Ο_Aquarii
Omicron Aquarii
Variable B-type star in the constellation Aquarius
Omicron Aquarii, Latinized from ο Aquarii, is the Bayer designation for a variable star in the equatorial constellation of Aquarius. Visible to the naked eye, it has an apparent visual magnitude of +4.71.[2] Parallax measurements put it at a distance of roughly 440 light-years (130 parsecs) from Earth.[1] It is drifting further away with a radial velocity of +11 km/s.[5] The star is a candidate member of the Pisces-Eridanus stellar stream.[12]
It has the traditional star name Kae Uh, from the Chinese 蓋屋 (Mandarin pronunciation Gài Wū).[13] In Chinese astronomy, 蓋屋 is the rooftop, an asterism consisting of ο Aquarii and 32 Aquarii.[14] Consequently, the Chinese name for ο Aquarii itself is 蓋屋一 (Gài Wū yī, English: the First Star of Roofing.)[15]
The spectrum of Omicron Aquarii fits a stellar classification of B7 IVe;[3] the luminosity class of IV suggests that this is a subgiant star that is exhausting the supply of hydrogen at its core and is in the process of evolving into a giant star. The 'e' suffix on the class indicates that the spectrum shows emission lines of hydrogen, thus categorizing this as a Be star. It is rotating rapidly with an equatorial rotational velocity of 368 km/s, which is ~96% of the star's critical rotation velocity of 391 km/s.[7] The emission lines are being generated by a decreted circumstellar disk of hot hydrogen gas.[10] This disk has been globally stable for at least twenty years, as of 2020.[7] It is likely a single star, with no stellar companions.[17]
Omicron Aquarii has 4.2 times the mass of the Sun, four[7] times the Sun's radius, and is radiating 340[6] times the luminosity of the Sun from its photosphere at an effective temperature of 13,464 K.[9] It is classified as a Gamma Cassiopeiae type[4] variable star and its brightness varies from magnitude +4.68 down to +4.89.[18]