Ursa Minor

Ursa Minor (Latin for 'Lesser Bear', 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" (MULMAR.GÍD.DA.AN.NA, 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. 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). 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. 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. 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"). 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

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