Epsilon Aquarii

Epsilon Aquarii

Location of ε Aquarii (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Aquarius
Right ascension	20h 47m 40.54957s[1]
Declination	−09° 29′ 44.79693″[1]
Apparent magnitude (V)	3.77[2]
Characteristics
Evolutionary stage	Main sequence[3] or subgiant[4]
Spectral type	A1 V[2] + KV[4]
U−B color index	+0.029[5]
B−V color index	–0.001[5]
Variable type	Constant[6]
Astrometry
Radial velocity (Rv)	−19.82±0.58[1] km/s
Proper motion (μ)	RA: +33.923 mas/yr[1] Dec.: -34.926 mas/yr[1]
Parallax (π)	13.3619 ± 0.3737 mas[1]
Distance	244 ± 7 ly (75 ± 2 pc)
Absolute magnitude (MV)	−0.46[7]
Orbit[4]
Primary	A
Companion	B
Period (P)	1.2 years
Semi-major axis (a)	25.9 mas 1.7 AU
Details
A
Mass	2.98±0.03[3] M☉
Radius	4.2[4] R☉
Luminosity	161±9[8] L☉
Surface gravity (log g)	3.66±0.14[9] cgs
Temperature	9,622±327[9] K
Metallicity [Fe/H]	−0.08±0.35[6] dex
Rotational velocity (v sin i)	118.0±5.9[7] km/s
Age	388[9] Myr
B
Mass	0.61[4] M☉
Radius	0.57[4] R☉
Temperature	4,070[4] K
Other designations
Albali, ε Aqr, 2 Aquarii, BD–10°5506, FK5 781, HD 198001, HIP 102618, HR 7950, SAO 144810[10]
Database references
SIMBAD	data

Epsilon Aquarii, Latinized from ε Aquarii, is a binary star in the equatorial zodiac constellation of Aquarius, located near the western constellation border with Capricornus. It has the proper name Albali /ælˈbeɪli/, now formally recognized by the IAU.^[11][12] It is visible to the naked eye with an apparent visual magnitude of 3.77,^[2] and has an absolute magnitude of −0.46.^[7] Based upon parallax measurements taken by the Gaia spacecraft, it is located at a distance of approximately 244 light-years (75 pc) from Earth.^[1] The star is drifting closer with a radial velocity of −20 km/s.^[1]

The primary is an A-type main-sequence star with a stellar classification of A1 V.^[2] It is estimated to be estimated 388^[9] million years old with a high rate of spin, showing a projected rotational velocity of around 118 km/s.^[7] The elemental abundances in the stellar atmosphere are close to solar, with pronounced underabundances of aluminium and strontium.^[6] The star has three^[3] times the mass of the Sun and about 4.2^[4] times the Sun's radius. It is radiating 161^[8] times the luminosity of the Sun from its photosphere at an effective temperature of 9,622 K.^[9]

The secondary component is a K-type main-sequence star with 0.6 times the mass and 0.57 times the mass of the Sun. Located at a projected separation of 26 milliarcseconds from the primary, this translates to a physical separation of 1.7 astronomical units. Albali A and B complete an orbit around their center of mass each 1.2 years.^[4]

ε Aquarii (Latinised to Epsilon Aquarii) is the star's Bayer designation.

It bore the traditional name, Albali, from the Arabic البالع (albāli‘), meaning "the swallower". (See also Albulaan.) Along with Mu Aquarii (Albulaan) and Nu Aquarii (also Albulaan), they were al Bulaʽ (البلع), meaning "the Swallower".^[13][14] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)^[15] to catalogue and standardize proper names for stars. The WGSN approved the name Albali for this star on 12 September 2016, and it is now so included in the List of IAU-approved Star Names.^[12]

In the catalogue of stars in the Calendarium of Al Achsasi al Mouakket, it was designated Nir Saad Bula (نير سعد ألبلع nayyir sa'd al bulaʽ), which was translated into Latin as Lucida Fortunæ Dissipantis, meaning "the brightest of luck of the swallower".^[16]

In Chinese, 女宿 (Nǚ Sù), meaning Girl (asterism), refers to an asterism consisting of Epsilon Aquarii, Mu Aquarii, 4 Aquarii, 5 Aquarii and 3 Aquarii.^[17] Consequently, the Chinese name for Epsilon Aquarii itself is 女宿一 (Nǚ Sù yī, English: the First Star of Girl).^[18]

• Image Epsilon Aquarii