Ursa Minor, also known as the Little Bear, is a constellation in the Northern Sky. Like the Great Bear, the tail of the Little Bear may also be seen as the handle of a ladle, hence the North American name, Little Dipper: seven stars with four in its bowl like its partner the Big Dipper. It was one of the 48 constellations listed by the 2nd-century astronomer Ptolemy and remains one of the 88 modern constellations. Ursa Minor has traditionally been important for navigation, particularly by mariners, because of Polaris being the North Star. Ursa Minor is bordered by Camelopardalis and Draco to the west and Cepheus to the east. Covering 256 square degrees, it ranks 56th of the 88 constellations in size. The official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of 22 segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 08h 41.4m and 22h 54.0m, while the declination coordinates range from the north celestial pole south to 65.40°. Its position in the far northern celestial hemisphere means that the whole constellation is only visible to observers in the northern hemisphere.
The German cartographer Johann Bayer used the Greek letters alpha to theta to label the most prominent stars in the constellation, while his countryman Johann Elert Bode subsequently added iota to phi. Only lambda and pi remain in use, likely because of their proximity to the north celestial pole. Within the constellation’s borders, there are 39 stars brighter than or equal to apparent magnitude 6.5. Marking the Little Bear’s tail, Polaris, or Alpha Ursae Minoris, is the brightest star in the constellation, varying between apparent magnitude 1.97 and 2.00 over a period of 3.97 days. Located around 432 light-years away from Earth, it is a yellow-white supergiant that varies between spectral types F7Ib and F8Ib and has around 6 times the Sun’s mass, 2,500 times its luminosity and 45 times its radius. Polaris is the brightest Cepheid variable star visible from Earth. It is a triple star system, the supergiant primary star having two yellow-white main-sequence star companions that are 17 and 2,400 astronomical units (AU) distant and take 29.6 and 42,000 years respectively to complete one orbit. Traditionally called Kochab, Beta Ursae Minoris at apparent magnitude 2.08 is only slightly less bright than Polaris. Located around 131 light-years away from Earth, it is an orange giant—an evolved star that has used up the hydrogen in its core and moved off the main sequence—of spectral type K4III. Slightly variable over a period of 4.6 days, Kochab has had its mass estimated at 1.3 times that of the Sun via measurement of these oscillations. Kochab is 450 times more luminous than the Sun and has 42 times its diameter, with a surface temperature of approximately 4,130 K. Estimated to be around 2.95 billion years old, give or take 1 billion years, Kochab was announced to have a planetary companion around 6.1 times as massive as Jupiter with an orbit of 522 days.
Ursa Minor is rather devoid of deep-sky objects. The Ursa Minor Dwarf, a dwarf spheroidal galaxy, was discovered by Albert George Wilson of the Lowell Observatory in the Palomar Sky Survey in 1955. Its centre is around 225000 light-years distant from Earth. In 1999, Kenneth Mighell and Christopher Burke used the Hubble Space Telescope to confirm that it had a single burst of star formation that lasted around 2 billion years that took place around 14 billion years ago, and that the galaxy was probably as old as the Milky Way itself. NGC 6217 is a barred spiral galaxy located some 67 million light-years away, an 11th magnitude object about 2.5° east-northeast of Zeta Ursae Minoris. It has been characterized as a starburst galaxy, which means it is undergoing a high rate of star formation compared to a typical galaxy. NGC 6251 is an active supergiant elliptical radio galaxy more than 340 million light-years away from Earth. It has a Seyfert 2 active galactic nucleus and is one of the most extreme examples of a Seyfert galaxy. This galaxy may be associated with gamma-ray source 3EG J1621+8203, which has high-energy gamma-ray emission. It is also noted for its one-sided radio jet—one of the brightest known—discovered in 1977.
The Ursids, a prominent meteor shower that occurs in Ursa Minor, peaks between December 18 and 25. Its parent body is the comet 8P/Tuttle. Credits: Wikipedia.