The New Student's Reference Work/Nebulæ

Nebulæ (nĕb′ ū́-lḗ), are celestial bodies resembling, in appearance, small patches of white cloud.  Hence the name, which is merely the Latin word for small cloud.  Many thousands of these nebulæ have been measured and catalogued, but with the exception of two or three all are invisible to the naked eye. Until 1864 — five years after the invention of the spectroscope by Kirchhoff and Bunsen — nebulæ were considered to be very distant star-clusters, or clusters made up of stars so small as not to be resolvable by any existing telescope.  But Sir William Huggins then examined a number of nebulæ with the spectroscope and found that they are not stars, but bodies composed of luminous gas, giving a spectrum of six or seven bright lines.  Two of these lines are fairly bright and are due, as has been proved by Keeler at Lick Observatory, to a substance not yet discovered on the earth.  The brightest of all nebulæ is the one in the girdle of Andromeda; the one in the sword-handle of Orion can also be seen at times by the naked eye.  Planetary nebulæ are those which show a more or less well-defined disc.  It is not impossible that nebulæ are merely stars in their early stages of development, later to pass through the phases of planetary nebulæ and nebulous stars.  See Schemer’s Astronomical Spectroscopy, translated by Frost.