Clouds Of Magellan

Press, Volume CVII, Issue 31542, 2 December 1967, Page 19

Clouds Of Magellan

High in the southern thy are two hazy patches, which appear rather like detached portions of the Milky Way. These are the Clouds of Magellan, named after the explorer, Ferdinand Magellan, who recorded them in his log in 1521 during the first circumnavigation of the globe. Because of the glare from lights these objects are not well seen from cities, but In the dark skies of the countryside they stand out elearly. To locate the clouds, during December evenings, face due south. Look for the Southern Cross to the southeast Imagine a line drawn from Gamma, the third brightest star in the Cross, to Alpha, the brightest Extend this in the same direction for four times the distance between Gamma and Alpha Cruds. The point now reached marks the South Celestial Pole, which, unlike the North Celestial Pole, has no bright star near it Continue the imaginary line two and a half times the distance between Gamma and Alpha Crucis, and you will then see one of the elouds. Somewhat east of this you will discern a second and larger misty patch. These two areas of luminosity are respectively the small and large Magellanic Clouds. To the naked eye each appears to consist of a bar, into which the stars are concentrated, surrounded by an irregular patch of light. These clouds are considered to be the most important celestial objects in the southern sky and most of the telescope time of the large instruments in Australia and South Africa is devoted to studying them. The recent decision to erect two large telescopes, each of 150 in aperture, one in Australia and the other in Chile, owes a great deal to the need to study these objects in greater detail. There are several reasons for the desire of astronomers to probe these celestial wonders with larger instruments. First they are twin galaxies to our own and the nearest to us in space. Second, they provide unique laboratories for the study of the birth and evolution of stars because they include the full range Of stars, dusters and nebulae. They are much easier to study than many parts of the Milky Way since they are not obscured by the cosmic dust that often blurs our view of the Milky Way. Yardsticks

Amongst the many different types of variable stars—those that change in brightness—are a class called cepheids. These stars pulsate very regularly, repeating their cycles year after year with little change. Around 1912 Miss Henrietta Leavitt, of Harvard College Observatory,

discovered hundreds of these pulsating stars in the Magellanic Clouds. When she arranged a group of these stars in order of increasing brightness at maximum light, she unexpectedly found that the periods were also in order, increasing from 1 to 150 days. Since the stars in the Magellanic Clouds are all at approximately the same distance it did not take long to determine what this distance was since it was possible to compare their apparent magnitudes with those of cepheids in our neighbourhood. These latter had already had their distances determined by other methods. Once it was found that a relationship existed between the period of the cepheids in the Magellanic Clouds and their apparent brightness it was obvious that the relationship held for the periods and absolute brightness since the stars were all at approximately the same distance. The absolute magnitude of a star is the brightness it would have if it were 32.6 light years from us. In this way the cepheid variables became the yardsticks that enabled the distances of galaxies to be measured, because this type of star is found in many of these systems. Size Of Clouds

It was found that both the Magellanic Clouds are roughly 160,000 light years away. A light year is the distance that light travels in one year at a speed of 186,000 miles a second. It is slightly less than six million million years. By stating distances in light years astronomers avoid having to use extremely large numbers.

In diameter the Large Cloud is around 60,000 light years and the Small Cloud 40,000 light years. To equal the radiation emitted by the objects making up the small cloud would require one hundred million stars similar to our Sun. To match the radiation from the Large Cloud would require six hundred million Suns.

Whilst the two clouds appear well separated to the eye, radio observations indicate that they are joined together by a tenuous bridge of gas. Indeed, in photographs taken with the larger instruments available there appears to be a connecting link formed by blue white super giant stars and star clusters. It appears as if the clouds are not unlike our own galaxy, being comparatively thin rotating discs. They include almost every known type of star, cluster and nebula. Young Stan

There are in the clouds a number of young stars, which include several blue-white super giants. In real bright ness these stars range from 100,000 times that of the Sun to almost a million times. They are thought to be be-

tween 70 and 100 times as massive as the Sun. Such stars eat up their fuel at a terrific rate so that it is unlikely that their life span is more than a million years at the most. Probably it is considerably less. Thus in the Magellanic Clouds the birth and evolution of the stars proceeds continuously. They thus provide unique laboratories for the study of these processes. As an example the Large Cloud contains the nebula known as 30 Doradus, which is one of the largest gaseous nebulae known. It is a mass of gas and stars including a large cluster of giant blue stars. Such stars were probably formed in the nebula and have not moved far from their birthplace because they are considered to be very young. Thus astronomers are able to study them in the place from which they originated.