A white-lightlight curve for Pi Aquarii, adapted from Nazéet al. (2020)[13]
This is a binary star system with aperiod of 84.1 days in a circular orbit.[14][15] The primary component is aB1giant orsubgiant star.[4] This is a massive star with 11 times the mass of the Sun, and is luminous with 8,300 times theSun's luminosity. It is spinning rapidly with aprojected rotational velocity of271 km/s. The fast rotation make the star ellipsoidal rather than spherical, its radius at equator is 5.8 timesthat of the Sun, while its polar radius is 5.0 times solar.Gravity darkening make the temperature at the equator to be about2,600 K colder than the temperature at the poles.[16]
Pi Aquarii is notable for having undergone a transition from aBe star (showing hydrogenemission lines) into an ordinaryB-type star.[15] It is classified as aGamma Cassiopeiae type[3]variable star and its brightness varies from magnitude +4.45 to +4.71; a range of 0.28. The dominant variability period,83.8±0.8 days, is nearly the same as the orbital period.[15] Pi Aquarii has a reasonable chance of becoming a supernova some day.[14]
False-color image of the emission nebula (green and blue) surrounding π Aqr (image center), obtained by the Northern Sky Narrowband Survey.
Infrared images from WISE at a wavelength of 22 µm reveal a bow shock located approximately 100″ from the star (51,000 au at 340 pc) in the direction of its proper motion.[17] In optical range, a weakly ionized emission nebula with a diameter of about 5° is visible (green to blue in the image). There also appears to be some interaction with the surrounding interstellar medium (red in the image). Notably, the rectangular shape of the nebula is not aligned with the current orientation of the circumstellar disk (see the infobox).
In the catalogue of stars in theCalendarium ofAl Achsasi al Mouakket, this star was designated Wasat al Achbiya (وسط الأخبية -wasath al ahbiyah), which was translated intoLatin as Media Tabernaculorum, meaningthe middle of luck of the homes (tents).[20] This star, along withγ Aqr (Sadachbia),ζ Aqr (Sadaltager / Achr al Achbiya) andη Aqr (Hydria), were al Aḣbiyah (الأخبية), the Tent.[21][22][23]
^abSlettebak, A. (1982), "Spectral types and rotational velocities of the brighter Be stars and A-F type shell stars",Astrophysical Journal Supplement Series,50: 55,Bibcode:1982ApJS...50...55S,doi:10.1086/190820.
^abcNicolet, B. (1978), "Photoelectric photometric Catalogue of homogeneous measurements in the UBV System",Astronomy and Astrophysics Supplement Series,34:1–49,Bibcode:1978A&AS...34....1N.
^abWielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions",Veröff. Astronomisches Rechen-Institut Heidelberg,35 (35), Astronomisches Rechen-Institut Heidelberg: 1,Bibcode:1999VeARI..35....1W.
^abConcha, D.; Arcos, C.; Turis-Gallo, D.; Souza, T. B.; Curé, M.; Levenhagen, R. S.; Araya, I. (2025). "New insight into the variability of the Be starπ Aquarii: Determination of stellar and disk parameters".Astronomy & Astrophysics.697: A209.arXiv:2505.13700.Bibcode:2025A&A...697A.209C.doi:10.1051/0004-6361/202452708.
^abcPollmann, Ernst (May 2012), "Period analysis of the Halpha line profile variation of the Be binary star pi Aqr",Information Bulletin on Variable Stars,6023: 1,Bibcode:2012IBVS.6023....1P.
^Knobel, E. B. (June 1895), "Al Achsasi Al Mouakket, on a catalogue of stars in the Calendarium of Mohammad Al Achsasi Al Mouakket",Monthly Notices of the Royal Astronomical Society,55 (8): 429,Bibcode:1895MNRAS..55..429K,doi:10.1093/mnras/55.8.429.
^Davis Jr., G. A. (October 1944), "The Pronunciations, Derivations, and Meanings of a Selected List of Star Names",Popular Astronomy,52 (3): 12,Bibcode:1944PA.....52....8D.
^γ Aqr asAoul al Achbiya orPrima Tabernaculorum (the first of luck of the homes or tents), π Aqr asWasat al Achbiya orMedia Tabernaculorum (the middle of luck of the homes or tents) andζ Aqr asAchr al Achbiya orPostrema Tabernaculorum (the end of luck of the homes or tents).η Aqr should be designated asal Achbiya consistently, but it was not designated as theArabic name except the nameHydria (Greek) orDeli (Hebrew)
