Ursa Minor

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Ursa Minor
Constellation
Ursa Minor
Abbreviation UMi[1]
Genitive Ursae Minoris[1]
Pronunciation /ˌɜrsə ˈmnər/,
genitive /ˌɜrs mɨˈnɒrɨs/
Symbolism the Little Bear[1]
Right ascension 15
Declination +75
Family Ursa Major
Quadrant NQ3
Area 256 sq. deg. (56th)
Main stars 7
Bayer/Flamsteed
stars
23
Stars with planets 4
Stars brighter than 3.00m 3
Stars within 10.00 pc (32.62 ly) 0
Brightest star Polaris[2] (1.97m)
Nearest star UU UMi
(42.60 ly, 13.06 pc)
Messier objects 0
Meteor showers Ursids
Bordering
constellations
Draco
Camelopardalis
Cepheus
Visible at latitudes between +90° and −10°.
Best visible at 21:00 (9 p.m.) during the month of June[2].

Ursa Minor (Latin: "Smaller She-Bear", contrasting with Ursa Major), 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 name Little 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 is notable as the location of the north celestial pole, although this will change after some centuries due to the precession of the equinoxes.

Polaris, the brightest star in the constellation, is a yellow-white supergiant and brightest Cepheid variable star in the night sky, ranging from apparent magnitude 1.97 to 2.00. Beta Ursae Minoris, also known as Kochab, is only slightly fainter, with its apparent magnitude of 2.08. An ageing star that has swollen and cooled to become an orange giant, it has a been found to have a planet orbiting it in 2014. It and magnitude 3 Gamma Ursae Minoris have been called the 'guardians of the pole star'. Three other stellar systems have been discovered to contain planets.

History and mythology[edit]

Ursa Minor, with Draco looping around it, as depicted in Urania's Mirror,[3] a set of constellation cards published in London c.1825

In the Babylonian star catalogues, Ursa Minor was known as MAR.GID.DA.AN.NA, the Wagon of Heaven, Damkianna. It appeared on a pair of tablets containing canonical star lists that were compiled around 1000 BC, the MUL.APIN, and was one of the "Stars of Enlil"—that is, the northern sky.[4] The possible origin of its name was its appearing to circle around the north celestial pole.[5]

The first mention of Ursa Minor in Greek texts was by Thales of Miletus in approximately 600 BCE.

Homer had previously only referred to one "bear",[6] leading to speculation over what he saw the stars of Ursa Minor as, or whether they were recognised at all.

Ursa Minor and Ursa Major were related by the Greeks to the myth of Callisto and her son Arcas, both placed in the sky by Zeus.[5] However, in a variant of the story, in which it is Boötes that represents Arcas, Ursa Minor represents a dog. This is the older tradition, which explains both the length of the tail and the obsolete alternate name of Cynosura (the dog's tail) for Polaris, the North Star.[7] Cynosura is also described as a nurse of Zeus, honoured by the god with a place in the sky.[5] An alternate myth tells of two bears that saved Zeus from his murderous father Cronus by hiding him on Mt Ida. Later Zeus set them in the sky, but their tails grew long from being swung by the god.[8]

In Hungarian mythology the constellation is called 'Little Goncol cart' (Göncöl szekér) after a legendary shaman. (Ursa Major is 'Big Goncol cart.') The shaman's knowledge knew no limit. He invented the cart: His nation was wandering, so the cart was the biggest gift of the Gods to the country. Legends claim he knew everything about the world. Nobody saw his death; his body simply disappeared among the stars.

In Inuit astronomy, the three brightest stars—Polaris, Kochab and Pherkad—were known as Nuutuittut "never moving", though the term is more frequently used in the singular to refer to Polaris alone. The pole star was too high in the sky at far northern latitudes to be of use in navigation.[9]

Because Ursa Minor consists of seven stars, the Latin word for "North" (i.e. where Polaris points) is septentrio, from septem (seven) and triones (oxen), from seven oxen driving a plow, which the seven stars also resemble. This name has also been attached to the main stars of Ursa Major.[5]

Characteristics[edit]

Ursa Minor is notable as the location of the north celestial pole, although this will change after some centuries due to the precession of the equinoxes.[10] It is bordered by Camelopardalis to the west, Draco to the west, and Cepheus to the east. Covering 256 square degrees, it ranks 56th of the 88 constellations in size. Ursa Minor is colloquially known as the Little Dipper because its seven brightest stars seem to form the shape of a dipper (ladle or scoop). The star at the end of the dipper handle is Polaris, the North Star. Polaris can also be found by following a line through the two stars that form the end of the "bowl" of the Big Dipper, a nearby asterism found in the constellation Ursa Major. The four stars constituting the "bowl" of the little dipper are unusual in that they are of second, third, fourth, and fifth magnitudes. Hence, they 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 International Astronomical Union in 1922, is 'UMi'.[11] The official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of 22 segments (illustrated in infobox). 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°.[12] Its position in the far northern celestial hemisphere means that the whole constellation is only visible to observers in the northern hemisphere.[13][a]

Notable features[edit]

The constellation Ursa Minor as it can be seen by the naked eye.

Stars[edit]

The German cartographer Johann Bayer used the Greek letters Alpha through Theta to label the most prominent stars in the constellation, while his countryman Johann Elert Bode subsequently added Iota to Phi. However, only Lambda and Pi remain in use, likely because of their closeness to the North Celestial Pole.[5] Within the constellation's borders, there are 39 stars brighter than or equal to apparent magnitude 6.5.[b][13]

Marking the Little Bear's tail,[15] 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.[16] Located around 432 light-years away from Earth,[17] it is a 'yellow-white' supergiant that varies between spectral types F7Ib and F8Ib,[16] and has around 6 times the Sun's mass, 45,000 times its luminosity and 45 times its radius. Polaris is the brightest Cepheid variable star visible to Earth. It is a triple star system, the supergiant primary star having two yellow-white main sequence star companions that are 17 and 2400 astronomical units (AU) distant and take 29.6 and 42,000 years respectively to complete one orbit.[18]

Beta Ursae Minoris, traditionally called Kochab, is only slightly less bright than Polaris with its apparent magnitude of 2.08.[19] Located around 131 light years away from Earth,[c][20] 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.[19] 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.[21] Kochab is 450 times more luminous than the Sun and has 42 times its diameter, with a surface temperature of approximately 4130 K.[22] 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.[23]

Gamma Ursae Minoris, traditionally known as Pherkad, has an apparent magnitude that varies between 3.04 and 3.09 roughly every 3.4 hours.[24] A white bright giant of spectral type A3II-III,[24] with around 4.8 times the Sun's mass, 1050 times its luminosity and 15 times its radius,[25] it is 487±8 light-years distant from Earth.[20] Pherkad belongs to a class of stars known as Delta Scuti variable[24]—short period (six hours at most) pulsating stars that have been used as standard candles and as subjects to study astroseismology.[26] Also possibly a member of this class is Zeta Ursae Minoris,[27] a white star of spectral type A3V,[28] which has begun cooling, expanding and brightening. It is likely to have been a B3 main sequence star and is now slightly variable.[27] At magnitude 4.95. the dimmest of the seven stars of the Little Dipper is Eta Ursae Minoris.[29] An F-type main sequence star of spectral type F5V, it is 97 light-years distant.[30] It is double the Sun's diameter and is 1.4 times as massive and shines with 7.4 times its luminosity.[29] Nearby Zeta lies 5.00-magnitude Theta Ursae Minoris. Located around 855 light-years distant,[31] it is an orange giant of spectral type K5III that has expanded and cooled off the main sequence, and has an estimated diameter around 4.8 times that of the Sun.[32]

Making up the handle of the Little Dipper are Delta and Epsilon Ursae Minoris. Just over 3.5 degrees from the North Celestial Pole, Delta is a white main sequence star of spectral type A1V with an apparent magnitude of 4.35,[33] located 172±1 light-years from Earth.[20] Bearing the proper name of Yildun, it has around 2.8 times the diameter and 47 times the luminosity of the Sun.[34] A triple star system,[35] Epsilon Ursae Minoris shines with a combined average light of magnitude 4.22.[36] A yellow giant of spectral type G5III,[36] the primary is an RS Canum Venaticorum variable star. It is a spectroscopic binary, with a companion 0.36 AU distant, and a third star—an orange main sequence star of spectral type K0—8100 AU distant.[35]

Located close to Polaris is Lambda Ursae Minoris, a red giant of spectral type M1III. It is a semiregular variable varying from magnitudes 6.35 to 6.45.[37] The northerly nature of the constellation means that the variable stars can be observed all year: the red giant R Ursae Minoris is a semiregular variable varying from magnitude 8.5 to 11.5 over 328 days, while S Ursae Minoris is a long period variable that ranges between magnitudes 8.0 and 11 over 331 days.[38] Located south of Kochab and Pherkad towards Draco is RR Ursae Minoris,[39] a red giant of spectral type M5III that is also a semiregular variable ranging from magnitude 4.44 to 4.85 over a period of 43.3 days.[40] T Ursae Minoris is another red giant variable star that has undergone a dramatic change in status—from being a long period (Mira) variable ranging from magnitude 7.8 to 15 over 310–315 days to a semiregular variable.[41] The star is thought to have undergone a helium flash—a point where the shell of helium around the star's core reaches a critical mass and ignites—in 1979.[42] 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.[43] RW Ursae Minoris is a cataclysmic variable star system that flared up as a nova in 1956, reaching magnitude 6. In 2003, it was still two magnitudes brighter than its baseline, and dimming at a rate of 0.02 magnitude a year. Its distance has been calculated as 5000 ± 800 parsecs (16300 light-years), which puts its location in the galactic halo.[44]

Taken from the villain in The Magnificent Seven, Calvera is the nickname given to an X-ray source known as 1RXS J141256.0+792204 in the ROSAT All-Sky Survey Bright Source Catalog (RASS/BSC).[45] It has been identified as an isolated neutron star, one of the closest of its kind to Earth.[46]

Kochab aside, three more stellar systems have been discovered to contain planets. 11 Ursae Minoris is an orange giant of spectral type K4III around 1.8 times as massive as the Sun. Around 1.5 billion years old, it has cooled and expanded since it was an A-type main sequence star. Around 390 light-years distant, it shines with an apparent magnitude of 5.04. A planet around 11 times the mass of Jupiter was discovered orbiting the star with a period of 516 days in 2009.[47] HD 120084 is another evolved star, this time a yellow giant of spectral type G7III, around 2.4 times the mass of the Sun. 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.[48] HD 150706 is a sunlike star of spectral type G0V some 89 light-years distant from our Solar System that was thought to have a planet as massive as Jupiter at a distance of 0.6 AU that was subsequently discounted in 2007.[49] Further study published in 2012 showed that it did in fact have 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 sun.[50]

WISE 1506+7027 is a brown dwarf of spectral type T6 that is a mere 11.1+2.3
−1.3
light-years away from Earth.[51]

Deep-sky objects[edit]

NGC 6217

Ursa Minor is rather devoid of many 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.[52] Its centre is around 225,000 light years distant from Earth.[53]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,[54] and that the galaxy was probably as old as the Milky Way itself.[55]

NGC 6217 is a barred spiral galaxy located some 67 million light years away,[56] which can be located with a 10 cm (4 in) or larger telescope as an 11th magnitude object about 2.5° east-northeast of Zeta Ursae Minoris.[57] 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,[58] 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.[58] It is also noted for its radio lobe.

Meteor showers[edit]

The Ursids are Ursa Minor's prominent meteor shower and peak between December 18 and 25. Its parent body is the comet 8P/Tuttle.[59]

See also[edit]

Notes[edit]

  1. ^ While parts of the constellation technically rise above the horizon to observers between the equator and 24°S, stars within a few degrees of the horizon are to all intents and purposes unobservable.[13]
  2. ^ Objects of magnitude 6.5 are among the faintest visible to the unaided eye in suburban-rural transition night skies.[14]
  3. ^ or more specifically 130.9±0.6 light years by parallax measurement.[20]

Citations[edit]

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  3. ^ "Urania’s Mirror c.1825 – Ian Ridpath's Old Star Atlases". Ianridpath.com. Retrieved 2012-02-13. 
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Cited texts[edit]

  • Wagman, Morton (2003). Lost Stars: Lost, Missing and Troublesome Stars from the Catalogues of Johannes Bayer, Nicholas Louis de Lacaille, John Flamsteed, and Sundry Others. Blacksburg, Virginia: The McDonald & Woodward Publishing Company. ISBN 978-0-939923-78-6. 

Coordinates: Sky map 15h 00m 00s, +75° 00′ 00″