Ursa Major (also known as the Great Bear) is a constellation in the northern sky, whose associated mythology likely dates back into prehistory. Its Latin name means “greater (or larger) she-bear,” referring to and contrasting it with nearby Ursa Minor, the lesser bear. In antiquity, it was one of the original 48 constellations listed by Ptolemy in the 2nd century AD. Today it is the third largest of the 88 modern constellations. Ursa Major is primarily known from the asterism of its main seven stars, which has been called the “Big Dipper,” “the Wagon,” “Charles’s Wain,” and “the Plough,” among other names. It is visible throughout the year from most of the northern hemisphere and appears circumpolar above the mid-northern latitudes. From southern temperate latitudes, the main asterism is invisible, but the southern parts of the constellation can still be viewed. Ursa Major covers 1279.66 square degrees or 3.10% of the total sky. In 1930, Eugène Delporte set its official International Astronomical Union (IAU) constellation boundaries, defining it as a 28-sided irregular polygon. In the equatorial coordinate system, the constellation stretches between the right ascension coordinates of 08h 08.3m and 14h 29.0m and the declination coordinates of +28.30° and +73.14°. Ursa Major borders eight other constellations: Draco to the north and northeast, Boötes to the east, Canes Venatici to the east and southeast, Coma Berenices to the southeast, Leo and Leo Minor to the south, Lynx to the southwest and Camelopardalis to the northwest.
The “Big Dipper” (a term mainly used in the United States and Canada; Plough and (historically) Charles’ Wain are often used in the United Kingdom) is an asterism within Ursa Major composed of seven bright stars (six of them of second magnitude or higher) that together comprise one of the best-known patterns in the sky, resembling either a ladle, an agricultural plough or wagon; in the context of Ursa Major, they are commonly drawn to represent the hindquarters and tail of the Great Bear. Starting with the “ladle” portion of the dipper and extending clockwise (eastward in the sky) through the handle, these stars are the following:
- α Ursae Majoris, known by the Arabic name Dubhe (“the bear”), which at a magnitude of 1.79 is the 35th-brightest star in the sky and the second-brightest of Ursa Major.
- β Ursae Majoris, called Merak (“the loins of the bear”), with a magnitude of 2.37.
- γ Ursae Majoris, known as either Phecda or Phad (“thigh”), with a magnitude of 2.44.
- δ Ursae Majoris, or Megrez, meaning “root of the tail,” referring to its location as the intersection of the body and tail of the bear (or the ladle and handle of the dipper).
- ε Ursae Majoris, known as Alioth, a name which refers not to a bear but to a “black horse,” the name corrupted from the original and mis-assigned to the similarly named Alcor, the naked-eye binary companion of Mizar. Alioth is the brightest star of Ursa Major and the 33rd-brightest in the sky, with a magnitude of 1.76. It is also the brightest of the “peculiar A (Ap) stars,” magnetic stars whose chemical elements are either depleted or enhanced and appear to change as the star rotates.
- ζ Ursae Majoris, Mizar, the second star in from the end of the handle of the Big Dipper, and the constellation’s fourth-brightest star. Mizar, which means “girdle,” forms a famous double star, with its optical companion Alcor (80 Ursae Majoris), the two of which were termed the “horse and rider” by the Arabs. The ability to resolve the two stars with the naked eye is often quoted as a test of eyesight, although even people with quite poor eyesight can see the two stars.
- η Ursae Majoris, known as either Alkaid or Benetnash, both meaning the “end of the tail”. With a magnitude of 1.85, Alkaid is the third-brightest star of Ursa Major.
The stars Merak (β Ursae Majoris) and Dubhe (α Ursae Majoris) are known as the “pointer stars” because they are helpful for finding Polaris, also known as the North Star or Pole Star. By visually tracing a line from Merak through Dubhe (1 unit) and continuing for 5 units, one’s eye will land on Polaris, accurately indicating true north.
Several bright galaxies are found in Ursa Major, including the pair Messier 81 (one of the brightest galaxies in the sky) and Messier 82 above the bear’s head, the Pinwheel Galaxy (M101), a spiral northeast of η Ursae Majoris. The spiral galaxies Messier 108 and Messier 109 are also found in this constellation. The bright planetary nebula Owl Nebula (M97) can be found along the bottom of the bowl of the Big Dipper. M81 is a nearly face-on spiral galaxy 11.8 million light-years from Earth. Like most spiral galaxies, it has a core made up of old stars, with arms filled with young stars and nebulae. Along with M82, it is a part of the galaxy cluster closest to the Local Group. M82 is a galaxy that is interacting gravitationally with M81. It is the brightest infrared galaxy in the sky. SN 2014J, an apparent Type Ia supernova, was observed in M82 on 21 January 2014. M97, also called the Owl Nebula, is a planetary nebula 1,630 light-years from Earth; it has a magnitude of approximately 10. It was discovered in 1781 by Pierre Méchain. M101, also called the Pinwheel Galaxy, is a face-on spiral galaxy located 25 million light-years from Earth. It was discovered by Pierre Méchain in 1781. Its spiral arms have regions with extensive star formation and have strong ultraviolet emissions. It has an integrated magnitude of 7.5, making it visible in both binoculars and telescopes, but not to the naked eye. NGC 2787 is a lenticular galaxy at a distance of 24 million light-years. Unlike most lenticular galaxies, NGC 2787 has a bar at its centre. It also has a halo of globular clusters, indicating its age and relative stability. NGC 3079 is a starburst spiral galaxy located 52 million light-years from Earth. It has a horseshoe-shaped structure at its centre that indicates the presence of a supermassive black hole. The structure itself is formed by superwinds from the black hole. NGC 3310 is another starburst spiral galaxy located 50 million light-years from Earth. Its bright white colour is caused by its higher than usual rate of star formation, which began 100 million years ago after a merger. Studies of this and other starburst galaxies have shown that their starburst phase can last for hundreds of millions of years, far longer than was previously assumed. NGC 4013 is an edge-on spiral galaxy located 55 million light-years from Earth. It has a prominent dust lane and has several visible star-forming regions. NGC 5474 is a peculiar dwarf galaxy in Ursa Major, located near the Pinwheel Galaxy (M101), with which it interacts. The galaxy is often classified as a dwarf spiral galaxy because it shows signs of a spiral structure. As a result of the tidal interactions with M101, the galaxy’s disk is offset from the nucleus, and so is the star formation. NGC 5474 is the closest companion to M101. It has a visual magnitude of 11.3 and is approximately 22 million light years distant from the solar system. I Zwicky 18 is a young dwarf galaxy at a distance of 45 million light-years. The youngest-known galaxy in the visible universe, I Zwicky 18 is about 4 million years old, about one-thousandth the age of the Solar System. It is filled with star forming regions which are creating many hot, young, blue stars at a very high rate. The Hubble Deep Field is located to the northeast of δ Ursae Majoris.
The Kappa Ursae Majorids is a newly discovered meteor shower, peaking between November 1 and November 10. Credits: Constellation Guide, Wikipedia.