Northerners who visit the Southern Hemisphere experience a whole new set of stars, deep-sky objects, and constellations never visible from northerly latitudes. Among these is a pair of companion galaxies to the Milky Way: the Large Magellanic Cloud, in the constellation Dorado, the Goldfish, and the Small Magellanic Cloud, in Tucana, the Toucan.
When the survivors of Ferdinand Magellan's round-the-world voyage returned to Europe in 1521, they reported sighting two ghostly clouds in the night skies of the Southern Hemisphere. Around 1930 astronomers discovered that these "clouds" were galaxies. Not only were they members of our Local Group, these irregular galaxies were satellite galaxies of our Milky Way, gravitationally bound and orbiting around it.
The Large Magellanic Cloud (LMC), 170,000 light-years from Earth, extends 60,000 light-years at its widest point and contains perhaps 15 billion stars. The Small Magellanic Cloud is a dwarf galaxy 30,000 light-years farther from Earth.
Some astronomers speculate that the Magellanic Clouds were barred spiral galaxies until our galaxy pulled them asunder. A hydrogen bridge connects the LMC with the outer disk of the Milky Way, and a huge loop of hydrogen gas, called the Magellanic Stream, surrounds both Magellanic Clouds. These factors suggest that the LMC has passed through the Milky Way's disk. It's possible that the two clouds split apart during the passage. Both galaxies exhibit active star formation, which may have been stirred up by their encounter with the Milky Way.
Planets of the Week
Mercury has been appearing above the western horizon after sunset; it sets earlier each night this week and will soon be lost in the sun's glare again. Mars is also above the western horizon in the evening hours, setting at about 10:00 p.m. On the opposite side of the sky at sunset, Saturn shines against the glittering backdrop of the constellation Taurus. Jupiter appears below it in the constellation Gemini and is the brightest object in the sky after the moon.
At 9:00 p.m. on January evenings the last of the late-summer and fall constellations are just about to set. Deneb, the bright alpha star in the constellation Cygnus, is just above the northwestern horizon. The Great Square of Pegasus is almost due west, and the constellations Cepheus, Cassiopeia, Perseus, and Andromeda, somewhat higher, are still visible. Ursa Minor, also known as the Little Bear or the Little Dipper, hangs almost straight down from Polaris, the North Star.
Leo, the Lion, and its bright alpha star Regulus are just rising in the east-northeast. Between Leo and Gemini lies the faint zodiacal constellation Cancer, the Crab. To the north is Ursa Major, the Great Bear. The Big Dipper, a part of that large constellation, stands upright on its handle.
The southeastern quadrant of the sky holds Orion, the Hunter, one of the brightest and most easily recognized constellations. Orion also serves as a pointer to several other constellations. The three equally bright, evenly spaced stars of Orion's belt point roughly toward Aldebaran, the alpha star in Taurus, to Orion's west, and toward Sirius, the alpha star in Canis Major, to the southeast. Sirius, also called the Dog Star, is the brightest star in the sky. Above Orion's belt are the stars of its shoulders: the brighter, eastern star of the two is the red star Betelgeuse; the other is called Bellatrix. Below the belt are two stars marking Orion's knees: the brighter, western one is the bluish-white star Rigel; the other is Saiph.
A line drawn from Rigel through Orion's belt to Betelgeuse and then extended northward points just north of Gemini, the Twins, and that constellation's two brightest stars, Castor and Pollux. Between Gemini and Canis Major lies the bright star Procyon, the alpha star in Canis Minor, the Little Dog.
There are few bright stars in the southwestern portion of the sky. The dim but large constellations of Pisces (the Fish), Cetus (the Sea Monster), and Eridanus (the River) span most of the western and southwestern sky. Near the meridian high in the south is Taurus, the Bull. The bright orange star Aldebaran marks its eye, and the V-shaped star cluster the Hyades marks its face. High in the southwest is the lovely star cluster known as the Pleiades, or the Seven Sisters. Almost overhead is Capella, the bright star in Auriga, the Charioteer.