Ursa Minor


The four stars constituting the bowl of the Little Dipper are of second, third, fourth, and fifth magnitudes, and provide an easy guide to determining what magnitude stars are visible, useful for city dwellers or testing one's eyesight. The three-letter abbreviation for the constellation, as adopted by the IAU (International Astronomical Union) in 1922, is "UMi".


The official constellation boundaries, as set by Belgian astronomer Eugène Delporte in 1930, are defined by a polygon of 22 segments (illustrated in infobox).


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.


It is a semidetached system, as the secondary star is filling its Roche lobe and transferring matter to the primary. RW Ursae Minoris is a cataclysmic variable star system that flared up as a nova in 1956, reaching magnitude 6.


A slight change in the orbital period in 1973 suggests there is a third component of the multiple star system—most likely a red dwarf—with an orbital period of 62.2±3.9 years.


It is also noted for its one-sided radio jet—one of the brightest known—discovered in 1977. ===Meteor showers=== The Ursids, a prominent meteor shower that occurs in Ursa Minor, peaks between December 18 and 25.


The star is thought to have undergone a shell helium flash—a point where the shell of helium around the star's core reaches a critical mass and ignites—marked by its abrupt change in variability in 1979.


Z Ursae Minoris is a faint variable star that suddenly dropped 6 magnitudes in 1992 and was identified as one of a rare class of stars—R Coronae Borealis variables. Eclipsing variables are star systems that vary in brightness because of one star passing in front of the other rather than from any intrinsic change in luminosity.


In 2003, it was still two magnitudes brighter than its baseline, and dimming at a rate of 0.02 magnitude a year.


It was thought to have a planet as massive as Jupiter at a distance of 0.6 AU, but this was discounted in 2007.


A planet around 11 times the mass of Jupiter was discovered orbiting the star with a period of 516 days in 2009.


A faint object of magnitude 14, it was discovered by the Wide-field Infrared Survey Explorer (WISE) in 2011. Kochab aside, three more stellar systems have been discovered to contain planets.


A further study published in 2012 showed that it has a companion around 2.7 times as massive as Jupiter that takes around 16 years to complete an orbit and is 6.8 AU distant from its star. ===Deep-sky objects=== Ursa Minor is rather devoid of deep-sky objects.


It has a planet 4.5 times the mass of Jupiter with one of the most eccentric planetary orbits (with an eccentricity of 0.66), discovered by precisely measuring the radial velocity of the star in 2013.

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Page generated on 2021-08-05