thousand, periods without irregularities manifesting themselves.

It may be added that this regularity of the period, taken in connection with the case of Eta Aquila, already mentioned, affords a strong presumption that the variations in the light of these stars are in some way connected with the revolution of bodies round them, or of one star round another. Yet it is certain that the types are not of the Algol class and that the changes are not due merely to one star eclipsing another. That such condensed clusters should have a great number of close binary systems is natural, almost unavoidable, we might suppose. It is probable that among the stars in general, single stars are the exception rather than the rule. If such be the case, the rule should hold yet more strongly among the stars of a condensed cluster.

Perhaps the most important problem connected with clusters is the mutual gravitation of their component stars. Where thousands of stars are condensed into a space so small, what prevents them from all falling together into one confused mass? Are they really doing so, and will they ultimately form a single body? These are questions which can be satisfactorily answered only by centuries of observation; they must, therefore, be left to the astronomers of the future.

12

CHAPTER XI

NEBULÆ

Some tumultuous cloud

Instinct with fire and nitre.

MILTON.

THE

'HE first nebula, properly so called, to be detected by an astronomical observer was that of Orion. Huyghens, in his Systema Saturnium, gives a rude drawing of this object, with the following description :

"There is one phenomenon among the fixed stars worthy of mention which, so far as I know, has hitherto been noticed by no one, and, indeed, cannot be well observed except with large telescopes. In the sword of Orion are three stars quite close together. In 1656, as I chanced to be viewing the middle one of these with the telescope, instead of a single star, twelve showed themselves (a not uncommon circumstance). Three of these almost touched each other, and, with four others, shone through a nebula, so that the space around them seemed far brighter than the rest of the heavens, which was entirely clear, and appeared quite black, the effect being that of an opening in the sky, through which a brighter region was visible."

For a century after Huyghens made this observation it does not appear that these objects received special attention from astronomers. The first to observe them systematically on a large scale was Sir

Wm. Herschel, whose vast researches naturally embraced them in their scope. His telescopes, large though they were, were not of good defining power and, in consequence, Herschel found it impossible to draw a certain line in all cases between nebulæ and clusters. At his time it was indeed a question whether all these bodies might not be clusters. This question Herschel, with his usual sagacity, correctly answered in the negative. Up to the time of the spectroscope, all that astronomers could do with nebula was to discover, catalogue, and describe them.

Several catalogues of these objects have been published. The one long established as a standard is the General Catalogue of Nebula and Clusters, by Sir John Herschel. With each object Herschel gave a condensed description. Recently Herschel's catalogue has been superseded by the general catalogue of Dreyer, based upon it and published in the Memoirs of the Royal Astronomical Society.

Some of the more conspicuous of these objects are worthy of being individually mentioned. At the head of all must be placed the great nebula of Orion. This is plainly visible to the naked eye and can be seen without difficulty whenever the constellation is visible. Note the three bright stars lying nearly in an east and west line and forming the belt of the warrior. South of these will be seen three fainter ones, hanging below the belt, as it were, and forming the sword. To a keen eye, which sharply defines the stars, the middle star will appear hazy. It is the nebula in question. Its character will be strongly brought out

by the smallest telescope, even by an opera-glass. Drawings of it have been made by numerous astronomers, the comparison of which has given rise to the question whether the object is variable. It cannot be said that this question is yet decided; but the best opinion would probably be in the negative. In recent times the improvements of the photographic process

THE GREAT NEBULA OF ORION, AS PHOTOGRAPHED BY A. A. COMMON, F.R.S., WITH HIS FOUR-FOOT REFLECTOR

have led to the representation of the object by photography. A photograph made by Mr. A. A. Common, F.R.S., with a reflecting telescope, gives so excellent an impression of the object that by his consent we reproduce it.

The most remarkable feature connected with the

nebula of Orion is the so-called Trapezium, already described. That these four stars form a system by themselves cannot be doubted. The darkness of the nebula immediately around them suggests that they were formed at the expense of the nebulous mass.

Great interest has recently been excited in the spiral form of certain nebulæ. The great spiral nebula M. 51 in Canes Venatici has long been known. We reproduce a photograph of this object and another. It is found by recent studies at the Lick Observatory that a spiral form can be detected in a great number of these objects by careful examination.