Alpha1 Capricorni

Alpha¹ Capricorni is a binary star system dominated by a highly luminous star in the constellation of Capricornus, north of the ecliptic. It has the traditional star names Prima Giedi (/ˌpraɪmə ˈdʒiːdi/) and Algiedi Prima (/æˈdʒiːdi ˈpraɪmə/). The Bayer designation Alpha¹ Capricorni is Latinized from α¹ Capricorni, and abbreviated Alpha¹ Cap or α¹ Cap. The system is separated from the brighter Alpha² Capricorni by 0.11° of the sky, a gap resolvable with the naked eye,^[8] similar to Mizar and Alcor. Both are not to be confused with much fainter 3 Capricorni nor somewhat fainter Nu Capricorni which are 3 to 6 times the angular distance apart than separate the two Alpha stars, respectively.

For other stars with this Bayer designation, see α Capricorni.

α¹ Capricorni

Location of the α Capricorni pair in the red circle.
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Capricornus[1]
Right ascension	20h 17m 38.869s[2]
Declination	−12° 30′ 29.56″[2]
Apparent magnitude (V)	4.27[3] + 8.60[4]
Characteristics
Evolutionary stage	Supergiant[5]
Spectral type	G3 Ib[6]
U−B color index	+0.70[3]
B−V color index	+1.07[3]
Variable type	Constant[1]
Astrometry
Radial velocity (Rv)	−25.79±0.15[5] km/s
Proper motion (μ)	RA: +21.709 mas/yr[2] Dec.: +1.643 mas/yr[2]
Parallax (π)	4.0157±0.1072 mas[2]
Distance	810 ± 20 ly (249 ± 7 pc)
Absolute magnitude (MV)	−1.90[1]
Details[5]
Mass	5.3 M☉
Radius	36.3±2.0 R☉
Luminosity	1,047 L☉
Surface gravity (log g)	1.75 cgs
Temperature	5,119±15 K
Metallicity [Fe/H]	−0.02 dex
Rotational velocity (v sin i)	7.3[6] km/s
Other designations
Prima Giedi, Algiedi Prima[7], α1 Cap, 5 Capricorni, BD−12°5683, HD 192876, HIP 100027, HR 7747, WDS 20176-1230
Database references
SIMBAD	data

The primary star is a yellow hued supergiant star with a stellar classification of G3 Ib.^[6] It has an apparent magnitude of +4.3; bright enough to make it visible to the naked eye. The star is located at a distance of approximately 810 light-years (250 pc) 870 light years from the Solar System based on parallax measurements,^[2] but is drifting closer with a radial velocity of −26 km/s.^[5] The star is past first dredge-up and has already evolved through the Cepheid instability strip; it may be about to do so a second time.^[5] It has 5.3 times the mass and 36 times the radius of the Sun, and is radiating around 1,047 times the Sun's luminosity from its photosphere at an effective temperature of 5,119 K.^[5]

The Hipparcos satellite in about the year 2000 found a previously undetected companion at an angular separation of 0.65″ from the primary.^[9] This magnitude 8.60 star forms a binary pair with α¹ Capricorni.^[4] Three other faint visual companion stars lie within one arc-minute, so are unresolveable in small telescopes. The brightest of these is 10th magnitude and on this basis it has often been considered as an optical binary. Separation is increasing rapidly due to great proper motion of the primary star.^[9]