Canis Major

by Tammy Plotner on October 15, 2008

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Canis Major

Canis Major

The constellation of Canis Major represents one of the dogs following the great hunter Orion, and was including in the original list of 48 constellations of Ptolemy. It endured through time to be included into the list of the 88 modern constellations and Canis Major has the honor of containing Sirius, the brightest star in the night sky. It is bordered by Monoceros, Lepus, Columba and Puppis. Canis Major contains 8 main stars and has 32 stars with Bayer/Flamsteed designations.

In mythology, the constellation of Canis Major has played a role since the beginning of time. Its Alpha star, Sirius, is the brightest object in the sky (besides the Sun, Moon, Jupiter, and Venus) as seen from Earth. It is also one of the nearest. The star’s name means scorching, since the summer heat occurred just after Sirius’ helical rising. The Ancient Greeks referred to such times in the summer as dog days, as only dogs would be mad enough to go out in the heat, leading to the star being known as the Dog Star. Consequently, the constellation was named after it, as a Big Dog.

In early European classical days, this constellation represented Laelaps, gift from Zeus to Europa; or sometimes the hound of Procris, Diana’s nymph; or the one given by Aurora to Cephalus, so famed for its speed that Zeus elevated it to the sky. Most commonly, Canis Major is Orion’s hunting dog, pursuing Lepus the Hare or helping Orion fight Taurus the Bull, and is referred to in this way by Aratos, Homer and Hesiod. The ancient Greeks refer only to one dog, but by Roman times, Canis Minor appears as Orion’s second child. Roman myth also refers to Canis Major as Custos Europae, the dog guarding Europa but failing to prevent her abduction by Jupiter in the form of a bull; and as Janitor Lethaeus, the watchdog of Hell.

Depending on the faintness of stars considered, Canis Major resembles a dog facing either above or below the ecliptic. When facing below, since Sirius was considered a dog in its own right, early Greek mythology sometimes considered it to be two headed. As such, together with the area of the sky that is deserted (now considered as the new and extremely faint constellations Camelopardalis and Lynx), and the other features of the area in the Zodiac sign of Gemini (i.e. the Milky Way, and the constellations Gemini, Orion, Auriga, and Canis Minor), this may be the origin of the myth of the cattle of Geryon, which forms one of The Twelve Labours of Heracles. Sirius was an object of wonder and veneration to all ancient peoples throughout human history. In the ancient Vedas this star was known as the Chieftain’s star; in other Hindu writings, it is referred to as Sukra, the Rain God, or Rain Star. The Dog is also described as “he who awakens the gods of the air, and summons them to their office of bringing the rain.” Sirius was revered as the Nile Star, or Star of Isis, by the ancient Egyptians. Its annual appearance just before dawn at the Summer Solstice, June 21, heralded the coming rise of the Nile, upon which Egyptian agriculture depended. This helical rising is referred to in many temple inscriptions, where the star is known as the Divine Sepat, identified as the soul of Isis. In the temple of Isis-Hathor at Dedendrah, Egypt, appears the inscription, “Her majesty Isis shines into the temple on New Year’s Day, and she mingles her light with that of her father on the horizon.” The Arabic word Al Shi’ra resembles the Greek, Roman, and Egyptian names suggesting a common origin in Sanskrit, in which the name Surya, the Sun God, simply means the “shining one.”

Now, stop and take a look at Sirius. In 1862, Alvan Graham Clark, Jr. made an unusual discovery. While watching Sirius, Clark uncovered the intense star’s faint companion while testing an 18″ refractor for Dearborn Observatory. The scope itself was built by Clark, his father and his brother. Imagine his excitement when it turned up the white dwarf – Sirius B! Based on the strange way Sirius A wobbles in the sky, Friedrich Bessel proposed its B’s existence back in 1844, but this is the first time it was confirmed visually. Sirius B is nicknamed “the Pup,” and tonight we’ll have a serious look at Sirius, and see what it takes to uncover its little companion. Sirius is the brightest star that normally graces the night sky. At magnitude -1.6, it produces so much light that the atmosphere won’t stand still for it – sometimes even flashing in vibrant colors! This means that poor “Pup” hardly stands a chance of being seen. At magnitude 8.5 it could easily be caught in binoculars if it were on its own. So how do you find it? First, you’ll need a mid-to-large telescope with a high power eyepiece. Second, add a stable evening – not night – sky around the time Sirius is as high up as possible. Third, you’ll have to train your eye to perceive something that will cause you to say “I could hardly believe my eyes!” – because it’s that faint. Seeing the Pup is a Sirius matter, but practice will help you walk “the Pup” out of the evening sky.

Keep binoculars handy and lets move on to Beta Canis Majoris and learn more. Its traditional name is Murzim “The Heralder” and it is a variable magnitude blue-white giant star. The brightness of Murzim varies between 1.95 and +2.00 in a six-hour period. It is a Beta Cephei variable star. It is a star which is currently in the final stages of using hydrogen gas for fuel. It will eventually exhaust its supply and begin using helium for fuel instead. Beta Canis Majoris is located near the far end of the Local Bubble – a cavity in the local Interstellar medium though which the Sun is traveling. Now move on down to the southern border for Eta Canis Majoris. Aludra shines brightly in the skies in spite of a large distance from Earth due to being intrinsically many times brighter (absolute magnitude) than the Sun. A blue supergiant, Aludra has only been around a fraction of the time our Sun has, yet is already in the last stages of its life. It is still expanding and may be becoming a red super giant, or perhaps has already passed that phase, but in either case it will become a supernova within the next few million years!

Return to Sirius and shift four degrees south (about one binocular field or half a fist width) south to pick out one fine open cluster – Messier 41 (NGC 2287). This stellar concentration was discovered by Giovanni Batista Hodierna before 1654 and was perhaps known to Aristotle about 325 BC. Containing about 100 stars, telescopes will find there are several red giant stars in this impressive star cluster! The brightest of the red super giant stars is spectral type K3, and located near M41′s center. The cluster is estimated to be moving away from us at 23.3 km/s and its diameter is estimated to be between 25 and 26 light years. Its age? The stellar age is approximately 190 and 240 million years old.

Also for binoculars and small telescopes, head off to NGC 2362. First seen by Giovanni Hodierna in 1654 and rediscovered William Herschel in 1783, this magnificent galactic star cluster may be less than 5 million years old and is still associated with nebulosity – the remains of the gas cloud from which it formed. What makes it even more special is the presence of Tau – Tau Canis Major that is! Easily distinguished as the brightest star in the cluster, Tau is a luminous supergiant of spectral type O8, visual magnitude 4.39, and absolute magnitude -7, 50,000 times more luminous than Sol. Tau CMa is also brighter component of a spectroscopic binary and studies of NGC 2362 suggest that it will survive longer before breaking up than will the Pleiades but not as long as the Hyades.

Shift southwest for NGC 2354. At magnitude 6.5, it is achievable with binoculars, but needs a telescope to fully resolve its members. Although it is very loose and hard to distinguish from the rich, Milky Way background, it’s still worth studying. Oddly enough, it has an eclipsing blue straggler star in it! While at best this will appear as a small, hazy patch to binoculars, NGC 2354 is actually a rich galactic cluster containing around 60 metal-poor members. As aperture and magnification increase, the cluster shows two delightful circle-like structures of stars, similar to a figure 8. Be sure to make a note… You’ve captured another Herschel 400 object!

For large telescopes and GoTo telescopes, here’s a couple of cool things you won’t want to miss. First – the Canis Major Dwarf Galaxy (RA 7 12 30 Dec -27 40 00). he Canis Major dwarf galaxy is classified as an irregular galaxy and is now thought to be the closest neighbouring galaxy to our location in the Milky Way, being located about 25,000 light-years away from our Solar System and 42,000 light-years from the Galactic Center. It has a roughly elliptical shape and is thought to contain as many stars as the Sagittarius Dwarf Elliptical Galaxy, the previous contender for closest galaxy to our location in the Milky Way. The galaxy was first discovered in November 2003 by an international team of French, Italian, British and Australian astronomers. Although closer to the Earth than the centre of the galaxy itself, the Canis Major Dwarf galaxy was difficult to detect as it is located behind the plane of the Milky Way, where concentrations of stars, gas and dust are densest. This, along with its small size, explains why it was not discovered sooner.

The team of astronomers that discovered it were collaborating on analysis of data from the Two-Micron All Sky Survey (2MASS), a comprehensive survey of the sky in infrared light, which is not blocked by gas and dust as severely as visible light. Because of this technique, scientists were able to detect a very significant over-density of class M giant stars in a part of the sky occupied by the Canis Major constellation, along with several other related structures composed of this type of star, two of which form broad, faint arcs. Astronomers believe that the Canis Major Dwarf Galaxy is in the process of being pulled apart by the gravitational field of the more massive Milky Way galaxy. The main body of the galaxy is extremely degraded. Tidal disruption causes a long filament of stars to trail behind it as it orbits the Milky Way, forming a complex ringlike structure sometimes referred to as the Monoceros Ring which wraps around our galaxy three times. The stream of stars was first discovered in the early 21st century by astronomers conducting the Sloan Digital Sky Survey. It was in the course of investigating this ring of stars, and a closely spaced group of globular clusters similar to those associated with the Sagittarius Dwarf Elliptical Galaxy, that the Canis Major dwarf galaxy was discovered.

Globular clusters thought to be associated with the Canis Major Dwarf galaxy include NGC 1851, NGC 1904, NGC 2298 and NGC 2808, all of which are likely to be a remnant of the galaxy’s globular cluster system before its accretion, or swallowing, into the Milky Way. NGC 1261 is another nearby cluster, but its velocity is different enough from that of the others to make its relation to the system unclear. The Canis Major Dwarf galaxy may also have associated open clusters, including Dol 25 and H18, and possibly AM-2. It is thought that the open clusters may have formed due to the dwarf galaxy’s gravity perturbing material in the galactic disk and stimulating star formation. The discovery of the Canis Major Dwarf Galaxy and subsequent analysis of the stars associated with it has provided some support for the current theory that galaxies may grow in size by swallowing their smaller neighbors. Martin et al. believe that the preponderance of evidence points to the accretion of a small satellite galaxy of the Milky Way which was orbiting roughly in the plane of the galactic disk.

Now set your mark on VY Canis Majoris. (This would make an excellent astronomy outreach presentation when combined with the “Size of Our Universe Video” located in our Outreach Resources category!) It’s located at RA 7 22 58 Dec -25 26 03. VY Canis Majoris (VY CMa) is a red hypergiant star located in the constellation Canis Major. It is the largest known star and also one of the most luminous known. It is located about 1.5 kiloparsecs (or 5,000 light-years) away from Earth. University of Minnesota professor Roberta M. Humphreys estimates the radius of VY CMa at 1800 to 2100 solar radii. To illustrate, if our Sun were replaced with VY Canis Majoris, its surface would extend to the orbit of Saturn. Assuming the upper size limit of 2100 solar radii, light would take more than 8 hours to travel around the star’s circumference. Dr. Humphreys recently estimated that the largest possible star is approximately 2,600 times the radius of the Sun.

Source: Chandra
Constellation Chart Provided by Your Sky.


Tammy is a professional astronomy author, President Emeritus of Warren Rupp Observatory and retired Astronomical League Executive Secretary. She’s received a vast number of astronomy achievement and observing awards, including the Great Lakes Astronomy Achievement Award, RG Wright Service Award and the first woman astronomer to achieve Comet Hunter's Gold Status.

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