Ursa Minor (‘Little Bear’) is situated opposite to Ursa Major (‘Big Bear’), and like The Big Dipper also contains an asterism of seven main stars, in this case known as The Little Dipper. Unlike The Big Dipper, however, just three of The Little Dipper’s stars are bright, namely Polaris, Kochab and Pherkad, and you will need a dark night sky to see all seven stars clearly.
Ursa Minor is the 56th largest constellation, occupies a surface area of 256 sq/degrees, and throughout history has been invaluable for navigation as it contains Polaris, also known as the North Star. The constellation is circumpolar, and can be seen by observers located between +90° and -10° of latitude.
Ursa Minor belongs to the Ursa Major family of constellations, along with Coma Berenices, Boötes, Camelopardalis, Canes Venatici, Corona Borealis, Draco, Leo Minor, Ursa Major and Lynx.
[http://www.astronomytrek.com/star-constellation-facts-ursa-minor/]
Ursa Minor from the Atlas Coelestis of John Flamsteed (1729). Polaris, the north pole star, lies at the tip of its unnaturally long tail.
The Little Bear was said by the Greeks to have been first named by the astronomer Thales of Miletus, who lived from about 625 to 545 BC. The earliest reference to it seems to have been made by the poet Callimachus of the third century BC, who reported that Thales ‘measured out the little stars of the Wain [wagon] by which the Phoenicians sail’. We have the word of Eratosthenes that the Greeks also knew Ursa Minor as Φοινίκη (Phoenike), i.e. the Phoenician. Aratus points out that although the Little Bear is smaller and fainter than the Great Bear, it lies closer to the pole and hence provides a better guide to true north. Aratus called the constellation Κυνόσουρα (i.e. Kynosoura, or Cynosura in Latin transliteration), Greek for ‘dog’s tail’.
According to Aratus the Little Bear represents one of the two nymphs who nursed the infant Zeus in the cave of Dicte on Crete. Apollodorus tells us that the nurses’ names were Adrasteia and Ida. Ursa Minor commemorates Ida while Adrasteia, the senior of the two, is Ursa Major.
In Ptolemy’s Almagest it appears under the Greek name Ἄρκτος Μικρά (Arktos Mikra). Ptolemy catalogued seven stars as part of the Little Bear, four in the body and three in the tail. These seven stars have a similar ladle shape to the Big Dipper of Ursa Major, and so are popularly termed the Little Dipper. At the end of the Little Bear’s tail (or the dipper’s handle) is the star Alpha Ursae Minoris, commonly known by the Latin name Polaris because it is currently the nearest bright star to the north celestial pole, although that has not always been the case. In Ptolemy’s day (2nd century AD) there was no bright star near the north celestial pole. The star we know as Polaris was then some 11° away.
According to the German star-name expert Paul Kunitzsch, the first known usage of the name ‘stella polaris’ applied to this star in print was in an edition of the Alfonsine Tables published in Venice in 1492. The name Stella Polaris also occurs on a globe of 1493 by Johannes Stöffler (1452–1531) and in books by Stöffler and Peter Apian (1495–1552) in the early 16th century. At that time, Polaris was still around three and a half degrees from the celestial pole, five times farther than it is at present, but was clearly becoming accepted as the pole star. Prior to that, the closest star of any note to the pole was 5th-magnitude Struve 1694 (aka HR 4893) in Camelopardalis.
Contrary to common belief, Polaris is not particularly bright. It is in fact of magnitude 2.0, just on the fringe of the top 50 brightest stars as seen by the naked eye. Currently it lies within a degree of the exact north celestial pole, close enough to make it an excellent guide star for navigators. Polaris will reach its closest to the north celestial pole around AD 2100, when the separation will be less than half a degree. After that, precession will just as inexorably move the celestial pole away from it again.
In ancient China, the polar region of the sky took on immense symbolism because of its literally pivotal position, mirroring the central authority of the Emperor on Earth. The area including the modern Ursa Minor plus parts of Camelopardalis, Draco, Cepheus, and Cassiopeia was known to the Chinese as Ziwei, the Purple Palace Enclosure or Central Palace. Surrounding it was a wall delineated by 15 stars, divided into western and eastern sections. The eastern wall started in present-day Draco and went through Cepheus into Cassiopeia, while the western section was in Draco, Ursa Major, and Camelopardalis.
[http://www.ianridpath.com/startales/ursaminor.htm]
Ursa Minor
[https://www.iau.org/public/images/detail/umi/]
Ursa Minor Constellation
[http://www.go-astronomy.com/constellations.php?Name=Ursa%20Minor]
The North Star, Polaris, is depicted as a triple star system in this artist’s conception, based on images taken with the Hubble Space Telescope.
[https://www.space.com/15567-north-star-polaris.html]
Alpha Ursae Minoris (Polaris), the closest bright star to the north celestial pole since the High Middle Ages, is the brightest star in Ursa Minor. It has an apparent magnitude of 1.985 and belongs to the spectral class F7:Ib-II. The star is approximately 434 light years distant from Earth.
Alpha Ursae Minoris is actually a multiple star, consisting of the bright giant Alpha Ursae Minoris A, two smaller and dimmer companion stars Alpha UMi B and Alpha UMi Ab, and two more distant stars, Alpha UMi C and Alpha UMi D. The brightest component in the Alpha UMi system is a bright giant (II) or supergiant (Ib) star belonging to the spectral class F8. It has a mass six times that of the Sun. Alpha UMi B, or Polaris B, discovered by William Herschel in 1780, is a main sequence star of the spectral type F3, and Alpha UMi Ab is a dwarf star in a very close orbit.
Because of its brightness and proximity to the pole, Polaris is an important star in celestial navigation and has been known by many different names. The easiest way to find Polaris in the night sky is to follow Dubhe and Merak, the two bright stars at the end of the Big Dipper asterism in Ursa Major, upwards and then look for the nearest bright star.
Beta Ursae Minoris (Kochab) is a giant star belonging to the spectral type K4 III. It has a visual magnitude of 2.08 and is 130.9 light years distant from the solar system. It is the brightest star in the bowl of the Little Dipper. Kochab is 130 times more luminous than the Sun and has about 2.2 solar masses. The star’s traditional name comes from the Arabic ‘al-kawkab,’ which means ‘the star.’
Kochab and Pherkad (Gamma Ursae Minoris) are sometimes called the Guardians of the Pole because they appear to be rotating around Polaris. From 1500 BC to 500 AD, the two stars served as twin pole stars, being the closest bright stars to the north celestial pole. Neither of the two, however, was as close to the pole as Polaris currently is.
Gamma Ursae Minoris (Pherkad) is an A-type star with an apparent magnitude of 3.05, approximately 487 light years distant. It has the stellar classification A3 lab, which means that it is an intermediate luminosity supergiant. It is a very fast rotating star, with a rotational velocity estimated at 180 kilometres per second. Its radius is 15 times solar and it is 1,100 times more luminous than the Sun. It is classified as a shell star, one that has a disk of gas surrounding its equator which causes variations in the star’s magnitude. The star’s traditional name is derived from the Arabic ‘farqad,’ meaning ‘calf.’
Delta Ursae Minoris (Yildun) is a white main sequence dwarf of the spectral type A1V, approximately 183 light years from Earth. It has a visual magnitude of 4.35. The star’s traditional name comes from the Turkish word for ‘star,’ ‘yıldız.’
Zeta Ursae Minoris (Akhfa) is a main sequence dwarf belonging to the spectral type A3Vn. It has a visual magnitude of 4.32 and is 380 light years distant. The star is in fact on the verge of becoming a giant star, with 3.4 times the mass of the Sun, 200 times the luminosity, and a surface temperature of 8,700 kelvins. Zeta Ursae Minoris is also classified as a suspected Delta Scuti variable. The star’s traditional name comes from the Arabic ‘aḫfa al-farqadayn,’ which means ‘the dimmer of the two calves.’
Eta Ursae Minoris (Anwar) is a yellow-white main sequence dwarf star belonging to the spectral class F5 V. It is 97.3 light years distant from Earth and has a visual magnitude of 4.95. It is visible to the naked eye. The star’s traditional name, ‘Anwar al Farkadain,’ comes from the Arabic phrase ‘anwar al-farqadayn,’ which means ‘the brighter of the two calves.’
Epsilon Ursae Minoris (Urodelus) is a triple star system consisting of Epsilon Ursae Minoris A, a yellow G-type giant which is also classified as an eclipsing spectroscopic binary star, and another component, the 11th magnitude star Epsilon Ursae Minoris B, which is orbiting the primary binary star system from a distance of 77 arc seconds. It is approximately 347 light years distant from Earth. It is also classified as an RS Canum Venaticorum type variable star. The luminosity of the binary system changes as a result of the components eclipsing each other, and the overall brightness varies from magnitude 4.19 to 4.23 with a period of 39.48 days.
[http://www.constellation-guide.com/constellation-list/ursa-minor-constellation/]
NGC 6217 is a barred spiral galaxy in Ursa Minor:
Barred spiral galaxy NGC 6217
Many spiral galaxies have bars across their centers. Even our own Milky Way Galaxy is thought to have a modest central bar. Prominently barred spiral galaxy NGC 6217, pictured above, was captured in spectacular detail in this recently released image taken by the newly repaired Advanced Camera for Surveys on the orbiting Hubble Space Telescope. Visible are dark filamentary dust lanes, young clusters of bright blue stars, red emission nebulas of glowing hydrogen gas, a long bright bar of stars across the center, and a bright active nucleus that likely houses a supermassive black hole. Light takes about 60 million years to reach us from NGC 6217, which spans about 30,000 light years across and can be found toward the constellation of the Little Bear (Ursa Minor).
[http://apod.nasa.gov/apod/ap091228.html]
The Ursa Minor Dwarf is one of the satellite galaxies of our Milky Way galaxy:
Dwarf spheroidal galaxy Ursa Minor Dwarf (UGC 9749)
The Ursa Minor Dwarf is one of the small satellite galaxies of our Milky Way Galaxy, at a distance of about 240,000 light years. It is a dwarf elliptical or dwarf spheroidal galaxy, consisting of an old stellar population only and no significant amount of interstellar matter. The Ursa Minor Dwarf was discovered by A.G. Wilson of Lowell Observatory in 1954, who had already discovered dwarf galaxy Leo I some years earlier, together with the Draco Dwarf, in 1955.
[http://spider.seds.org/spider/LG/umi_dw.html]
In the spring of 1996, Comet Hyakutake’s tail passed across the stars of Ursa Minor:
Hyakutake: Stars through a comet’s tail
Comets are cosmic icebergs. They follow very elongated orbits which carry them from the frozen, remote outer reaches of the Solar System to close encounters with the Sun. Heated by sunlight, they slough off layers of material as gas and dust, forming their characteristic awe-inspiring comas (heads) and tails. In the spring of 1996, Comet Hyakutake inspired Arizona photographers Rick Scott and Joe Orman to take this picture showing faint stars near the constellation Ursa Minor (the Little Dipper) shining through the comet’s long, graceful tail. Blown by the solar wind, comet tails generally point away from the Sun.
[https://apod.nasa.gov/apod/ap980717.html]
No comments:
Post a Comment