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, 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.

The primary star is a yellow hued supergiant star with a stellar classification of G3 Ib. 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, but is drifting closer with a radial velocity of −26 km/s. 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. 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.

The Hipparcos satellite in about the year 2000 found a previously undetected companion at an angular separation of 0.65″ from the primary. This magnitude 8.60 star forms a binary pair with α¹ Capricorni. 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.