vendredi 19 septembre 2008

Canis Major 5

The largest of Orion's two hunting dogs, might be chasing Lepus, the Rabbit, who is just in front of him. Or perhaps he is ready to help Orion battle the great bull.
The stories concerning Orion's dogs are not of mythic proportion, but the Greeks did have several interesting beliefs concerning Sirius, alpha Canis Majoris.
The Athenian New Year began with the appearance of Sirius. He was seen as two-headed, like the Roman God Janus: looking back at the past year and forward to the new one.
Sirius was sometimes confused with another two-headed beast called Orthrus. This was Geryon's watchdog; his job was to guard this tyrant's cattle. Heracles captured the cattle (as his Tenth Labour), killing Orthrus in the process.
In antiquity, as Homer and Hesiod were penning their stories, the Dog Star was already associated with the Sun, since the Sun enters that part of the sky in the hot summer months. While the brightest of stars, it hadn't the best of reputations in antiquity as it was said to bring sickness and death. Perhaps this was due to the fact that July and August were habitually the times of drought and disease.
The name Sirius may come from the Greek meaning "scorching", or it may not. Burnham's Celestial Handbook (as always) offers a wide background into the matter of etymology. The star is mostly thought of now as a winter star, accompanying Orion, rather than as the summer home of the sun.
Some facts about Sirius:
Although the brightest star, Sirius is rather Sun-like in size and luminescence; certainly it is no giant at an estimated 1.5 Sun diameters.
Its brightness comes from the fact that it is very close to us: at 8.56 light years away it ranks as the sixth closest star.
The star is a notable binary, but with a companion which is very dim and very close. The companion is a white dwarf, and its presence wasn't really discovered at first; it was just a hypothesis.
In 1834 Friedrich Bessel noticed a slight oscillation in Sirius's orbit. He made the calculations and predicted the existence of an unseen companion. But by his death, in 1846, the companion hadn't yet been discovered. It was only in 1862 that verification came.
This white dwarf has since been the subject of much study. Named Sirius B or The Pup, it is an eighth-magnitude star with an estimated radius of only 10,000 km (about twice the size of the earth). Yet its mass is nearly equal to that of our Sun's, which creates a density so high that a tablespoon full of its matter would weight over a ton.
Such a small dense object is the first phase of the collapse of the so-called main-sequence stars. First white dwarfs, as they continue to cool they become yellow dwarfs then red dwarfs. Finally they die completely and are known as black dwarfs.
Beta Canis Majoris is also of some interest. Its name, "Murzim" means "The Announcer", as its appearance on the horizon signifies the approach of Sirius. This is a pulsating giant that has become the prototype of a class of variable stars (see below).
The Bayer stars are quite bright, ranging from -1.5 to fifth magnitude, with a dozen stars of third magnitude or better.

Double stars in Canis Major
Sirius B: The companion describes an orbit of 50.09 years. At 1 January 2000, it will have a PA of 150 degrees and a separation of 4.6".
Mu CMa is a fixed multiple binary, with components B, C, and D at these fixed spots: B: 340º, 3", C: 288º, 88.5", and D: 61º, 101".
h3945 is a gorgeous yet rather unknown binary: gold and blue. It isn't terribly difficult to find nor to resolve, and when you do find it you will keep coming back to enjoy its colours.
The primary is a fairly bright 5.0; the companion has a visual magnitude of 6.1 and is found at PA 55º and separation 26.6".
To locate the primary, first find tau CMa, which is just to the northeast of delta CMa. Now look north of tau CMa, about 1.75 degrees and very very slightly to the west of due north. You should find the fairly bright primary with no problem. Focus carefully and study this star. Its companion should be quite visible, particularly if you enjoy clear dark skies. You will know when you find it; the colours are unmistakable.

Variable stars in Canis Major
Beta CMa is a pulsating giant star, and the prototype of a small class of variables. Its variations are too slight to be noticed by the naked eye, as it changes from only 1.93 to 2.00 every 6h, 2.6s.
This class of variable is also called the "beta Cepheid type", as this particular star was the first in this class to be discovered (in 1901).
These are all young stars, with a spectra of O or B. Characteristically, they have extremely small changes in magnitude over very short periods (the longest period is ES Vul, with a period of 14h 38.4m). Interestingly, the radial velocity appears to fluctuate with the same period, often quite dramatically (e.g. more than 100 km/s). Reasons for this phenomenon are still not understood.
Other beta CMa variables are iota Canis Majoris and xi1 CMa. Both of these stars fluctuate only about 0.04 visual magnitude; in iota's case, in every 1h 55m, while xi takes almost 5h to make the cycle.
There are no long-period Mira-type variables of any consequence in Canis Major. Indeed, unless one is studying cepheid variables, this is not a particularly fruitful area of the sky for the student of variable stars.

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