The Magellanic Clouds

Just 15° and 36° southwest of Canopus, respectively, lie two bar-shaped patches of light that, to the naked eye, look for all appearances like detached pieces of the Milky Way. These clouds are not fragments of our galaxy, but separate, nearby galaxies that astronomers think may be gravitationally bound to our own, like the Moon is to the Earth. They are named the Large and Small Magellanic Clouds, after the Portuguese explorer Ferdinand Magellan, who recorded them in 1519 during his voyage in search of the strait - also subsequently named for him - at the extreme tip of South America.

Because of their proximity to the south celestial pole, the Magellanic Clouds can only be seen from the Southern Hemisphere (though they may be glimpsed very near the southern horizon after sunset from latitude 10°N in late December and early January). As seen from Sydney, Australia, this time of year, the Large Magellanic Cloud (LMC) straddles the southern meridian 55° above the horizon, just after sunset. The Small Magellanic Cloud (SMC) lies about 21° toward the southwest. In a dark sky, they're impossible to miss, and from the Cimmerian seas off the eastern coast of South America, they must have been a startling apparition to Magellan in the early sixteenth century.

Both the LMC and SMC belong to a class of galaxies called 'irregulars,' because they exhibit no definitive shape, unlike those of the spiral and elliptical classes. The LMC is about 33,000 light-years in diameter and lies about 171,000 light-years away. The SMC is about 20,000 light-years across and lies even further away, some 200,000 light-years. Both galaxies are rich in a class of young stars called population I, which are metal-rich stars forged from gas processed in the nuclear fires of previous generations of stars, an older group known as population II stars.

Both galaxies are enveloped in a cloud of cool, electrically neutral hydrogen gas that radio astronomers have traced over 110° of the sky back toward our galaxy. This runnel of gas, called the Magellanic Stream, may have been gravitationally drawn out of one or both of the Magellanic Clouds when they passed too near the Milky Way over 200 million years ago. Some astronomers think the Stream may have been created during the Clouds' first interaction with our galaxy; others think it may be the product of many such interactions.

The Magellanic Clouds are located in an area of sky fairly devoid of bright stars, although these two objects more than make up for this deficit. A pair of 7X50 binoculars is all you need to scan the Clouds for star clusters and nebulae.

In the LMC, the most striking feature is the Tarantula Nebula, designated NGC 2070,* clearly visible to the naked eye at the southeast end of the Cloud. The nebula contains a group of about a dozen very hot, massive stars packed into a region less than a light-year across. This massive stellar bonfire lies at the center of the spider-like tendrils of hot gas that give the nebula its name. If this cluster of hot stars were as far away as Pollux (35 light-years), in our skies it would appear brighter than the full Moon.

In and around the Tarantula Nebula, and, indeed, scattered throughout the LMC itself, are many bright gaseous star-forming regions. This area is a hotbed for star birth - and death, as was seen in 1987, when a star exploded on the extreme western end of the Tarantula Nebula. Supernova 1987A was the first supernova seen with the naked eye since 1604, reaching magnitude 2.9 and remaining visible for months. The remnants and outflowing shock wave generated by the supernova are still being studied by astronomers.

Although the demure SMC doesn't exhibit the obvious celestial treasures of its larger counterpart, it nevertheless contains a fair share of star clusters and wispy nebulosity, including the globular cluster NGC 362 and the open cluster NGC 346.

Most impressive is the beautiful globular cluster 47 Tucanae (NGC 104), lying just west of the SMC. Visible to the naked eye as a 4th-magni-tude fuzzy star, this object is resolved into a seething ball of stars when seen with a small telescope. Being one of our nearest globular clusters, however, only 20,000 light-years away, makes 47 Tucanae a part of our galaxy, rather than the SMC, as it appears.

* NGC stands for New General Catalog, which is a vast compilation of thousands of star clusters, nebulae, and galaxies.

For those of us in the Northern Hemisphere, it is well worth a trip down under to behold the Large and Small Magellanic Clouds in the same sky with Canopus, Alpha and Beta Centauri, Achernar (Alpha Eridani), and the Southern Cross. Seeing these celestial wonders for the first time is like seeing a different universe.

Also this week:

• Capella, sixth-brightest star in the night sky, is overhead for observers at mid-latitudes in the Northern Hemisphere around 8 o'clock.

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