D  e  s  e  r  t     E  x  p  o  s  u  r  e     March 2005

Constellation of the Month: Canis Major, the Big Dog

Last month our constellation was Orion the Hunter. This month, we look to his southeast toward one of his hunting dogs, the big one, Canis Major. Greek mythology has Canis Major about to leap on his neighboring constellation, Lepus, the Hare. This makes a celestial tableau with Orion, Canis Major, Canis Minor and Lepus forming a hunt across the winter sky. In Native American mythology, however, Canis Major represented a deer hunter, with Orion a stag with the three stars of Orion's belt an arrow lodged in its side.

The winter constellations light up our southern sky in March. The Constellation of the Month, Canis Major, with its brilliant star, Sirius, is due south around 8 p.m. This chart is centered about a third of the way up from the horizon. Click the image for a larger map.

The gem of this constellation is the brightest star in the sky, Sirius, commonly known as the "Dog Star." In ancient Egypt, people noticed that Sirius would appear in the early morning sky shortly before sunrise in mid-July. Within a week or two, the August floods would come down the Nile and deposit a fresh layer of fertile silt along the Nile so the next year's crop could be planted. Since these were the hottest days of the year, they were referred to as the "Dog Days" of summer. The walls of Egyptian monuments and temples dating back as far as 3000 BC clearly depict Sirius' hieroglyph, the dog. The Egyptians associated Sirius with the goddess Isis, wife and sister of Osiris, the sun god, and linked the appearance of Sirius in the morning sky with Osiris' return from the dead.

Sirius is a white giant star (spectral class A0) that is only 50 trillion miles (8.6 light years) away. This allows it to shine at magnitude -1.46, making it the brightest object in the night sky after the Moon, Venus, Jupiter and Mars.
In the 1830s and 1840s, astronomer Frederick Bessel analyzed the motion of Sirius against more distant stars and determined that it was not moving in a straight line, but curved back and forth as the gravity of an invisible companion tugged on it. The orbit of the companion was computed, but the search for it long proved fruitless.

Being such a bright star, Sirius is used by telescope makers for testing new telescopes. In 1862, famed telescope maker Alvan Clark was testing a new lens, an 18 1/2-inch that was the largest in the world at that time, and spotted the elusive companion star—10,000 times fainter than Sirius. Measurements eventually showed that the companion, dubbed Sirius B (sometimes called "The Pup"), is the same temperature as our Sun and about the same mass, but the entire star is contained in only 90 percent of the diameter of the Earth. Things on the surface of Sirius B weigh 400,000 times what they do on the surface of the Earth.

Astronomers could not understand how such an object could exist. In the 1920s, R.H. Fowler of Cambridge University, using the newly developed theory of quantum mechanics, suggested that "white dwarf" stars like Sirius B were made of normal matter that was so tightly packed that the electrons were pushed as close to the nucleus as they could go. After using up its nuclear fuel, the remnants of a star like our Sun contract into a white dwarf.

This gave a general answer to the riddle of the white dwarf. The details were worked out in the 1930s by astronomer Subrahmanyan Chandrasekhar. By combining quantum mechanics and the theory of relativity he showed that the electrons could keep a white dwarf from collapsing further, if the mass of the white dwarf was less than 1.4 times that of our Sun, called the Chandrasekhar Limit.

The Planets for March 2005

Making an appearance this month in the west just after sunset is the planet Mercury. Our speedy little Messenger of the Gods will be visible only for the first three weeks of the month, being farthest from the Sun on Feb. 13, when it will be eight degrees above the western horizon. At magnitude 0.4, Mercury will be visible 45 minutes after sunset that night.

Higher in the sky, in fact almost overhead, is Saturn. Still in Gemini, Saturn is drifting slowly westward among the stars. Since the planets (and the Moon) normally move eastward, this westward movement is called "retrograde motion." The Ringed Planet continues to shrink and fade slightly at magnitude -0.4. Through the telescope, Saturn's ball is down to 19.2 seconds of arc across. The rings are continuing to open at 23.9 degrees, with the southern face showing, and they are 43.6 seconds of arc across.

Virgo remains home to Jupiter this month. Rising around 8 p.m., Jupiter is about 10 degrees above (west) of the brightest star in Virgo, Spica. Jupiter brightens to magnitude -2.4. The King of the Planets has an equatorial diameter at 43.3 seconds of arc.

The morning sky sees Mars rise about 3:30 a.m. Right at the beginning of the month, Mars is in Sagittarius and moves into Capricornus on the March 20. Still brightening to magnitude +1.0, Mars is only 5.56 seconds of arc across.
The Moon passes in front of the star Antares on the morning of March 3. The bright side of the Moon covers the star around 3:21 a.m., and the star will come out from behind the dark side of the Moon around 4:25 a.m. Binoculars will make this event more easily visible, but you should be able to see the reappearance with the naked eye.

In the northern hemisphere, spring begins with the Vernal Equinox, which occurs on March 20 at 5:33 a.m. when the Sun passes through the celestial equator headed north. With the beginning of spring and warmer weather, it will be more comfortable to go out and look at the sky in the evening, so "keep watching the sky"!

An amateur astronomer for more than 35 years, Bert Stevens is
co-director of Desert Moon Observatory in Las Cruces.

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