Canis Major, the Big Dog

The hunter who always catches her prey


Look for the brightest star in March’s evening sky and you will have found the constellation of Canis Major, the Big Dog. Located just east of Orion, the Hunter, Canis Major’s bright star is Sirius, which is also the brightest star in the sky. While Sirius is intrinsically bright, its distance of only 8.6 light-years enhances its brightness. It is also one of the closest stars to us.

Canis Major is associated with the Greek mythological dog Laelaps, a hunting dog that always caught her prey. Laelaps, which means “hurricane”, was a gift from Zeus, the King of the gods, to Europa. Europa had been abducted by Zeus in the form of a bull and taken to the island of Crete. There, she became the first queen of Crete. Zeus gave her a necklace made by the god of metalworking, Hephaestus. He also gave her a three other gifts: a bronze automaton named Talos that was to protect her from pirates and invaders, a javelin that never missed and Laelaps.

Later, Laelaps was passed down to Europa’s son, King Minos. She later became the property of Procris, a princess of Athens. Her husband, Cephalus, used Laelaps to hunt the Teumessian fox, a fox which could never be caught. With a fox that cannot be caught and a dog that cannot fail, the chase went on for a long time. Perplexed by this paradox, Zeus turned them both into stone and then placed them in the sky, Laelaps as Canis Major and the Teumessian fox as Canis Minor.

Sirius is a white main sequence star of spectral class A0 that is twice the mass of our Sun and radiating twenty-five times its energy. On Jan. 31, 1862, Alvan Clark, the famed telescope maker, was testing an 18.5-inch lens that he was constructing for the great refractor to be installed in the Dearborn Observatory at Northwestern University in Evanston, Illinois. In testing, he pointed it at Sirius and was surprised to find an 8.4 magnitude star just a few seconds-of-arc away.

Dubbed Sirius B, that faint star was determined to be about the size of the Earth, but it has the mass of the Sun. This compact body is a white dwarf star, the burnt-out core of a red giant star. Most of its mass was ejected into space after nuclear fusion ceased, leaving a hot object composed of oxygen and iron. It has the density of around two tons per cubic inch, the result of its tremendous gravity that compresses the atomic nuclei much closer together than normal. Electrons floating between the nuclei keep them from getting any closer together due to the Pauli Exclusion Principle. 

Sirius B is still glowing from the heat it absorbed when it was still actively generating energy from nuclear fusion. It will radiate its remaining heat into space, eventually becoming a black dwarf. This cool-down will take longer that the universe has been in existence, so no white dwarf has had time to become a black dwarf yet.

While Sirius B is one of the smallest stars, another star in Canis Major is one of the largest. VY Canis Majoris (VY CMa) is a variable star in far eastern Canis Major. It is a red hypergiant star, 1,420 times the diameter of our Sun. If this star were in our Solar System, its surface would be between the orbits of Jupiter and Saturn. As a variable star, it changes brightness from magnitude +6.5 to +9.6, so most of the time it is a telescopic object.

VY CMa is less than ten million years old, but it has already entered its old age. It had been a spectral class O9 star when it was on the main sequence, fusing hydrogen into helium in its core. At roughly ten time the mass of our Sun, it quickly ran through its supply of hydrogen and started fusing helium into carbon. This produced an outpouring of energy that pushed the surface of this star far into space. Some of its surface became so extended that it drifted away from the star all together, forming a small nebula around it. The nebula is about ten seconds-of-arc across, a complex of jets and shock waves that probably originated from explosions in VY CMa’s atmosphere.

VY CMa was first reported in a star catalog from French astronomer Jérôme Lalande on March 7, 1801. It was listed as seventh magnitude, but later records indicate that it has been fading since 1850. This star is about 3,830 light-years away, but even so, it would be naked eye if it were not for the intervening nebula. Like all red super/hypergiants, VY CMa will someday end its existence in a supernova explosion or perhaps the even larger superluminous supernova explosion.

The Planets for March 2020.

As the sky darkens on March evenings, Venus is the first “star” to appear out of the twilight. It is the only planet in the evening sky, shining at magnitude -4.3. During the month, Venus travels from eastern Pisces, through all of Aries, and just over the border into far western Taurus. Now a “half-moon”, Venus’s disc is fifty-six percent illuminated and is 21.5 seconds-of-arc across at midmonth. Venus is thirty-eight degrees above the western horizon as it gets dark and it sets at almost 11 p.m. MDT.

Mars rises around 4 a.m. in the east-southeast ending the night twenty-seven degrees above the south-southeastern horizon. Mars’s disc is 5.8 seconds-of-arc across at midmonth, shining at magnitude +1.0. The God of War moves from central Sagittarius to far western Capricornus during the month, passing Jupiter on March 20 and Saturn on March 31.

The King of the Gods is unmistakable in the southeastern morning sky, shining at magnitude -2.1. It rises around the same time as Mars and it is about the same altitude as it gets light. Jupiter is moving slowly eastward in eastern Sagittarius. At midmonth, its disc is 35.4 seconds-of-arc across.

Saturn is also moving eastward in far eastern Sagittarius, reaching the border with Capricornus on March 21 and ending the month at the far western end of that constellation. As it gets light, Saturn is twenty-three degrees above the southeastern horizon, after having risen at 4:30 a.m. Shining at magnitude +0.7, the Ringed Planet’s disc is 15.7 seconds-of-arc across while the Rings are 35.6 seconds-of-arc across, tilted 21.3 degrees downward with the northern face showing.

Mercury pops out of the morning twilight at the end of the first week of the month. It reaches its greatest distance from the Sun of twenty-eight degrees on March 24, shining at magnitude +0.3. On that day, the Messenger of the Gods has a disc that is 7.4 seconds-of-arc across and fifty-three percent illuminated as it goes from crescent to almost full during the month. Mercury rises at 6:00 a.m. and is only eight degrees above the east-southeastern horizon as it gets light. It moves from southwestern Aquarius into Capricornus at the beginning of the month. Mercury immediately turns back east into Capricornus where it ends the month in the southeastern part of that constellation.

Earth’s polar axis is perpendicular to the Earth-Sun line on March 19 at 9:50 p.m., marking the March Equinox. The Sun will cross the celestial equator moving northward, starting the astronomical season of Spring. Day and night will be of equal length (hence an equinox). With nights continuing to get shorter, enjoy the longer March nights and "keep watching the sky"!

An amateur astronomer for more than 45 years, Bert Stevens is co-director of Desert Moon Observatory in Las Cruces.