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Canis Major, 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.
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.
Mu CMa is a fixed multiple binary, with component B the most accessible:
AB: 5.3, 7.1; 344º, 2.8".
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. See the Binocular Section, below, for details of this star and several others.
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.
M41 is a globular cluster easily located four degrees south of Sirius.
Perhaps a hundred or so stars make up this bright group, fifty of them
bright enough to be easily seen in binoculars. At the centre of the
group is a red giant. The group is thought to be about 2500 light years
away.
For a closer appreciation of Canis Major, visit the Binocular Section.
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