Phi Aquarii

Phi Aquarii is a binary star^[8] system in the equatorial constellation of Aquarius. Its name is a Bayer designation that is Latinized from φ Aquarii, and abbreviated Phi Aqr or φ Aqr. This system is visible to the naked eye as a point of light with a combined apparent visual magnitude of +4.223.^[2] Parallax measurements indicate its distance from Earth is approximately 227 light-years (70 pc).^[1] It is drifting further away with a radial velocity of +2.5 km/s.^[4] The system is positioned 1.05 degrees south of the ecliptic so it is subject to lunar occultations.^[9]

Phi Aquarii

Location of φ Aquarii (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Aquarius
Right ascension	23h 14m 19.358s[1]
Declination	−06° 02′ 56.42″[1]
Apparent magnitude (V)	+4.223[2]
Characteristics
Spectral type	M1.5 III[3]
U−B color index	+1.897[2]
B−V color index	+1.563[2]
Astrometry
Radial velocity (Rv)	+2.48±0.32[4] km/s
Proper motion (μ)	RA: +36.575 mas/yr[1] Dec.: −195.441 mas/yr[1]
Parallax (π)	14.3482±0.2594 mas[1]
Distance	227 ± 4 ly (70 ± 1 pc)
Absolute magnitude (MV)	0.30±0.120[5]
Details
A
Mass	1.00±0.03[6] M☉
Radius	34.77+1.83 −2.04[6] R☉
Luminosity	207.7±25.2[6] L☉
Surface gravity (log g)	1.5[4] cgs
Temperature	3,715±48[6] K
Rotational velocity (v sin i)	6.7[4] km/s
Age	10.97±0.83[6] Gyr
Other designations
φ Aqr, 90 Aquarii, BD−06°6170, FK5 1607, GC 32346, HD 219215, HIP 114724, HR 8834, SAO 146585, PPM 207311[7]
Database references
SIMBAD	data

This is a spectroscopic binary star system with an estimated period of 2,500 days (6.8 yr).^[10] The primary component is an aging red giant star with a stellar classification of M1.5 III.^[3] At an estimated age of 11 billion years,^[6] it has exhausted the supply of hydrogen at its core and evolved away from the main sequence. The star has the same mass as the Sun,^[6] but has expanded to 35 times the Sun's girth.^[6] It is radiating 208 times the luminosity of the Sun from its enlarged photosphere at an effective temperature of 3,715 K,^[6] giving it the reddish hue of an M-type star.^[11]

On 6 September 2019, it had a close conjunction (geocentric separation <1') with Neptune.^{[12] : 167}