jeudi 4 septembre 2008

Centaurus 7

Centaurus is one of the largest constellations with a clearly discernible asterism: the huge form faces east, with a sword waving menacingly toward Lupus the Wolf on the west.
The constellation has an almost complete list of Bayer stars except for omega, which isn't a star, but a well known globular cluster, NGC 5139 (see below).
The front hooves (or feet, if you wish) are formed by two bright stars: alpha and beta Centauri, known also by the Arabian names of Wazn and Hadar.
Alpha Centauri is best known by the name "Rigil Kentaurus", or the Centaur's foot. This is a triple system, three stars which are the closest to our own Sun.
Alpha1 and alpha2 Centauri form a noted binary (see below). They are 4.393 light years away, and each is approximately the size of the Sun.
The closest star is actually alphaC, known as Proxima Centauri. This is a red dwarf of visual magnitude 11.01 and distance 4.221 light years.
The star has a diameter of about 65,000 km (40,000 miles), or about five times that of the Earth. It is a great distance from the other two (perhaps as far as a sixth of a light year away) and the orbit's period is estimated to be hundreds of thousands of years; Burnham suggests "perhaps ... half a million years".
Proxima Centauri is a flare star and is therefore also known by its variable designation of V645 Centauri. See below for details.
Beta Centauri (Hadar) is the tenth brightest star in the heavens, at 0.61 visual magnitude (which is actually the combined values of its two components). It's 525 light years distant and is a rather difficult visual binary (see below).
Double stars:
Alpha1 and alpha2 Centauri form a wide double with an orbit of 79.92 years: -0.04, 1.2. The 2000.0 values are PA 222º and separation 14.1".
Beta Centauri is a difficult double because of the primary's brightness compared to the companion: 0.58, 3.95; PA 251º, separation 1.3". The orbit has not been calculated, but is thought to be at least several hundred years.
Gamma Centauri is a visual double of two nearly identical stars, with orbit of 84.5 years: 2.9, 2.9. The 2000.0 values are a PA of 347º and separation 1.0".
Eta Centauri is a binary with very faint companion: 2.3, 13; PA 270º, separation 5.6".
Kappa Centauri also has a faint companion: 3.1, 11; PA 82º, separation 3.9"
Variable stars:
AlphaC Centauri (V645 Centauri) is a flare star. That is, its visual magnitude may change rapidly, perhaps taking only several seconds to change its magnitude.
The prototype of this kind of variable is UV Ceti, which has been known to change 3.5 magnitudes within seven seconds!
Since these stars are extremely dim, only the closest ones have been investigated. There are twenty or so such stars within twenty light years of our Sun; they all have an M4.5-M6.5 spectra. Only two have visual magnitudes brighter than 10.
R Centauri is a Mira type variable, 5.3 to 11.8, with 546.2 year period. In 1999 the maximum should arrive in mid-February; the same in 2002.
As the chances of seeing this star at its maximum are therefore not very likely, you might find Burnham's finder's chart (p. 559) of some use.

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