In the midst of the early turmoil over the distance scale and the internal structure of stars, a few vague attempts were made to examine a whole new aspect of astronomy. Until this time, astronomers had been observing only in the optical part of the spectrum, that part ranging from about 300 to 1000 nm. […]

Bubble had shown definitively that spiral nebulae were external galaxies. In the 1930s he continued work on these objects with the Mount Wilson 2.5-m reflector, which was then the world’s largest telescope. Studies over the next decade revealed that the extra-galactic nebulae could be divided into four main classes: irregular, elliptical, normal spiral and barred […]

Progress on the nebulae in the early twentieth century was stim lated by the construction of large telescopes in good climates and the regular use of permanent records such as photographic plates and spectrograms. The Harvard telescope in Peru, the Crossley 1-m reflector at the Lick Observatory and the Mount Wilson 1.5-m played an important […]

Herschel’s discovery of stars embedded in nebulosity, the so-called planetary nebulae, reinforced a possible interpretation of the nebulae as planetary systems in the making. Such an interpretation correlated very well with the accepted theory of the origin of the Solar System developed by Pierre Simon de Laplace towards the end of the eighteenth century. According […]

The Nature of Nebulae Certain nebulous patches, such as the Magellanic Clouds in the south and the Andromeda Nebula in the north, are clearly visible to the naked eye and have undoubtedly been observed from early times. Comments on the Andromeda Nebula date back to the Arabs in the middle of the tenth century. Similarly, […]

The Draper catalogues provided astrophysicists with the massive numbers of spectra needed for statistical studies of the distribution of stellar types, which in turn might provide clues on stellar evolution. Two independent studies of particular interest, one by E.Hertzsprung, a Danish astronomer, the other by the American astronomer H.N.Russell, resulted in one of the corner-stones […]

In the mid-nineteenth century, very little was known about stars other than positions, apparent magnitudes, and, in a few cases distances. The method of determining distance, by trigonometrical parallax, was limited to those stars within a hundred parsecs of the Sun ; thus, even by 1890 fewer than 100 stars had known distances. Spectral analysis […]

The dark lines in the solar spectrum, noted by Wollaston in 1802, were first mapped systematically by the physicist Joseph Fraunhofer. By 1817 Fraunhofer had described hundreds of lines and bands, the principal ones to which he assigned letters A, B, C and so on, which are used even today. Furthermore, he believed that the […]

Nineteenth-century ideas on the nature of the Sun can he divided into two distinct periods: the first characterized by the simple visual observations of sunspots, the second, by the application of photography and spectroscopy to the Sun. In the early nineteenth century, sunspots were the major clue to the nature of the Sun. The common […]

The discovery at the turn of the nineteenth century that the light from the Sun dispersed through a spectroscope produced not only a continuous spectrum, as Newton had shown in 1666. But one crossed by dark lines – the Fraunhofer lines as they came to be known – provoked much concern among astronomers, physicists and […]