NEBULYE. A class of objects in the sidereal heavens, distinguish able by a dim light and the absence of any definite outline. The earliest notice of nebulae is to be found in the Syntax's ' of Ptolemy, who has inserted five objects in his catalogue under the appellation of cloudy stars. The application of the telescope to astronomical observa tion revealed the real nature of the earlier nebulae. It was found that in every instance they really consisted of an aggregate number of stars, apparently so close to each other that they failed to produce an im pression of their individual existence upon the eye. In 1614, Simon Marius discovered a nebula in the constellation Andromeda, which seemed to be of a totally different nature to any of the nebnlx hitherto discovered, inasmuch as when observed in the telescope it was found to be irresolvible into stars. Its discoverer compared the aspect presented by it to that exhibited by the light of a candle when viewed through a transparent horn. In 1618 a similar object was discovered in the constellation Orion, by Cyxatus, a Swiss astronomer. Huyghens, who recognised this nebula in 1656, without having been aware of its pre vious discovery, was struck with amazement at its singular aspect. The region in the vicinity of the nebula being intensely black, he con ceived that the phenomenon was occasioned by looking through an aperture in the heavens into the luminous region beyond.
About the middle of the last century, the number of nebulae dis covered by astronomers amounted to little more than twenty. In 1755, Lacaille, the French astronomer, announced to the Academy of Sciences the existence of 42 additional nebulae in the southern hemi sphere. He divided them into three distinct classes. The first class comprised those objects which, when viewed in the telescope, were found to retain their nebulous aspect without affording any indication of sidereal structure. The second class consisted of such nebulm as were resolvable by the telescope into stars. The third class consisted
of stars surrounded by a nebulous substance. Messier, another French astronomer, was the discoverer of no fewer than 103 new nebulm. His observations are recorded in the 'Memoirs of the Academy of Sciences' fer 1771, and in the volumes of the ' Connaissance des Temps' for 1783 and 1784.
The elder Herschel was the next astronomer who contributed towards the existing knowledge respecting nebulre. In 1786 be communicated to the Royal Society a catalogue of 1000 new nebulte. This was fol lowed by a second catalogue, communicated to the same Society in 1789. and containing 1000 additional nebuhe. In 1S02, he communi cated a third catalogue of nebula', containing 500 new objects. On this last occasion ho distributes the various objects of the stellar regions into twelve distinct classes.
I. Insulated stars, such as Arcturus, Capella, Sirius, &c. • II. Binary stars, or in other words, systems of two stars revolving in each case nixed their common centre of gravity.
III. Triple and Multiple stars.
IV. Clustering collections and the Milky ny. Herschel was of opinion that in many parts of the heavens the stars afford indications of collecting together and forming distinct clusters from the efl'ect of their mutual attraction. Ile supposes that the Milky Way is under going a process of disruption from this cause.
V. Groups of stars. This consisted of aggregrations of stars which did not exhibit any regularity of outline, nor condensation in any part of their structure indicative of their forming separate systems.
VI. Clusters of stars.—These were objects of a round form, presents lug in all cases a gradual condensation towards the centre.
VII. .Viteche.—These were supposed to be objects consisting in reality of aggregate collections of stare.
'VI If. Stellar nebula.— These were imagined by Herschel to be clusters of stars situate at such immense distances as to put on the aspect of merely luminous points with burs.