NEBULZE (Lat. nom. pl., clouds, vapors, mists). Patches of luminous matter occurring in considerable numbers in various parts of the sky. and differing from the stars in that they do not present simply small definite points of light, but rather large surfaces of widely varying bril liancy. We have acquired much exact knowl edge about the nebula', especially in recent years, since the application of photographic processes to astronomical observation; and they have also been made the subject of extensive philosophical theories as to the origin, development, and con struction of the universe, of which some are based upon too many hypothetical considerations to receive the unqualified assent of astronomers.
Only two neludre are visible to the naked eye: that in Andromeda and the great nebula in Orion. The former consists of an oval mass of light, about three times as long as it is broad, and surrounded with several more or less dis tinct disconnected oval rings. There is also a central condensation in the middle of the whole mass. The Orion nebula, on the other hand, is of quite irregular shape, contains a number of stars, and is indented• with at least one very large non-luminous break known as the 'fish Both these large nelmlx, in the tele scopic field of cover a space of more than one square degree on the surface. When tested by long-ex posure photographs, they are found to lie of truly gigantic size. It is now admitted that the entire constellation of Orion is included in the outlying parts of the great nebula. Condensations appear to fasten about the stars of the constellation : and indeed the conclusion seems irresistible that we are looking ujmon a Ilniverse in course of formation out of plastic and gaseous material. Many of the less conspicuous nebula• are round or oval; often have a stellar central condensation, and sometimes one of a disk-like form with uniform surface luminosity. These are the so-called `planetary Then there are or ring-shaped tiebuhr, and spiral nelmbr. These last look like great whirlpools of matter ap parently rotating rapidly and trailing long streamers of light. The very latest researches in ph. tograrhing 11C1411:1' seems to indicate that the spiral form is in tact the most prevalent ne. he of t he spectroscope, and especially I he photographic ,pectro,ope, to lily has proved that many nebular are really gaseous in constitution. and not simply ag.rregations of stars too small and close together to be separated even by our most powerful Icie st, pes. It has also been possible to measure
with the spectroseope the Aelocity with which Hill' of the• nebuhr are moving through space in the direction of our solar system; and it is found that nebular velocities are of about the same order of magnitude as those observed in the case of ordinary stars. As to the changes of the shape and f01111 of individual nebulae we have no decisive evidence. Indeed, the outlines of these objects are so hazy that no reliance can br placed 111a111 Ordinary drawings; and even photographs van mislead us, since what we See 1111011 them iti great measure upon the length of time during which the sensitive plate was exposed to the nebula's light. The number of the nebulae at present known is not far from 111,000; and the best catalogue of them is by Dreyer. published in the Memoirs of the loyal Astronomical Society of London.
is to their distance our knowledge is still in the conjectural stage. No one has yet meas ured a nebular parallax (see t'ARAl.t..lx) suc cessfully; but it is extremely probable that their distances. like their velocities of motion, are of the same order of magnitude as the correspoml ing quantities for the fixed stars.
The theory of cosmic evolution known as the nebula• hypothesis of Laplace intends to ae count for the origin and development of our solar systP711, beginning with a presupposed mass of formless matter. This It rotation under the induence of natural forces, might be expected to take upon itself the form of a, flattened sphere; and if a contraction of it, hulk should oeeur after the lapse of ages of time, certain rings of matter might be left be hind, as it were. by the contracting mass. Such rings have indeed a certain analogy in the ..1ndromed8 nebula; and their possible formation once granted. there is nothing to prevent the gradual breaking tip of a ring under the action of gravitational forces. Its matter might then Le gathered into a single ball. and thus would he formed a planet circulating in an orbit around the central condensing mass destined ill later ages to form the sim itself. It will be seen that this hypothesis. while plausible. rests up. n a basis of imagined possibilities rather than ascertained facts. therefore, it is most interesting. it must on no account be reck oned among those astronomical theories (such as gravitathm) whose foundation is as strong as that underlying any of the scientilie truths com ing within the range of human knt.wh.dge. Nye Cos 111110 (Nr.