mardi 23 septembre 2008

Ursa Minor 2

Ursa Minor is a fainter version of the Big Dipper (or Plough, in the UK), and is home to the North Star.
The constellation dates from antiquity, and is said to have been introduced by the Greek philosopher Thales around 600 BC.
For as long as ships have sailed the seas Polaris has been an essential guiding star.
However the Pole Star isn't, as one might think, constant, but rather it changes gradually through several thousands of years.
The earth's axis moves very slowly, like a top, completing a circular path every 25,800 years. During this "precessional cycle" several stars take turns becoming the Pole Star.
The present pole star, alpha Ursae Minoris, will be at the closest to the pole in 2102 AD, at which time it will only be 27' 31" from the north pole. However beta UMi (Kochab), the brightest star in the constellation, is sometimes closer to the pole than is alpha, as it was 3000 years ago.
The brightest Pole Star is Vega (alpha Lyrae), which will resume this title in another 12,000 years. Another star which periodically becomes the Pole Star is Thuban (alpha Draconis), as it was some 4600 years ago, at the time of the pyramid building in Egypt.
Traditionally amateur astronomers have used the constellation as a rough guide on the clarity of the evening's sky. The stars range from second magnitude down to fifth (and even sixth); if these latter stars are clearly seen, it's a good night for viewing.
Ursa Minor has one notable binary and a few variables.

Double stars:
Alpha UMi is a well-known double star with a wide ninth magnitude companion: 2.1, 9.1; PA 218º, separation 18.4".

Variable stars:
Alpha UMi (Pole Star) is a cepheid varying from 1.92 to 2.07 every 3d 23h 16m 28.8s.
Gamma UMi is a delta Scuti type variable with extremely small range (3.04 - 3.09) every 3h 26m.
Epsilon UMi is an EA type variable: 4.19 - 4.23, period 39.48d

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