Nebula

NEBULA, in astronomy, the name given to certain luminous patches in the sky. (Latin for "mist," Greek pE4EX77). Dark nebulae are also known. The smaller luminous nebulae resemble distant comets when seen in a telescope of moderate power but can be easily distinguished from them, because the latter, being within the confines of the solar system, appear to move among the stars, while the nebulae, being at stellar distances, have no such motion. They are almost without exception invisible to the unaided eye, and, while a few hundred of them can be seen with a small telescope, it requires the combined aid of a very large instrument and the photographic plate to study these faint ob jects at all adequately. The two largest and brightest are the spiral nebula in Andromeda and the irregular nebula in Orion.

The nebulae are very numerous, their number being reckoned in hundreds of thousands, but only a comparatively few of these appear large enough to show much structure even on photographs taken with our most powerful telescopes. As the nebulae present to us luminous surfaces, as opposed to the point-image of a star, even the largest telescope, though it makes them appear larger, does not make them any brighter than they are to the unaided eye. But the photographic plate has the advantage over the human eye of being able to accumulate the effect of exposure to a faint source of light, and thus by means of long exposures, sometimes extending over many hours, it is possible to photograph nebular detail far too faint to be observed visually in any telescope.

Historical.—Bef ore the days of photography the nebulae were observed by Messier; he recorded 103 nebulae and star clusters; by Sir William Herschel at Slough, and by Sir John Herschel both there and at the Cape of Good Hope, who raised the number of known nebulae to 4,000 ; and in the middle of the 19th century by the third Earl of Rosse, with his giant reflecting telescope at Parsonstown. They were first observed with the spectroscope in 1864 by Sir William Huggins, whose discovery that some of them had a gaseous spectrum dispelled the idea then becoming current that all nebulae were aggregations of stars, and that it was merely a matter of obtaining more powerful telescopes to show them as such. The catalogues formed by J. L. E. Dreyer

contain about i o,000 nebulae discovered visually. They have not yet been superseded by any general catalogue based upon photo graphic observations.

Amongst the pioneers of photography as applied to the study of the nebulae were A. A. Common and Isaac Roberts in England, and later E. E. Barnard and J. E. Keeler in America. During the present century the nebulae have been extensively photographed at the Lick, Mount Wilson, and Lowell observatories in America, These nebulae, which are only found in or near the Milky Way, are of three kinds: (a) Dark Nebulae, which do not shine, and can only be detected because they obscure the light coming from the stars that lie behind them; they are irregular in shape; (b) Diffuse Nebulae, also irregular in shape ; they are often of very intricate structure; (c) Planetary Nebulae, which are round, or nearly so, and have sharply defined edges.

Dark Nebulae.

There are in the Milky Way a great number of dark patches, some of them entirely devoid of stars. A few of these were discovered by Sir John Herschel, who thought them to be holes through which we looked into empty space beyond; and Barnard, whose studies of these dark nebulae finally showed that they were caused by the presence of clouds of non-luminous matter, was at first loth to believe they were other than holes or spaces actually containing no stars. They vary in size from small patches only a minute of arc in diameter to the large lanes in Ophiuchus and Scorpius. Even the huge rift, 120° long, which divides the Milky Way from Cygnus to Centaurus, must be con sidered to be of a similar nature. It stretches right through the region where most of the globular clusters are found, and yet there is not a single one of these clusters within its boundaries.