Pi1 Gruis

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π1 Gruis
The surface of the red giant star π1 Gruis from PIONIER on the VLT.jpg
Surface of the red giant star π1 Gruis.[1]
Observation data
Epoch J2000      Equinox J2000
Constellation Grus
Right ascension  22h 22m 44.20571s[2]
Declination −45° 56′ 52.6115″[2]
Apparent magnitude (V) 5.31 - 7.01[3] + 10.4[4]
Spectral type S5,7:[5] + G0V[4]
Variable type SRb[6]
Parallax (π)6.13 ± 0.76[2] mas
Distanceapprox. 530 ly
(approx. 160 pc)
Absolute magnitude (MV)G0: +4.3[4]
Period (P)6,200 years
Semi-major axis (a)2.8"
(460 AU)
Mass1.5[4] M
Radius350[1] R
Luminosity7,244[4] L
Temperature3,100[4] K
Mass1.0[4] M
Other designations
CD−46° 14292, HR 8521, HD 212087, SAO 231105, HIP 110478
Database references

π1 Gruis (Pi1 Gruis) is a semiregular variable star in the constellation Grus around 530 light-years from Earth. It forms a close naked-eye double with π2 Gru four arc-minutes away.

π1 Gruis is an asymptotic giant branch (AGB) star of spectral type S5. It is one of the brightest members of a class of stars known as S Stars. It is also a semi-regular variable star ranging from apparent magnitude 5.3 to 7.0 over a period of 198.8 days,[7] It is an ageing star, thought to 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.[8] 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.[9]

π1 Gruis 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 18.37 milliarcseconds,[10] corresponding to a size 350 times that of the Sun.[1]

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 "π Gruis" to π2. It was Thomas Brisbane who designated this star as π1.[11] 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.[12] π1 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.[13] Analysis of its spectrum showed bands indicating the presence of technetium,[14] as well as oxides of zirconium, lanthanum, cerium and yttrium but not titanium nor barium (which have been recorded in other S stars).[15]


  1. ^ a b c "Giant Bubbles on Red Giant Star's Surface". www.eso.org. Retrieved 21 December 2017.
  2. ^ a b c 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.
  3. ^ Pojmanski, G. (2002). "The All Sky Automated Survey. Catalog of Variable Stars. I. 0 h - 6 h Quarter of the Southern Hemisphere". Acta Astronomica. 52: 397–427. arXiv:astro-ph/0210283. Bibcode:2002AcA....52..397P.
  4. ^ a b c d e f g h Mayer, A.; Jorissen, A.; Paladini, C.; Kerschbaum, F.; Pourbaix, D.; Siopis, C.; Ottensamer, R.; Mečina, M.; Cox, N. L. J.; Groenewegen, M. A. T.; Klotz, D.; Sadowski, G.; Spang, A.; Cruzalèbes, P.; Waelkens, C. (2014). "Large-scale environments of binary AGB stars probed by Herschel. II. Two companions interacting with the wind of π1 Gruis". Astronomy & Astrophysics. 570 (113): A113. arXiv:1408.3965. Bibcode:2014A&A...570A.113M. doi:10.1051/0004-6361/201424465.
  5. ^ Stewart, Paul N.; Tuthill, Peter G.; Nicholson, Philip D.; Sloan, G. C.; Hedman, Matthew M. (2015). "An Atlas of Bright Star Spectra in the Near-infrared from Cassini-VIMS". The Astrophysical Journal Supplement Series. 221 (2): 30. arXiv:1511.01670. Bibcode:2015ApJS..221...30S. doi:10.1088/0067-0049/221/2/30.
  6. ^ Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/gcvs. Originally Published In: 2009yCat....102025S. 1: B/gcvs. Bibcode:2009yCat....102025S.
  7. ^ Tabur, V.; Bedding, T. R.; Kiss, L. L.; Moon, T. T.; Szeidl, B.; Kjeldsen, H. (2009). "Long-term photometry and periods for 261 nearby pulsating M giants". Monthly Notices of the Royal Astronomical Society. 400 (4): 1945. arXiv:0908.3228. Bibcode:2009MNRAS.400.1945T. doi:10.1111/j.1365-2966.2009.15588.x.
  8. ^ Young, K.; Phillips, T. G.; Knapp, G. R. (1993). "Circumstellar shells resolved in IRAS survey data. II - Analysis" (PDF). Astrophysical Journal. 409: 725–38. Bibcode:1993ApJ...409..725Y. doi:10.1086/172702.
  9. ^ Sacuto, S.; Jorissen, A.; Cruzalèbes, P.; Chesneau, O.; et al. (2008). "The close circumstellar environment of the semi-regular S-type star π 1 Gruis" (PDF). Astronomy & Astrophysics. 482 (2): 561–74. arXiv:0803.3077. Bibcode:2008A&A...482..561S. doi:10.1051/0004-6361:20078306.
  10. ^ Paladini, C; Baron, F; Jorissen, A; Le Bouquin, J.-B; Freytag, B; Van Eck, S; Wittkowski, M; Hron, J; Chiavassa, A; Berger, J.-P; Siopis, C; Mayer, A; Sadowski, G; Kravchenko, K; Shetye, S; Kerschbaum, F; Kluska, J; Ramstedt, S (2017). "Large granulation cells on the surface of the giant star π1 Gruis". Nature. 553 (7688): 310–312. doi:10.1038/nature25001. PMID 29258298.
  11. ^ Wagman, Morton (2003). Lost Stars: Lost, Missing and Troublesome Stars from the Catalogues of Johannes Bayer, Nicholas Louis de Lacaille, John Flamsteed, and Sundry Others. Blacksburg, VA: The McDonald & Woodward Publishing Company. pp. 360–62. ISBN 978-0-939923-78-6.
  12. ^ Merrill, Paul W. (1955). "Red Stars". Publ. Astron. Soc. Pac. 67 (397): 199–213. Bibcode:1955PASP...67..199M. doi:10.1086/126804.
  13. ^ Merrill, Paul W. (1922). "Stellar spectra of class S". Astrophysical Journal. 56: 457–82. Bibcode:1922ApJ....56..457M. doi:10.1086/142716.
  14. ^ Little, Stephen J.; Little-Marenin, Irene R.; Bauer, Wendy Hagen (1987). "Additional late-type stars with technetium". Astronomical Journal. 94: 981–995. Bibcode:1987AJ.....94..981L. doi:10.1086/114532.
  15. ^ Murty, P.S. (1983). "Pi Gruis - Molecular identifications and spectral classification" (PDF). Astrophysics and Space Science. 94 (2): 295–305. Bibcode:1983Ap&SS..94..295M. doi:10.1007/BF00653719. ISSN 0004-640X.