Alpha1 Capricorni, Alpha2 Capricorni - Algedi

α1 Capricorni and α2 Capricorni are a wide naked-eye double at the northwest corner of Capricornus' boat-shaped outline. They also have the traditional names Algiedi, Al Giedi, Algedi, or Giedi; however, Giedi is sometimes also associated with β Cap.

α1 Cap is referred to as Algedi Prima, and α2 as Algedi Secunda. The name Algedi is derived from the Arabic word "al-jadiyy", meaning "the billy goat" or "kid," and also refers to the entire constellation of Capricornus. The stars are known as "the Second Star of the Ox" in Chinese.

Properties

The two stars are separated by 0.11° in the sky - 1/6 the angular diameter of the full Moon - and are resolvable by the naked eye. They are an optical double star, physically unrelated to each other, and merely appear along the same line of sight. The magnitude 3.6 α1 Cap is 109 light years away, while magnitude 4.2 α2 Cap is 690 light years distant.

Both stars shine with a rich yellowish-orange color from surfaces around 5000 K, rather similar in temperature to the Sun. Both are evolved, yellow stars: class G3 for α1, and G8 for α2. Both stars have stopped fusing hydrogen in their cores, and are preparing to fuse their internal helium to carbon, if they have not started doing so already.

The closer star (α2 Cap) is a giant 43 times more luminous than the Sun. It is deficient in metals; its iron abundance is between a tenth and half of the Sun's, indicating that it comes from an older population of stars.

The farther star (α1 Cap) is a supergiant 930 times more luminous than the Sun (and 21 times more luminous than α2 Cap), only seeming fainter because of its greater distance. It is also the larger of the two, with a radius of 40 times the Sun's (and 5 times α2 Cap's). It is also the more massive, containing 5 solar masses - double that of α2 Cap. Unlike α2 Cap, the supergiant has a normal, sunlike chemical composition.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]