mercredi 3 septembre 2008

Argo Navis 12, Vela

Vela is one of the three constellations that use to form the huge constellation Argo Navis, the ship of the Argonauts. In 1763 Nicolas Louis de Lacaille divided the gigantic Argo Navis into three constellations: Carina (the Keel), Puppis (the Stern, or Poop deck), and Vela (the Sail).

As mentioned in regard to "Carina", Jason and his Argonauts sailed off in the Argo Navis to capture the Golden Fleece. The constellation that commemorated that adventure is now broken up into three smaller constellations: Carina (the Keel), Puppis (the Stern) and Vela (the Sail).
The stars that make up the sail are widely dispersed. Some cartographers of the night sky add a few more stars to make the sail more billowy. Since Vela is part of the original Argo Navis, it has only a few of the Bayer stars of that larger constellation.
Sometimes observers associate the lower two stars, kappa Velorum and delta Velorum, with iota Carinae and epsilon Carinae, and believe they are looking at the Southern Cross. The real Southern Cross is in the nearby constellation of Crux; this cross shared by Vela and Carina goes by the name of the False Cross.
Although Vela does not make much of a sight in the southern skies, it does have a number of notable objects, including the brightest Wolf-Rayet star, an optical pulsar, and a pulsating variable which is the prototype of an entire class of cepheids.

Double stars in Vela:
Gamma Velorum is not only a fixed double (AB), but there are two other wide components. This is also a notable Wolf-Rayet star (see below).
AB: 2.2, 4.4; PA 220º, 41.2";AC: 8.5, 151º, 62.3"; D: 9.4, 141º, 94".
Delta Velorum is a multiple system as well, with component C having its own companion.
AB: 2.1, 5.1; PA 153º, 2.6";AC: 10.5, PA 61º, 69.5"; Cc: 10.5, 13; PA 102º, 6.2".
Psi Velorum is the most interesting binary in Vela. It is a close visual binary with a very rapid orbit. Presently the values are: PA 214º, separation 0.48".

Variable stars in Vela:
Lambda Velorum is a supergiant irregular variable which changes slightly in magnitude from 2.14 to 2.3.
Y Velorum is a Mira-type variable, from 8.0 to 14.2, every 444.61 days. In the year 2000 the maximum should occur around the second of January.
AI Velorum is a notable pulsating variable now grouped into a select number called dwarf cepheids. This star is the brightest of the 70 or so known dwarf cepheids, varying from 6.4 to 7.1, bright enough to be classified by the Webb Society as a "binocular variable".
Dwarf cepheids typically have a period of from 2.4 hours to 4.8 hours (i.e. 0.1-0.2 days), although as a group they range from as short as 1h20m to as long as 6h. AI Velorum's period is 2h40m.
Pulsating variables change in visual magnitude due to sporadic movement in their outer layers. The pulsation occurs when there is an imbalance between gravitational pull (inward) and gaseous pressure (outward), causing a continuous cycle of expansion-contraction. [See Valerie Illingworth, Facts on File Dictionary of Astronomy for a detailed explanation.]
While the brightest of the known dwarf cepheids, AI is not that easy to find. First locate gamma Velorum, then move up three degrees where you find a fairly bright but unnamed star (HD 68217). In the same view you should see AI to the southeast one degree. If this method proves difficult, you might try the finder's chart in Burnham, p. 2039.
Gamma2 Velorum (and Wolf-Rayet stars in general). Wolf-Rayets form a very rare type of star having extremely hot surfaces (perhaps up to 90,000 kelvin) and ejecting gas: WC eject predominately helium and carbon, WN nitrogen and helium, and the even more rare WO stars eject oxygen.
Named after C.J.E. Wolf and G. Rayet, who discovered the existence of this type of star in 1867 (at the Paris Observatory), these stars are essentially left-over centres of giant O stars, which have ejected their helium and nitrogen atmospheres. These stars are typically around ten solar masses, and many of them are binaries (such as gamma Velorum).
Most Wolf-Rayets are quite distant from us; gamma Velorum is the closest known W-R star at an estimated 550-800 light years, although some catalogues list a greater distance.

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