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Canis Major, the Big Dog

Plus the planets for March.


Our sky during winter evenings has some of the brightest constellations in the heavens. Orion, Taurus, Canis Minor and Gemini all have first-magnitude stars that light up our cold, chilly nights. But the constellation with the brightest star in the sky is Canis Major, the Big Dog, with its brilliant star Sirius. The name Sirius is derived from the Ancient Greek Seirios ("glowing" or "scorcher"), and at magnitude -1.4, it is a scorcher to the eye.

thor's helmet

Click the image for a larger map


About halfway up in the southern sky on these cool March nights is the constellation Canis Major and its brilliant star Sirius. Sirius is the brightest star in our sky, shining at magnitude -1.4. Sirius is often called “The Dog Star,” based on its home constellation. Sirius has an 8.3-magnitude white dwarf companion that takes 50 years to orbit its primary star. Even though Canis Major is in the Milky Way, there are few deep-sky objects to be found here.

Sirius is a slightly variable spectral class A1 star that is twice the mass of our Sun, but puts out 25 times the light and heat. But the thing that really makes this star shine in our sky is that it is only 8.6 light-years away from us. The Egyptians based their calendar on the helical rising of Sirius, the first day that Sirius appears in the morning just before the Sun. This always signaled the annual flooding of the Nile River.

In Greek mythology, Canis Major and Canis Minor are the big and little hunting dogs belonging to the great hunter, Orion. Both dogs are ready to help Orion find and catch his prey, which may be Lepus, the Hare, which crouches at Orion's feet. It is also possible that Canis Major is helping Orion fight Taurus, the Bull, who is also nearby.

One of the few deep-sky objects in Canis Major is NGC 2359 (New General Catalog number 2359), known more commonly as Thor's Helmet. This nebula is an emission nebula that is shaped like a helmet, with vertical sides and a rounded top. It also sports two wings, one on either side, similar to the helmet that the Norse god Thor is often pictured wearing. It is eight minutes-of-arc by eight minutes-of-arc in size, roughly a quarter of the size of the Moon in our sky.

At the center of the Thor's Helmet nebula is a Wolf-Rayet star. This type of star is massive, some 20 times the mass of our Sun or more. It has already passed the stable part of its life on the main sequence (where our Sun is now) and is starting down the road to being a supernova. As it gets older, a star starts blowing off the gas in its outer atmosphere. The gas travels outward at a high speed as a stellar wind until it encounters the gas and dust that exists throughout our galaxy. Thor's Helmet is the bubble formed by the stellar wind, with the surface of the bubble composed of the galactic gas and dust piling up as the bubble's surface expands into space, pushed by the stellar wind.


Wolf-Rayet stars get their name from astronomers Charles Wolf and Georges Rayet. In 1867, they were surveying the spectra of the stars in Cygnus with the 16-inch Foucault telescope at the Paris Observatory. They discovered three stars that had continuous spectra with broad emission lines. The light from a star can be broken up into its different colors. An incandescent light bulb has a continuous spectrum, looking much like a rainbow with no bright or dark lines. This is what Wolf and Rayet saw in the spectra of these three stars, but they also saw bright areas in the spectrum, like broad lines at specific colors.

Bright lines in a spectrum are generated by the atoms of specific elements (like hydrogen, helium, carbon, oxygen and nitrogen) that are excited by ultraviolet light from the star and release light in the particular color associated with that element. The lines are broadened by the rapid motion of the atoms in the gas, indicating that the gas around a Wolf-Rayet star was moving at an amazing 180 to 1,500 miles per second.

Normally, an element would glow in one very specific color (narrow spectrum line). But when you have that element's gas atoms moving at many different high speeds, each atom will have a different Doppler shift depending on its speed along our line of sight. If the atom happens to be moving perpendicular to our line of sight, its line-of-sight speed is zero; it will have no Doppler shift and the color is the element's normal color. An atom coming straight at us has all its speed, causing a large Doppler shift of the element's color toward the blue. An atom traveling directly away from us shifts the color toward the red. Atoms traveling at angles between directly toward us, perpendicular to us and directly away from us all spread out the narrow spectrum line into the broad spectrum lines seen in Wolf-Rayet stars.



The Planets for March 2013


This month we have two planets too near the Sun to be seen. Mars and Venus are both on the other side of the Sun from us and will not be visible for a few months.

Still high in the western sky as it gets dark is the planet Jupiter. Moving slowly eastward just north of the Hyades star cluster in Taurus, Jupiter shines at magnitude -2.3. Jupiter's disc is 37.3 seconds-of-arc across. The King of the Planets sets around 1 a.m. MDT.

Saturn is moving westward in western Libra. Glowing with its characteristic yellowish light, the magnitude +0.3 Ringed Planet rises around 10:45 p.m. MDT and is visible the rest of the night. Saturn's disc is 18.2 seconds-of-arc across and the Rings are 41.3 seconds-of-arc across. They are tilted down 19.1 degrees from our line of sight with the northern face showing.

Mercury is in the morning sky during the later half of the month. Having been in the evening sky last month, Mercury passes between the Sun and the Earth on March 4. The next day it leaves Pisces and enters central Aquarius, where it spends the rest of the month. Mercury will reach its greatest distance from the Sun on March 31, when it can be found eight degrees above the east-southeast horizon as it gets dark. At that time, it will be magnitude +0.3. The Messenger of the Gods' disc will be 7.6 seconds-of-arc across and 50% illuminated (half full). As we go into April it will become more illuminated (fuller).

Comet C/2011 L4 (PANSTARRS) will be in our evening sky this month. This comet. named after the telescope that discovered it in June 2011, has been moving up from the south and should be visible low in the west around March 12. Then it will be four degrees left of the 2% illuminated crescent Moon, just 10 degrees above the western horizon as it gets dark. PANSTARRS is expected to be first magnitude and sport a visible tail. This comet is "new" in that this is its first time in the inner Solar System, making its appearance very unpredictable. It could break apart and fizzle out, or produce a large quantity of dust, creating a spectacular tail. For the rest of the month, it will continues to move northward (to the right) as it moves away from the Sun and rapidly to fades from sight.

The astronomical season of spring begins on March 20, when the Sun passes through the celestial equator going north. In the southern hemisphere, the season of fall begins as the Sun becomes lower in their sky. But for us, denizens of the northern hemisphere, the days are getting longer and the nights shorter, so maker the most of the shortening nights and "keep watching the sky"!



Watch the Skies

(times MST/MDT)


March 4, 6 a.m. — Mercury between Earth and Sun        (inferior conjunction)
            2:53 p.m. — Last Quarter Moon
March 10, 2 a.m. — Daylight Savings Time begins
March 11, 1:51 p.m. — New Moon
March 19, 11:27 a.m. — First Quarter Moon
March 20, 5:02 a.m. — March Equinox, spring begins in        the Northern Hemisphere
March 27, 3:27 a.m. — Full Moon
March 31, 4 p.m. — Mercury greatest distance west of       Sun (28 degrees)





An amateur astronomer for more than 40 years, Bert Stevens is co-director
of Desert Moon Observatory in Las Cruces.


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