May 1 1964 - The Clouds of Magellan

Among the most fascinating objects in the sky are the two Clouds of Magellan, or Nubecula. They are too far south to be visible from Europe, but European observatories have been co-operating in studies of them. Actual photographs are taken from southern hemisphere stations, and the theoretical work is being undertaken on an international scale.

A great deal of this work is being carried out at the Armagh Observ­atory, in Northern Ireland. Armagh Observatory is extremely interesting in many ways, and so we decided to go there for a Sky at Night pro­gramme. Dr E. M. Lindsay, the Director, could not have been more helpful - indeed, he joined in the broadcast, and showed himself to be an expert in television technique as well as in his own sphere of astro­physics. I must also pay tribute to Mrs Lindsay and to the Armagh staff, who made our stay at the Observatory so very pleasant.

In the southern hemisphere of the sky, the famous sixteenth-century explorer Magellan noticed two strange luminous patches. They looked almost like two detached portions of the Milky Way, and both were easily visible to the naked eye. Today, these objects are known as the Magellanic Clouds or Nubecula, even though Magellan himself was not their actual discoverer.

The Clouds never rise in our own latitudes. They are therefore permanently out of view from great observatories such as Palomar and Mount Wilson. However, they are of great importance in astronomical study, and intensive research into their nature and characteristics has been going on during the past half-century or so. The visual work must naturally be done from a southern station, but much of the theoretical work is carried out in the north, and a part in the investigations has been played by the Observatory of Armagh.

Armagh Observatory, in Northern Ireland, has a long and honourable history. Indeed, it is the oldest British observatory still operating from its original site (Greenwich Observatory is now at Herstmonceux in Sussex). The founder was Lord Rokeby, better remembered as Archbishop Robinson, a great man in the history of Armagh; memorials to him are to be found all over the city. During the closing years of his life, Archbishop Robinson was anxious to found an Ulster university. This project was never realized, but the founding of the observatory, due entirely to Robinson, was probably connected with the university scheme. The Archbishop's generosity led to the setting-up of the observa­tory in 1790, and in July of that year the Rev. Dr James Hamilton was appointed Director. Since that time Armagh has maintained a fine record of observational and theoretical work. There have been seven Directors, including Dr E. M. Lindsay, the present holder of the office; special mention should be made of Dr Romney Robinson (no relation to the Archbishop) whose regime extended from 1823 to his death in 1882, and Dr J. L. E. Dreyer, who is particularly remembered for his work in connection with the New General Catalogue of star-clusters and nebulae.

There are many historical exhibits in the present observatory. There is, for instance, a mirror made by Sir William Herschel, discoverer of the planet Uranus and 'explorer of the heavens', together with letters written by his sister Caroline. Herschel was well acquainted with Archbishop Robinson, and the two used to visit Bath Spa together from time to time even after Herschel had ceased to be organist there and had begun to devote all his time to astronomy. Equally interesting is a small reflector which was owned and used by no less a person than King George III, who was keenly interested in astronomical science.

Together with this historical material, there are modern instru­ments in regular use. There is, for instance, a Schmidt telescope - better known, perhaps, as a Schmidt camera, since its use is entirely photographic. The Armagh Schmidt is employed mainly on variable star research. Another telescope is a 10-inch Grubb reflector, with which double star measures are being carried out.

Moreover, Armagh is a meteorological station as well as an observatory, and continuous weather records have been kept ever since 1790.

Excellent though they are so far as quality is concerned, the Armagh telescopes are small compared with the American giants such as the Palomar 200-inch reflector. Moreover, conditions in Ireland are not ideal for astronomical observation, although admittedly they are no worse than over many other parts of Europe (including England). For these reasons, a link was formed with the famous Harvard Observatory in the United States and with the Dunsink Observatory at Dublin. The result was the setting up of the 'A.D.H.' (Armagh-Dunsink-Harvard) telescope at the Boyden Station of the Harvard Observatory, near Bloemfontein in South Africa, with the set intention of studying objects which could never be seen from northern latitudes. The project dates from 1950. In 1955 the Swedish, Belgian, and West German Observatories joined with Harvard and the two Irish Observatories to form the Boyden Observatory, the Council of which consists of the Directors of Armagh, Brussels, Dunsink, Harvard, and Stockholm Observa­tories. Unlike most international projects, this scheme has been completely free from friction, and there has been full agreement in all respects.

The A.D.H. telescope at Boyden is a 36-inch reflector. Use is also made of other instruments at the observatory, notably the 60-inch reflector. Conditions are excellent, and the programme which has been undertaken is a very full one. Emphasis has been placed on the two Clouds of Magellan, and the results have been very encouraging, as Dr Lindsay has stressed.* There are two Clouds; the Large (Nubecula Major) and the Small (Nubecula Minor). They are not, in fact, detached parts of the Milky Way, but are galaxies in their own right. Each is about 180,000 light- years away, so that the Clouds are considerably nearer than any other external systems.

Our Galaxy is made up of perhaps 100,000 million stars, of which the Sun is one. The system is arranged in a flattened form, so that the familiar Milky Way effect is nothing more than an effect of perspective; when we look along the main axis of the Galaxy, we see many stars in much the same direction. Together with stars, the Galaxy contains huge clouds of gas (nebulas), and clusters of various kinds; there are open or loose clusters, of which the Pleiades group is the best known, and globular clusters, such as Messier 13 in Hercules. The two brightest globulars, Omega Centauri and 47 Tucanae, are, like the Clouds of Magellan, too far south to be seen in Europe or the northern United States. The closest of the really large outer galaxies is Messier 31m Andromeda, known as the Great Spiral, which is dimly visible to the naked eye on a clear night. This system appears to be considerably larger than our Galaxy, but is very remote, since it lies at a distance of more than 2,000,000 light-years. The Clouds of Magellan are seen, then, to be relatively near to us on the cosmical scale. To use Lindsay's analogy: if our Galaxy is represented by London, then the Clouds will be represented by Greenwich or Woolwich.

The Clouds are also relatively close to each other. The distance between their centres is about 75,000 light-years, and there is evidence that the two are actually connected, forming a twin system; for instance, radio astronomers have been able to show that the two Clouds are embedded in a common envelope of hydrogen gas. There have also been suggestions that the Clouds are satellites of our Galaxy, and move round it, taking an im­mensely long time to complete one revolution. This may or may not be the case - more information is needed - but at all events there is no doubt that the Clouds are true members of the so-called 'local group' of galaxies.

Broadly speaking, there are three main types of galaxies: elliptical, spiral, and irregular. In a spiral, such as Messier 31 in Andromeda, there is a central nucleus, relatively free from inter- stellar gas and dust, and with a high proportion of red giant stars, which are well advanced in their evolution. In the spiral arms there are many young, highly luminous, hot blue stars, together with clouds of gas and dust. Parts of the Nubecula Major are seen to be of the 'spiral-arm' type, and there is inconclusive evidence of an incipient spiral structure; there is certainly a main axis with a great concentration of stars. The smaller Cloud is different; there is no nucleus, no symmetry, and nothing in the nature of a spiral arm, so that Nubecula Minor is best classified as an irregular galaxy. There is one marked extension, but this is probably due to tidal effects caused by the Large Cloud. All things considered, efforts to find order and structure in the two Nubeculae have not, so far, proved very rewarding.

What is far more important is the fact that the Clouds contain objects of the same kind as are found in our own Galaxy, and to all intents and purposes we may assume that all objects in the Clouds are at the same distance from us. (To give a rough analogy; it is true that New York is closer to Southampton than to Portsmouth, but the difference is utterly unimportant.) It was by studying the short-period variables in the Small Cloud, fifty years ago, that Miss Henrietta Leavitt, at Harvard - using photographs taken from southern stations - made the discoveries that led on to the 'period-luminosity law' of Cepheid stars. These Cepheids brighten and fade regularly, and their behaviour can always be predicted. Miss Leavitt found that the longer the period of a Cepheid, the brighter it looked; since she could assume that all the Cloud Cepheids lay at the same distance, it followed that the longer- period stars were genuinely the more luminous. Without the Clouds, this far-reaching discovery would have been very difficult to make.

At Armagh, during the past few years, Dr Lindsay and his assistants have been completing a survey of various types of objects photographed in the Clouds with the A.D.H. telescope. There are clusters, both open and globular; there are planetary nebulae, and also gaseous nebulae. A planetary nebula consists of a central star surrounded by a ring or shell of gas, and is not appropriately named, since it is not, strictly speaking, a nebula - and is certainly not a planet. The brightest example in our own Galaxy, the Ring Nebula in Lyra (not far from the brilliant bluish Vega) is visible with a moderate" telescope. Planetary nebulae in the Clouds are too remote for their structure to be seen, but their spectra, as obtained with the A.D.H. telescope, reveal their true nature, and about 150 have now been found. Also to be seen are a few objects which look very much like the wrecks of supernovae, or stars which exploded long ago and never returned to their original state.

Of the gaseous nebulae, the most striking is known popularly as 'The Tarantula'. It is of basically the same type as the celebrated Orion Nebula in our own Galaxy, but is much larger. The Orion Nebula, which may be seen with the naked eye in the Hunter's Sword, is 1,600 light-years away and about 25 light-years in diameter; its mass is about 100 times as great as that of the Sun, and it seems to be one of the sites where fresh stars are being created from interstellar material. The Tarantula Nebula in the Large Cloud of Magellan, however, is 800 light-years across, and has a mass equivalent to 500,000 Suns. If it lay in our own system, it would indeed be a magnificent object. Embedded in it are clusters of hot, young, very luminous stars.

It is also interesting to note that the Large Cloud contains S Doradus, which has the distinction of being the most luminous star known. It is equal to about 1,000,000 Suns - and yet is so far away from us that without a telescope it cannot be seen at all. It is variable, and is using up its 'nuclear fuel' at a fantastic rate, so that it must have a relatively short expectation of life in its present form. The Sun will not change much for some thousands of millions of years to come; S Doradus can hardly continue squan­dering energy at its current rate for more than 1,000,000 years.

Great attention is being paid to the star-clusters in the Clouds, and by now many have been detected; about 120 in Nubecula Minor, almost 1,000 in Nubecula Major. Most of these are of the open type, though there are also numbers of globular clusters. From their distribution, it is possible to estimate the maximum dimensions of the systems, and it has been found that the diameters are 20,000 light-years for the Small Cloud and double this for the Large Cloud. Even the senior of the two is, therefore, much smaller than our Galaxy, which is about 100,000 light-years across: even so, the Clouds are certainly not dwarfs.

Astronomers consider that we are fortunate in having the two Nubeculae near at hand. Studies of them give us considerable insight into the anatomy or structure of galaxies in general, and also yield information about the evolutionary processes of the stars and star-systems. As Lindsay has commented: 'One of the aims of the astronomer, if not the ultimate aim, is .to find out how and when the universe began, how it is changing, and how and when it will end - if indeed there was a beginning or if there will be an end'.

It is probably true to say that without the Clouds our knowledge of the universe today would be markedly less than it actually is. There are no other comparable systems within range; the Andro­meda and Triangulum spirals are much more remote, so that they cannot be examined so easily, and the remaining galaxies of the local group are much less informative. There is an urgent need for a really large telescope in the southern hemisphere, since photo­graphs of the Clouds taken with a 150-inch or 200-inch reflector would be invaluable. Meanwhile, the A.D.H. telescope is playing its part well.

At Armagh, the 'old' meets the 'new'. The observatory itself is a charming old building set in picturesque surroundings, and the modern domes housing the Schmidt telescope and the 10-inch refractor fit well into the picture. Yet the work in progress there is extremely modern, and is of great scientific importance. Moreover, the complete success of the international project at Boyden augurs well for the future.