Canis Major - the Greater Dog
Bright Sirius has been part of the mythscape of worldwide cultures throughout time, long before it was part of a constellation. Although Canis Major has many bright stars, Sirius reigns supreme.
Thousands of years ago in Mesopotamia Sirius was an arrow, and nearby stars formed the bow. Even though the Greeks adopted their astronomy from Mesopotamia, they saw Sirius as a dog. Over time it developed into a constellation that was simply the Dog. Later, in Roman times, a second dog joined it, so the original constellation became Canis Major (Greater Dog) and the new one Canis Minor (Lesser Dog). When Ptolemy published the Almagest in the 2nd century, both were listed.
Many myths are attached to Canis Major. It's commonly represented as one of the hunting dogs of Orion, yet to the earlier Greeks it represented the dog Laelaps which always caught what it pursued. In a later Roman myth it was a dog that failed to stop Zeus, disguised as a bull, from abducting Princess Europa.
My favorite story is that of Laelaps and the Teumessian fox. The giant crimson fox was terrorizing Thebes in Greece. But what could they do about a fox destined never to be caught? Maybe if the owner of Laelaps were to send it in pursuit, the inescapable dog would prevail? An amazing chase ensued, almost too fast even for the eye of gods to follow, but Fate was caught in a paradox. What happens when the ineluctable dog is pitted against the uncatchable fox?
Zeus was usually more interested in attractive females than philosophy, but this paradox was giving him a headache. He resolved it in a straightforward way, ending the chase by turning both animals to stone. Some sources say he put them in the sky as Canis Major and Canis Minor. However it seems unlikely that Canis Minor was ever a fox. Certainly celestial mapmakers haven't reflected this, e.g., in this 19th century representation where Canis Minor looks like a sweet doggie, not a ferocious fox.
Stars and planets
Sirius (Alpha Canis Majoris) is a binary star. Sirius B is a faint white dwarf, the end result of the collapse of a sunlike star that's used up its nuclear fuel. However Sirius A is about 25 times brighter than the Sun, and because it's so close to us, it appears as the brightest star in the night sky. Interestingly, although Sirius seems to us to be the brightest star of all, most of the other stars of Canis Major are much more luminous – but they're also much farther away.
Delta Canis Majoris – also called Wezen – is a yellow-white supergiant 1600 light years away. We see it as a second magnitude star, but in actuality it's 50,000 times brighter than the Sun. If it were where Sirius is, it would be as bright as the half Moon. And if it were to replace the Sun, it would nearly fill Earth's orbit.
One of the biggest known stars is VY Canis Majoris, a red hypergiant. It's located around 4000 light years from Earth, and at that distance it's understandably difficult to work out its size and other characteristics. Estimates have differed wildly, such as size estimates varying from 600 to 3000 times the radius of the Sun. The best figure is probably that from observations with the Very Large Telescope. That gives a radius for VY Canis Majoris of 1420 (give or take 10%) solar radii, and a luminosity some 270,000 times that of the Sun.
As of June 2016, a total of ten planets had been found, distributed among seven Canis Major star systems. They include a super-Earth, a Hot Neptune, and a Saturn-mass planet. The others are Jupiter-sized or bigger. A Hot Neptune is a Neptune-sized planet close to its star – this one takes just four days to orbit.
Deep sky objects
The band of the Milky Way goes through the constellation and there are numerous deep sky objects. Here's a selection.
Messier 41 is an open star cluster about 2300 light years from us, the only Messier object in Canis Major. It's 25 light years across and contains about a hundred stars. Another open cluster, known as Caroline's Cluster, was discovered by Caroline Herschel in 1783. It's 15 light years in diameter and located about 3700 light years away. The cluster's age is estimated to be about 2.2 billion years. That's old for an open cluster, so the young bright blue-white stars are absent, having long since evolved past that stage.
Thor's Helmet NGC 2359 is an emission nebula 10,000 light years from Earth. (Image: Adam Block/Mount Lemmon SkyCenter/University of Arizona) The nickname comes from a resemblance to a cartoon Viking helmet. The nebula is formed around a large unstable star of a type known as a Wolf-Rayet star. It's 280,000 times brighter than the Sun and likely to explode as a supernova one day.
In 1835 English astronomer John Herschel (1792-1871) discovered NGC 2207 and IC 2163, which we now know as a beautiful pair of face-on spiral galaxies. (Image: NASA/ESA Hubble Space Telescope) Eventually they'll merge, but already their gravitational interaction is disturbing the gas and dust and initiating star formation.
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