mardi 23 septembre 2008

Ursa Minor 5

Polaris (α UMi / α Ursae Minoris / Alpha Ursae Minoris, commonly North(ern) Star or Pole Star, and sometimes Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole (42′ away as of 2006), making it the current northern pole star.
Polaris is about 430 light-years from Earth. Concerning the detailed physics, α UMi A is an F7 bright giant (II) or supergiant (Ib). The two smaller companions are: α UMi B, an F3V main sequence star orbiting at a distance of 2400 AU, and α UMi Ab, a very close dwarf with an 18.5 AU radius orbit. Recent observations show that Polaris may be part of a loose open cluster of type A and F stars.
Polaris B can be seen with even a modest telescope and was first noticed by William Herschel in 1780. In 1929, it was discovered by examining the spectrum of Polaris A that it had another very close dwarf companion (variously α UMi P, α UMi a or α UMi Ab), which had been theorized in earlier observations (Moore, J.H and Kholodovsky, E. A.). In January 2006, NASA released images from the Hubble telescope, directly showing all three members of the Polaris trinary system. The nearer dwarf star is in an orbit of only 18.5 AU (2.8 billion km;[2] about the distance from our Sun to Uranus) from Polaris A, explaining why its light is swamped by its close and much brighter companion.[3]
Polaris is a classic Population I Cepheid variable (although it was once thought to be Population II due to its high galactic latitude). Since Cepheids are an important standard candle for determining distance, Polaris (as the closest such star) is heavily studied. Around 1900, the star luminosity varied ±8% from its average (0.15 magnitudes in total) with a 3.97 day period; however, the amplitude of its variation has been quickly declining since the middle of the 20th century. The variation reached a minimum of 1% in the mid 1990s and has remained at a low level. Over the same period, the star has brightened by 15% (on average), and the period has lengthened by about 8 seconds each year.
Recent research reported in Science suggests that Polaris is 2.5 times brighter today than when Ptolemy observed it (now 2mag, antiquity 3mag). Astronomer Edward Guinan considers this to be a remarkable rate of change and is on record as saying that "If they are real, these changes are 100 times larger than [those] predicted by current theories of stellar evolution."

To the Bedouin people of the Negev and Sinai, Polaris is known as الجديّ al-jadiyy, "the billy goat". It and Suhail (= Canopus, α Car) are the two principal stars used for nomadic wandering at night. Because it was circumpolar and hence always visible, it became associated with a steadfast nature, as opposed to Suhail, which disappears below the horizon and hence 'flees'.
A monkey's head is the emblem of the Mayan god of the pole star.
Christopher Columbus didn't have to use Polaris for navigation because the compass was already invented. But he did check the direction of the compass needle against the glow of this star. After leaving Canary Islands he noticed that the compass needle pointed toward NW, discovering a phenomenon called variance where a secondary magnetic field is superimposed on the primary field of a dipole.

Beta Ursae Minoris (β UMi / β Ursae Minoris) is the second brightest star in the bowl of the "Little Dipper," the constellation Ursa Minor. It also has the traditional name Kochab. Kochab's magnitude is 2.07. It is 16 degrees from Polaris. The star is an orange giant and is 126.4 ± 2.5 light years from Earth. It is 130 times more luminous than the Sun. Kochab has a surface temperature of approximately 4,000 K.
Kochab and its neighbor Pherkad are both naked eye stars. They served as twin pole stars, Earth's North pole stars, from 1500 B.C. until 500 A.D. Neither star was as proximitous to the pole as Polaris is now. Due to precession of the equinoxes, the previous holder of the title was Thuban, and the next was the present-day Polaris. This succession of pole stars is a result of earth's precessional motion.
The origin of the name Kochab is indistinct. It has sometimes been associated with a Hebrew word for star. The name "Kochab" may originate from Arabic الكوكب al-kawkab and means simply "the star".

Aucun commentaire: