Ursa Minor (, contrasting with Ursa Major), also known as the Little Bear, is a constellation located in the far northern sky. As with 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. Ursa Minor 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 pole star.
Polaris, the brightest star in the constellation, is a yellow-white supergiant and the brightest Cepheid variable star in the night sky, ranging in apparent magnitude from 1.97 to 2.00. Beta Ursae Minoris, also known as Kochab, is an aging star that has swollen and cooled to become an orange giant with an apparent magnitude of 2.08, only slightly fainter than Polaris. Kochab and 3rd-magnitude Gamma Ursae Minoris have been called the "guardians of the pole star" or "Guardians of The Pole". Planets have been detected orbiting four of the stars, including Kochab. The constellation also contains an isolated neutron star—Calvera—and H1504+65, the second-hottest white dwarf yet discovered, with a surface temperature of 200,000 K.
History and mythology
In the Babylonian star catalogues, Ursa Minor was known as the "Wagon of Heaven" (, also associated with the goddess Damkina). It is listed in the MUL.APIN catalogue, compiled around 1000 BC, among the "Stars of Enlil"—that is, the northern sky.
According to Diogenes Laërtius, citing Callimachus, Thales of Miletus "measured the stars of the Wagon by which the Phoenicians sail". Diogenes identifies these as the constellation of Ursa Minor, which for its reported use by the Phoenicians for navigation at sea were also named Phoinikē.Hermann Hunger, David Edwin Pingree, Astral Sciences in Mesopotamia (1999), p. 68. The tradition of naming the northern constellations "bears" appears to be genuinely Greek, although Homer refers to just a single "bear". The original "bear" is thus Ursa Major, and Ursa Minor was admitted as the second, or "Phoenician Bear" (Ursa Phoenicia, hence Φοινίκη, Phoenice) only later, according to Strabo (I.1.6, C3) due to a suggestion by Thales, who suggested it as a navigation aid to the Greeks, who had been navigating by Ursa Major. In classical antiquity, the celestial pole was somewhat closer to Beta Ursae Minoris than to Alpha Ursae Minoris, and the entire constellation was taken to indicate the northern direction. Since the medieval period, it has become convenient to use Alpha Ursae Minoris (or "Polaris") as the North Star. (Even though, in the medieval period, Polaris was still several degrees away from the celestial pole. ) Now, Polaris is within 1° of the north celestial pole and remains the current Pole star. Its Neo-Latin name of stella polaris was coined only in the early modern period.
Another ancient name of the constellation is Cynosura (Greek Κυνοσούρα "dog's tail"). The origin of this name is unclear (Ursa Minor being a "dog's tail" would imply that another constellation nearby is "the dog", but no such constellation is known). 447f. "The origin of this word is uncertain, for the star group does not answer to its name unless the dog himself be attached; still some, recalling a variant legend of Kallisto and her Dog instead of Arcas, have thought that here lay the explanation. Others have drawn this title from that of the Attican promontory east of Marathon, because sailors, on their approach to it from the sea, saw these stars shining above it and beyond; but if there be any connection at all here, the reversed derivation is more probable; while Bournouf asserted that it is in no way associated with the Greek word for "dog." Instead, the mythographic tradition of Catasterismi makes Cynosura the name of an Oread nymph described as a nurse of Zeus, honoured by the god with a place in the sky.Condos, T., The Katasterismoi (Part 1), 1967. Also mentioned by Servius On Virgilius' Georgics 1. 246, c. AD 400; a mention of doubtful authenticity is Hyginus, De astronomia 2.2. There are various proposed explanations for the name Cynosura. One suggestion connects it to the myth of Callisto, with her son Arcas replaced by her dog being placed in the sky by Zeus. Others have suggested that an archaic interpretation of Ursa Major was that of a cow, forming a group with Boötes as herdsman, and Ursa Minor as a dog.265f. Robert Brown, Researches into the origin of the primitive constellations of the Greeks, Phoenicians and Babylonians (1899), "M. Syoronos (Types Mon. des anciens p. 116) is of opinion that in the case of some Kretan coin-types, Ursa Maj. is represented as a Cow, hence Boôtês as 'the Herdsman', and Ursa Min. as a Dog ('Chienne' cf. Kynosoura, Kynoupês), a Zeus-suckler." A supposed Latin tradition of naming Ursa Minor Catuli "whelps" or Canes Laconicae "Spartan dogs", recorded in Johann Heinrich Alsted (1649, 408), is probably an early modern innovation. George William Cox explained it as a variant of Λυκόσουρα, understood as "wolf's tail" but by him etymologized as "trail, or train, of light" (i.e. λύκος "wolf" vs. λύκ- "light"). Allen points to the Old Irish name of the constellation, drag-blod "fire trail", for comparison. Brown (1899) suggested a non-Greek origin of the name (a loan from an Assyrian An‑nas-sur‑ra "high-rising")."Very recently, however, Brown [Robert Brown, Researches into the origin of the primitive constellations of the Greeks, Phoenicians and Babylonians] has suggested that the word is not Hellenic in origin, but Euphratean; and, in confirmation of this, mentions a constellation title from that valley, transcribed by Sayce as An‑ta-sur‑ra, the Upper Sphere. Brown reads this An‑nas-sur‑ra, High in Rising, certainly very appropriate to Ursa Minor; and he compares it with Κ‑υν‑όσ‑ου‑ρα, or, the initial consonant being omitted, Unosoura." (Allen, Richard Hinckley. "Star Names: Their Lore and Meaning." New York, Dover Editions, 1963, p. 448.) Brown points out that Aratus fittingly describes "Cynosura" as "high-running" ("at the close of night Cynosura's head runs very high", κεφαλὴ Κυνοσουρίδος ἀκρόθι νυκτὸς ὕψι μάλα τροχάει v. 308f).
An alternative myth tells of two bears that saved Zeus from his murderous father Cronus by hiding him on Mount Ida. Later Zeus set them in the sky, but their tails grew long from their being swung up into the sky by the god.
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 plough, which the seven stars also resemble. This name has also been attached to the main stars of Ursa Major.
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 is too high in the sky at far northern latitudes to be of use in navigation. In Chinese astronomy, the main stars of Ursa Minor are divided between two asterisms: 勾陳 Gòuchén (Curved Array) (including α UMi, δ UMi, ε UMi, ζ UMi, η UMi, θ UMi, λ UMi) and 北極 Běijí (Northern Pole) (including β UMi and γ UMi).
Characteristics
Ursa Minor 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 in the US 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. Polaris can also be found by following a line through the two stars—Alpha and Beta Ursae Majoris, popularly called the Pointers—that form the end of the "bowl" of the Big Dipper, for 30 degrees (three upright fists at arms' length) across the night sky. The four stars constituting the bowl of the Little Dipper are of second, third, fourth, and fifth magnitudes, respectively, 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). In the equatorial coordinate system, the right ascension coordinates of these borders lie between and , while the declination coordinates range from the north celestial pole to 65.40° in the south. Its position in the far northern celestial hemisphere means that the whole constellation is visible only to observers in the northern hemisphere.
Features
Stars
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 through 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.
The traditional names of the main seven in Johann Bayer's ordering are: Polaris Kochab Pherkad Yildun Epsilon Ursae Minoris has no traditional name. Zeta Ursae Minoris has no traditional name. Eta Ursae Minoris has no traditional name.
Marking the Little Bear's tail, Polaris, or Alpha Ursae Minoris, is the brightest star in the constellation, varying between apparent magnitudes 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 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, ±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.