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Canis Major and Canis Minor, 2 Constellations for February

Contributed by Paul Rodmell, Southland Astronomical Society

CANIS MAJOR and CANIS MINOR, (pronounced KAY-niss MAY-jer and KAY-niss MY-ner)

Chart showing Canis Major and Canis Minor

These are ancient constellations, representing the two dogs following at the heels of Orion. To find them look north late evening, find Orion (Pot) and follow the map.

Canis Major, the large dog, is a conspicuous constellation lying mainly just south of the Milky Way between Orion and the long train of bright stars from the old Argo Navis. The constellation is dominated by the brilliant white Sirius (the Dog Star) (pronounced SIH-rih-uss), the brightest appearing star in the sky. The ancient Egyptians based their calendar on its yearly motion around the sky.

Canis Minor, the small dog, had two stars assigned to it in ancient times. The brightest of these is Procyon.

Chart showing the two constellations as seen high in the sky to the north mid evening in February.

Canis Major and Minor.

Constellation Caelum Constellation Columba Constellation Puppis Constellation Pyxis Constellation Monoceros Constellation Cancer Constellation Orion Constellation Lepus

Details of some of the objects shown in the chart.

α Canis Majoris, Sirius (from the Greek, meaning sparkling or scorching) at magnitude -1.46 is exceed only by some of the planets in apparent brightness. It owes its brilliance to being one of the Sun's closest neighbours at 8.7 light years away, and as well, is about 22 times more luminous than the Sun. Several other stars in Canis Major are in fact far more luminous, but not appear as bright due to their greater distance.

Sirius has a white dwarf companion that orbits it every 50 years. Closest approach was in 1993, while the widest separation will be in 2022. The white dwarf will not be visible to most backyard observers, however, until the early years of the 21st century. Even then it remains difficult to see because of the glare of Sirius. A white dwarf is only about the diameter of the Earth, but contains most (90%) of the star's original mass. White dwarfs are therefore exceptionally dense bodies. A teaspoonful of material would have a mass of thousands of kilograms or over 100,000 times that of water. Over thousands of millions of years they slowly cool off and fade.

β CMa, Mirzam is a magnitude 2.0 star which is 500 light years away. It has a luminosity more than 3000 times that of the Sun.

δ CMa, Wezen is a magnitude 1.8 star some 1800 light years distance. It has a luminosity more than 30000 to 40000 times that of the Sun.

ε CMa, Adara is a magnitude 1.5 star which is 430 light years away. It has a luminosity about 4000 times that of the Sun.

η CMa, Aludra is a magnitude 2.5 star and is 3000 light years away. This large distance means it has a luminosity some 9000 times that of the Sun.

M 41 (NGC 2287) is a fine open cluster recorded by Messier in 1765, but was known to Aristotle around 325 BC, being visible to the unaided eye. It is a beautiful object in binoculars and small telescopes, containing some delicate pairs and triplets, with a fine orange star near the centre. It lies about 2500 light years away.

NGC 2362 is a most beautiful smaller and fainter cluster than M 41, but it is easily found surrounding the 4.4 magnitude blue giant star τ CMa. Small telescopes and binoculars show about 40 stars in the cluster. τ CMa is also a member of the cluster which lies about 4000 light years away.

α Canis Minoris, Procyon (pronounced PROH-see-on), a brilliant yellow star about five times more luminous than the Sun lying 11.3 light years away. It's name comes from the Greek, meaning 'before the dog', referring to its rising before Canis Major. Like Sirius, Procyon has a white dwarf companion. This one orbits every 41 years, but is even more difficult to see than the companion to Sirius, requiring the use of large professional telescopes.

β CMi is a magnitude 2.9 star which is 170 light years away and has a luminosity more than 160 times that of the Sun.

Visibility

Sirius transits, when it is to the north and at its highest, at 10 pm (NZDT) about February 20. It, and Canis Major, remain visible in the evening sky until late June when they are low to the west at the end of evening twilight. By then Sirius will also be visible in the morning twilight low to the east. The constellation is in the late evening sky by late November, again to the east.

Canis Minor is considerably further north in the sky than Canis Major, so is above the horizon in New Zealand for a shorter time. 10 pm transit is also a little later, about 1 week into March. Procyon sets only about 15 minutes before Sirius, so also remains visible in the early evening until late June. On the other hand it rise about 2 hours after Sirius, so is a month later appearing in the morning sky. It is well into December before Canis Minor appears in the late evening sky.


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