Alpha Andromedae

α Andromedae

Alpha Andromedae
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Andromeda
Right ascension	00h 08m 23.25988s
Declination	+29° 05′ 25.5520″
Apparent magnitude (V)	2.06 (2.22 + 4.21)
Characteristics
U−B color index	−0.46
B−V color index	−0.11
R−I color index	−0.10
Primary
Spectral type	B8IVpMnHg
Secondary
Spectral type	A3V
Astrometry
Primary
Radial velocity (Rv)	−10.6 ± 0.3 km/s
Proper motion (μ)	RA: 135.68 mas/yr Dec.: −162.95 mas/yr
Parallax (π)	33.62 ± 0.35mas
Distance	97 ± 1 ly (29.7 ± 0.3 pc)
Absolute magnitude (MV)	−0.19 ± 0.30
Secondary
Absolute magnitude (MV)	2.00 ± 0.30
Orbit
Period (P)	96.7015 ± 0.0044 days
Semi-major axis (a)	24.0 ± 0.13 mas
Eccentricity (e)	0.535 ± 0.0046
Inclination (i)	105.6 ± 0.23°
Longitude of the node (Ω)	284.4 ± 0.21°
Periastron epoch (T)	MJD 47374.563 ± 0.095
Argument of periastron (ω) (secondary)	257.4 ± 0.31°
Details
Primary
Mass	3.8 ± 0.2 M☉
Radius	2.7 ± 0.4 R☉
Luminosity (bolometric)	240 L☉
Surface gravity (log g)	3.75 cgs
Temperature	13,800 K
Metallicity	[M/H] = 0.2
Rotation	2.38195 d
Rotational velocity (v sin i)	52 km/s
Age	60 Myr
Secondary
Mass	1.85 ± 0.13 M☉
Radius	1.65 ± 0.3 R☉
Luminosity (bolometric)	13 L☉
Surface gravity (log g)	4.0 cgs
Temperature	8,500 K
Metallicity	[M/H] = 0.2
Rotational velocity (v sin i)	110 ± 5 km/s
Age	70 Myr
Other designations
Alpheratz, Sirrah, Sirah, α And, Alpha Andromedae, Alpha And, δ Pegasi, δ Peg, Delta Pegasi, Delta Peg, 21 Andromedae, 21 And, H 5 32A, MKT 11, ADS 94 A, BD+28°4, CCDM J00083+2905A, FK5 1, GC 127, HD 358, HIP 677, HR 15, IDS 00032+2832 A, LTT 10039, NLTT 346, PPM 89441, SAO 73765, WDS 00084+2905A/Aa
Database references
SIMBAD	data

Alpha Andromedae (α Andromedae, abbreviated Alpha And or α And), also named Alpheratz, is located 97 light-years from the Sun and is the brightest star in the constellation of Andromeda. Located immediately northeast of the constellation of Pegasus, it is the northeastern star of the Great Square of Pegasus.

Although it appears to the naked eye as a single star, with overall apparent visual magnitude +2.06, it is actually a binary system composed of two stars in close orbit. The chemical composition of the brighter of the two stars is unusual as it is a mercury-manganese star whose atmosphere contains abnormally high levels of mercury, manganese, and other elements, including gallium and xenon. It is the brightest mercury-manganese star known.

α Andromedae (Latinised to Alpha Andromedae) is the star's Bayer designation. Ptolemy considered Alpha Andromedae to be shared by Pegasus, and Johann Bayer assigned it a designation in both constellations: Alpha Andromedae (α And) and Delta Pegasi (δ Peg). When the modern constellation boundaries were fixed in 1930, the latter designation dropped from use.

The star bore the traditional names Alpheratz or Alpherat and Sirrah deriving from the Arabic name, سرة الفرس surrat al-faras "the navel of the mare". (سرة alone is surrah.) The word horse reflects the star's historical placement in Pegasus. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Alpheratz for this star.