Ursa Major, or the Great Bear, is one of the most widely known of all the constellations. One reason is that Ursa Major is the third largest constellation in the sky and very difficult not to notice. A second reason is the seven brightest stars of Ursa Major form an asterism, or pattern, called the Big Dipper. The Big Dipper, in turn, points the way to Polaris, the North Star. Many cultures around the world, from the ancient Greeks to the Native Americans, saw this star pattern as a bear. This article will explore the highlights of this constellation, focusing on its brightest stars and deep sky objects.
For observers who live in the Northern Hemisphere, Ursa Major is visible on all clear nights of the year. From latitude 50 degrees N and northward, the stars of the Big Dipper are all circumpolar, meaning they never set below the horizon. As you can see from the diagram, the Big Dipper moves in a counterclockwise circle around the north pole of the sky. During winter, the Dipper is located in the northeast, standing upright relative to the northern horizon. In springtime, the Dipper is high overhead. Throughout summer, the Dipper is in the northwestern sky with its bowl pointed downward. For most of autumn, the Dipper can be found low in the northern sky just above the horizon.
In most constellations, the brightest star is designated alpha, the second brightest star beta, and so on. In the case of Ursa Major, however, its stars were assigned Greek letters starting from Dubhe (alpha) and Mirak (beta) in the Dipper’s bowl running to Alkaid (eta), at the end of the handle.
A line drawn through Dubhe and Mirak points the way to Polaris, a second magnitude star located about one degree from the north celestial pole. Continuing around the bowl, you come to the gamma and delta stars, named Phecda and Megrez, respectively. The epsilon star at the base of the handle is named Alioth, meaning "the goats" in Arabic.
The zeta star is named Mizar, which means loincloth or wrapping. Close to Mizar is a dimmer star called Alcor (the rider). Mizar and Alcor are a naked-eye double. Observers with sharp vision really can distinguish two stars here. In the medieval Arab world, the ability to see Mizar and Alcor distinctly served as a test of visual acuity. In Native American folklore, Mizar represented one of a trio of hunters pursuing the Bear, and Alcor was his cooking pot.
The eta star at the end of the Dipper’s handle is called Alkaid, or Benetnash. In Arabian mythology, this star represents a chief mourner who leads his followers in a perpetual circle around the star Al Jadi (Polaris), seeking revenge for the murders of the children of a character named Na’ash. The name Benetnash translates to children of Na’ash.
Deep sky objects are often given an M designation in honor of the 18th-century French astronomer Charles Messier. He and his students compiled a list of objects, now known to be galaxies, nebulae and star clusters, frequently mistaken for comets by amateur astronomers. All of the Messier objects in Ursa Major are galaxies, with the exception of M97, the Owl Nebula. M51 is called the Whirlpool galaxy. At magnitude 8, M51 is visible in a large pair of binoculars or any small telescope. M81 and M101 are also 8th magnitude spiral galaxies, while M82, the Cigar Galaxy, is an irregular galaxy located near M81. M108 and M109 are dim 10th-magnitude spiral galaxies, best seen through a larger telescope.