One of the wonders which most attracted the attention of early explorers in the southern hemisphere, and roused as much interest as the Southern Cross, was the pair of faint clouds, looking like detached pieces of the Milky Way, which are seen in the neighbourhood of the South Pole. Marco Polo made a sketch of the Greater Cloud, which he describes wonderingly as “a star as big as a sack.”
Although some star-maps show short branches of Milky Way pointing towards the two Clouds, this is incorrect, and they are quite separate from it. Herschel was struck by their isolation, especially in the case of the Little Cloud, which he described as situated in a “most oppressively desolate desert,” its only neighbour being the globular cluster 47 Toucani, which is near, but separated by a perfectly black sky.
The Greater Cloud is much brighter to the naked eye than the Lesser, and it is much more complex and interesting in the telescope. It contains, moreover, the wonderful Looped Nebula, of which we have already spoken.
Both Clouds consist of gaseous nebulae and star-clusters on a background of vague nebulosity and crowds of almost indistinguishable stars. But the white nebulae shun the Clouds, just as they shun the Milky Way.
An immense number of variable stars have been discovered in the Clouds of Magellan, of the same type as those in globular clusters. Miss Leavitt of Harvard Observatory catalogued from photographs no less than 969 in the Lesser Cloud and 800 in the Greater. In the latter the greatest number of variables was found in a stream of faint stars which connects a group of star-clusters with the Looped Nebula, and others occur locally in certain parts of the Cloud, but few are in its northern region or in parts where many of the brighter stars congregate. All the variables are very faint, the usual minimum in both Clouds being about fourteenth magnitude, and the maximum seldom more than one magnitude brighter. A few in the Lesser Cloud have been found with periods unusually long for this “cluster type” of variables, amounting to 32, 66, and even 127 days. These longer periods seem to belong to somewhat brighter stars, but they are quite as exact as the usual period of a few days or a single day.
XVII
THE MILKY WAY
Like a great river returning into itself, the Galaxy encircles the starry heavens, and those who know only its northern course have no idea of its brilliance and wonderful complexity in its brightest part.
Its light is soft, milky, and almost uniform, between Cygnus and Sirius, but when it enters Argo it becomes extremely broad, and spreads out like a river on a flat marshy plain, in many twisting channels with spaces between. Where Canopus shines on the bank there is a narrow winding ford right across its whole breadth, as if a path had been made by the crossing of a star.
After this it suddenly becomes extremely narrow, but so bright that all the light which was shining in the broad channel seems to be condensed in this narrow bed. In the brightest, richest part the Great Nebula of Argo is easily distinguished by the naked eye. Contrasting with this and other bright condensations are black gaps, the largest and blackest of which is the well-known Coal-Sack near the Southern Cross.
