Observations and the universe

The first detailed and extensive studies of nebulae were made by William and Caroline Herschel in the late eighteenth century. These researches led to wide-ranging debates on the nature of these objects: are they truly clouds of nebulous material or are they perhaps distant star systems? By the end of the nineteenth century, astronomers generally agreed that nebulae are either within or closely linked to our own stellar system, and that no galaxies beyond our own Galaxy had been sighted, even in the largest telescopes. But early in the twentieth century, astronomers managed to fashion novel ways to determine the distances to a class of nebulae known as spiral nebulae. With the aid of these distance indicators, the spiral nebulae were transformed into galaxies of stars. Modern extragalactic cosmology thereby came into being in the first few decades of the twentieth century.