A constellation of the zodiac, representing a centaur with a bow, popularly known as the Archer. The Sun passes through Sagittarius from the third week of December to the third week of January, and hence is in the constellation at the winter solstice. Its brightest star is Epsilon Sagittarii (Kaus Australis). Beta Sagittarii is a naked-eye double, consisting of unrelated stars of magnitudes 4.0 and 4.3. Sigma Sagittarii is Nunki. RY Sagittarii is a variable star of the R Coronae Borealis type, which periodically dips from 6th to 14th magnitude. The constellation contains dense Milky Way starfields that lie towards the centre of our Galaxy; the exact centre is believed to be marked by the radio source Sagittarius A. M8 is the Lagoon Nebula; M17 is the Omega Nebula; M20 is the Trifid Nebula. M22 is a rich globular cluster of 5th magnitude, the third-best in the heavens.