The Magellanic Clouds

The night sky in the Southern Hemisphere is very different from that in the North. One feature (more properly two) is the patches of fuzziness, easily visible to the naked eye, known as the Large and Small Magellanic Clouds.

They take their name from Ferdinand Magellan, the Portuguese mariner whose expedition circumnavigated the world in 1519-22, although he himself was killed during the voyage. The Clouds were mentioned in the journal of the voyage compiled by Antonio Pigafetta, who was one of only 18 men who completed the voyage.

However, the Clouds must have been known to inhabitants of the Southern Hemisphere millennia before they were given the name by which they are now known. The Large Magellanic Cloud is almost certainly the feature named the “White Ox” by the Persian astronomer Al Sufi in 964, who noted that it was visible from southern Arabia but not the north.

Until the early 20^th century, the Clouds were believed to be outlying regions of our own Milky Way galaxy, but they are now known to be more distant, with the Large Magellanic Cloud being about 150,000 light years away and the Small Magellanic Cloud being even further away at about 200,000 light years (the distance between the Clouds is about 75,000 light years). This makes them companion galaxies of the Milky Way, rather than constituent parts of it.

Recent research has cast doubt on the previously held belief that the Clouds are in orbit around our galaxy, but they still exert a tidal pressure on it, warping its shape. The presence of the Clouds may be the reason why the Milky Way has such well-pronounced spiral arms. In return, the Milky Way’s gravity draws out a long stream of hydrogen gas from both Clouds, this being known as the Magellanic Stream. There is also a stream of gas that connects the two Clouds.

As galaxies go, the Magellanic Clouds are quite small, and irregularly shaped, although they both have a vague spiral pattern to them. The Large Magellanic Cloud is about a quarter as luminous as the Milky Way and the Small Cloud about one twenty-fifth as luminous. The Small Cloud is only about one-sixth as massive as the Large Cloud.

Star formation is taking place within both Clouds, but at a slower rate than in our own galaxy. They are also noticeably bluer in color than the Milky Way, which indicates that many of their stars are younger than the redder stars that typify the Milky Way. The Clouds are very rich in gas but poor in heavy elements (i.e. heavier than carbon, nitrogen, etc). This suggests that star formation is slow, because such elements are formed in the interior of stars.

A particular feature of the Large Magellanic Cloud is the Tarantula Nebula (30 Doradus) which is one of the largest star-forming regions known, being about 30 times the size of the Orion Nebula. It is believed that it contains a star that is 1,000 times more massive than the Sun, which would make it ten times more massive than any star in the Milky Way.

In 1987, a supernova explosion near the Tarantula Nebula, the brightest seen for 300 years, proved useful in giving astronomers a means of checking the distances of galaxies from Earth. The calculations made at the time agreed with those made in relation to other indicators of distance such as variable stars.

As new tools, such as the Hubble telescope, enable astronomers to see even further into remote regions of the Universe, the presence of a local galaxy that can be studied in considerable detail has proved very useful for testing various theories. By knowing the processes that take place within galaxies, based on “close” observation, we can make better guesses as to what is going on in more distant parts of Space.

© John Welford