Nebula (Lat., mist, vapor), an aggregation of stars or stellar matter having the appearance, through an ordinary telescope, of a small, cloud-like patch of light. An enlargement of telescopic power usually converts this appearance into a cluster of innumerable stars, besides bringing to light other nebulae before invisible. These in turn yield to augmented magnifying power; and thus every increase in the capacity of the telescope adds to the number of clusters resolved from nebulas, and of nebulas invisible to lower powers. Nebulas proper, or those which have not been definitely resolved, are found in nearly every quarter of the firmament, though abounding especially near those regions which have fewest stars. Scarcely any are found near the milky way, and the great mass of them He in the two opposite spaces furthest removed from this circle. Their forms are very various, and often undergo strange and unexpected changes as the power of the telescope with which they are viewed is increased, so as not to be recognizable in some cases as the same objects. The spiral nebulas are an example of this transformation. This class was recognized by Lord Rosse through the use of his six-foot reflector.
Many of them had been long known as nebulas, but their characteristic spiral form had never been suspected. They have the appearance of a maelstrom of stellar matter, and are among the most interesting objects in the heavens.
There is another class of nebulas which bear a close resemblance to planetary disks, and are hence called planetary nebulas. They are very rare. Some of them present remarkable peculiarities of color. Sir John Herschel has described a beautiful example of this class, situated in the southern cross. But in telescopes of the highest power some of the so-called planetary nebulas assume a totally different appearance; and many of them are singularly complicated in structure, instead of being simple globes of nebulous matter, as was for-merlv supposed. There are several which have perfectly the appearance of a ring, and are called annular nebulas. A conspicuous and beautiful example is situated in Lyra. Some appear to be physically connected in pairs like double stars. -Most of the small nebulas have the general appearance of a bright central nucleus enveloped in a nebulous veil. This nucleus is sometimes concentrated as a star and sometimes diffused. The enveloping veil is sometimes circular and sometimes elliptical, with every degree of eccentricity between a circle and a straight line. There are some which, with a general disposition to symmetry of form, have great branching arms or filaments with more or less precision of outline. An example of this is Lord Rosse's Crab nebula.
Another remarkable object is the nebula in Andromeda, which is; visible with the naked eye, and is the only one which was discovered before the invention of the telescope. Simon Marius (1612) describes its appearance as that of a candle shining through horn. Besides the above, which have comparatively regular forms, there are others more diffused, and devoid of symmetry of shape. A remarkable example is the great nebula in Orion, discovered by Huygens in 1656. This nebula and that in Andromeda have been admirably delineated by the professors Bond of Harvard observatory. (See "Memoirs of the American Academy of Arts and Sciences," new series, vol. iii.) The great nebula in Argo, which Sir John Herschel has charted with exquisite care and elaborateness in his "Cape Observations," is another example of this class. In the southern firmament there are two ex-nebulous tracts known as the Magellanic clouds; the greater called Nubecula Major, and occupying an area of 42 square degrees; the smaller called Nubecula Minor, and covering about 10 square degrees. In these trad- are found multitudes of small nebula' and clusters.
The number of these wonderful objects which have been recognized in all the heavens is upward of 5,000. Of these fewer than 150 were known prior to the time of Sir William Herschel. In 1786 he communicated to the royal society a catalogue of 1,000 new nebulas and clusters; in 1789 a second catalogue of the same number of new objects; and in 1802 a third which included 500 more. In 1833 Sir John Herschel communicated to the royal society a catalogue of 2,306 nebulas and clusters in the northern hemisphere observed by him, 500 of which were new. In 1847 appeared his " Cape Observations," which contained catalogues of 1,708 nebulas and clusters in the southern heavens. - The application of spectroscopic analysis to these objects, by Huggins, Secchi, Vogel, and others, has resulted in the noteworthy, discovery that while some among the nebulas are really clusters of stars, others consist in the main of gaseous matter. The former give spectra resembling in their general characteristics the spectra of stars; the latter give a spectrum of three bright lines (occasionally four), one line corresponding in position to a line in the spectrum of hydrogen, another corresponding to a line in the spectrum of nitrogen.
The resolvable nebula? mostly give spectra of the former class, while the bright-line spectrum is given by all the irregular nebulas hitherto examined, and by the planetary nebulas. Of about 70 nebulas examined by Huggins, nearly one third gave the spectrum indicative of gaseity, the rest giving a stellar spectrum. - As to the nature of nebulas, two chief theories have been advanced. It was first suggested by Wright of Durham, and afterward maintained by Kant and Lambert, that the nebulas are stellar galaxies similar to our own star system. Sir W. Herschel, at the beginning of his researches into the constitution of the universe, adopted this view as respects certain nebulas which he regarded as external, while holding (contrary to the usual statement in our text books of astronomy) that many nebulas form parts of our own star system. At a later stage of his labors he advanced the hypothesis commonly known as Herschel's nebular hypothesis, which however related only to certain orders of nebulas. At this stage Herschel for the first time indicated his ideas respecting the arrangement of all orders of stellar aggregations and nebulous matter.
At the lower extremity of the scale he placed widely spread luminosity, such as he had first described in 1802. He passed from this irregularly spread luminosity, through all the orders of gaseous nebulas (irregular nebulas, planetary nebulas, nebulous stars) formed by the gradual condensation of the gaseous matter, until the star itself is formed; then he entered on the part of the series he had before recognized, passing on to the various orders of stellar aggregation, diffused clusters, ordinary stellar nebulas, and more and more condensed groups of stars, up to the richest star clusters. At this period (1814) we no longer find him speaking of external nebulas; not, it is to be presumed, that he no longer recognized the probability that other stellar galaxies besides our own exist, but that he no longer found it possible to discriminate those nebulas which are external from the far greater number which unquestionably form component parts of our own sidereal system. The researches of the present writer into the subject dispose him to believe that our sidereal system extends far beyond the limits which have ordinarily been assigned to it, and that there are no nebulas which can be regarded as external to it.
Fig. 1. - Spiral Nebula in Canes Venatici.
Fig. 2. - Stellar Nebula.
Fig. 3. - Planetary Nebula.
Fig. 4. - Annular Nebula in Lyra.
Fig. 5. - Great Nebula in Orion.
Fig. 6. - Large Magellanic Cloud.