Triangulum, constellation in the northern sky at about 2 hours right ascension and 30° north in declination. Its brightest star is Beta Trianguli, with a magnitude of 3.0. The brightest stars in the constellation form an obvious triangle. This constellation contains M33, one of the nearest spiral galaxies at a distance of 2.8 million light-years. The ancient Greeks compared it to the capital letter Δ (delta).