Terebellum

On the eastern fringes of Sagittarius, near the point where the Ecliptic crosses over into Capricornus, lies a small formation of four stars. None of these stars is particularly bright in the night sky, but the fact that they are all of a similar magnitude, as well as the lack of other bright stars in the immediate area, makes this tiny asterism remarkably distinctive.

These four stars have been recognised as a group since antiquity: to the Greeks they were the Tetrapleuron ('quadrilateral'), but today they are known by the Latin name Terebellum, meaning the auger or boring tool (the stars are arranged in an approximate cross shape, with one axis distinctly longer than the other, reminiscent of a handled tool). The name Terebellum is also sometimes seen applied to various of the individual stars within the asterism, and indeed is formally recognised as the proper name of the star Omega Sagittarii.

The four stars that make up the Terebellum are Omega, A, b and c Sagittarii (or respectively 58, 60, 59 and 62 Sagittarii by Flamsteed numbers). Apart from their apparent position in the sky, they are not related to one another, and their similar apparent brightness is simply a line-of-sight effect. In fact the stars are quite different in luminosity and distance: for example, Omega Sagittarii is the nearest of the four, a subgiant at a distance of about 80 light years from Earth, while b Sagittarii is an intrinsically much brighter supergiant star more than 1,200 light years distant.