not observed from Mount Wilson due to its southerly location in the sky.[7] Analysis of its spectrum showed bands indicating the presence of technetium,[8] as well as oxides of zirconium, lanthanum, cerium and yttrium but not titanium nor barium (which have been recorded in other S stars).[9]
References
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