In 1610, two years after the invention of the telescope, the Orion Nebula, which looks like a star to the naked eye, was discovered by a scientist named Nicolas -Claude Fabri Peiresc. In 1656, Christan Huygens, the Dutch scholar and scientist, using his own greatly improved instruments, was the first to describe the inner region of the nebula and to determine that its inner star is not a single but a compact quadruple system. At the beginning of the 18th century, observational astronomers gave high priority to the search for comets. One of the results of their research was the discovery of numerous bright nebulae. Several catalogs of special objects have been compiled by comet researchers; the best known by far is that of the Frenchman Charles Messier, who in 1781 drew up a catalog of 103 nebulous, or extended, objects in order to avoid their confusion with comets. Most are star clusters, 35 are galaxies and 11 are nebulae. Even today, many of these objects are commonly referred to by their Messier catalog number; M20, for example, is the large Trifid nebula, in the constellation Sagittarius. THE WORK OF THE HERSCHELS By far the greatest observers of the early and mid-19th century were the English astronomers William Herschel and his son John. Between 1786 and 1802, William Herschel, helped by his sister Caroline, compiled three catalogs totaling approximately 2,500 clusters, nebulae and galaxies. John Herschel later added to the catalogs another 1,700 nebulous objects in the southern sky visible from the Cape Observatory in South Africa but not from London and another 500 objects in the northern sky visible from England. The Herschel catalogs formed the basis of JL Dreyer's large New General Catalog (NGC), published in 1888. It contains the location and a brief middle of paper......at high spectral resolution (i.e. (say to distinguish slightly different wavelengths). For even higher spectral resolution, astronomers use Fabry-Pérot interferometers. Spectra provides powerful diagnostics of physical conditions within nebulae. Images and spectra provided by satellites orbiting Earth, including the Hubble Space Telescope, have provided data of unprecedented quality. Ground-based observations have also played a major role in recent advances in the scientific understanding of nebulae. Gas emission in the radio and submillimeter wavelength ranges provides crucial information on physical conditions and molecular composition. Large radio telescope arrays, in which several individual telescopes operate collectively as a single enormous instrument, provide spatial resolutions in the radio regime far superior to those achieved so far by optical means...