The Big Dipper is only the tail end of a much bigger bear
The seven stars called the Big Dipper in North America and the Plough in Britain form just the hindquarters and tail of Ursa Major, the Great Bear. The whole constellation spreads across more than 1,279 square degrees, making it the third largest of the 88 recognised today, and its mythology likely reaches back into prehistory.
Ptolemy included it among his 48 constellations in the 2nd century AD, building on Greek, Egyptian, Babylonian and Assyrian predecessors. Many cultures treat it as an emblem of the north, and it appears on Alaska's flag. From mid-northern latitudes it never sets, while from southern temperate latitudes the famous dipper stays hidden, although the constellation's southern reaches can be seen. In 1930 Eugène Delporte fixed its official boundary as a 28-sided polygon, bordering eight neighbours, among them Draco, Leo and Boötes.
Six of the dipper's seven stars are second magnitude or brighter. Extend the line from Merak through Dubhe about five times that gap and you reach Polaris, marking true north. Five of the seven, all except Dubhe and Alkaid, are drifting together toward a point in Sagittarius, part of a family known as the Ursa Major Moving Group. Along the constellation's southern edge, Arab astronomers saw three pairs of stars as a leaping gazelle's footprints; Chinese and Indian names for the same pattern both mean three steps.
Planet hunters know 47 Ursae Majoris, a Sun-like star about 46 light-years away with three giant planets found in 1996, 2001 and 2010; the first is 2.53 times Jupiter's mass. A faint star near the dipper has even been adopted as Delaware's state star, nicknamed the Delaware Diamond.
Deeper views reveal remarkable galaxies. M81, a nearly face-on spiral 11.8 million light-years away, is locked in a gravitational dance with M82, the brightest infrared galaxy in the sky, where a supernova appeared on 21 January 2014. The Pinwheel Galaxy, M101, lies 25 million light-years off and glows at magnitude 7.5, visible in binoculars but not to the naked eye. Pierre Méchain found both it and the Owl Nebula, a planetary nebula beneath the dipper's bowl, in 1781.
Source: Ursa Major
