The New Astronomy

During the nineteenth century, physics expanded beyond the traditional domain of mechanics to include the mathematical theory of optics, heat flow, and electromagnetism. Physics came to encompass a range of phenomena— infrared rays, the polarization of light, electromagnetic induction, and others—which had not even been known to exist in earlier science. In France in the first part of the century, and later, in Britain, Germany, and Italy, physicists devised sophisticated theories based on new physical concepts and powerful mathematical methods. Physics divided into experimental and theoretical branches and was increasingly applied in engineering and technology. Thermodynamics aided the analysis of heat engines, and the theory of electromagnetism was applied to the generation of energy and the transmission of waves. Physics also developed connections with chemistry. In electrolysis, chemical compounds were decomposed by an electrical current, while in spectral analysis the light emitted by burning elements was analyzed by optical methods.

The rapid growth of physics strongly influenced work in astronomy, particularly in the second half of the century. The term "the new astronomy" (self-consciously recalling Kepler's book of 1604) came to designate the study of the heavens by methods and theories of contemporary physics. Writers on astronomy understood themselves to be living in a new stage in the history of the subject, one in which the physical processes of the whole universe were opened up to detailed investigation. Astrophysics as a scientific discipline was well established by the end of the century.

Beginning in 1814 Frauhofer observed and cataloged a large number of dark lines in the spectrum of light from the Sun. In 1849 Léon Foucault (1819—1868) discovered that emission lines seen in the spectrum of a carbon arc in his laboratory appeared as absorption lines when sunlight was passed through a diffraction grating. Eleven years later, Gustav Robert Kirchoff (1824—1887) found that the solar D-lines seen in absorption coincided with the bright lines in the laboratory spectrum of sodium. The origin of the solar absorption lines was hypothesized by him to arise from a cooler layer of gas surrounding the Sun and containing sodium, through which the Sun's rays passed. Kirchoff and his fellow researcher Robert Bunsen (1811—1899) recorded the emission spectral lines of several common elements and, by matching these with absorption lines in the solar spectrum, were able to carry out the first chemical analysis of a celestial body.

Solar spectroscopy would lead to the discovery of a new element unknown on the Earth. Solar prominences are gaseous eruptions from the Sun first observed in the eighteenth century during total eclipses of the Sun. In the eclipse of 1868 Norman Lockyer (1836-1920) and Jules Janssen (1824-1907) identified a new spectral line in a prominence, which Lockyer called D3 because its wavelength was very close to that of sodium (D). Lockyer attributed this line to a new element, which he named helium (from the Greek helios, for "Sun.") Some 30 years later, helium gas was produced in the laboratory by burning a mineral, and much later yet, helium would be shown to play a crucial role in the energy processes in the centers of stars.

In the 1860s, two pioneers in the application of spectroscopy to the analysis of starlight were William Huggins (1824—1910) in England and Father Angelo Secchi (1818-1878) in Italy. Huggins attached a diffraction grating to his 15-inch refractor and fitted his observatory with chemical equipment for the burning of compounds and the recording of spectra. Although observation of stellar spectra was difficult, Huggins was able to show that different elements showed up in different stars and that the same elements found in stars were present on Earth. One of his most notable discoveries was the detection of emission lines in a planetary nebula in Draco, indicating that the nebula was gaseous in character and not an unresolved conglomeration of stars. Secchi pioneered the classification of stellar spectra, identifying four main types, and produced a catalog of over 4,000 stellar spectra.

The invention of photography in the 1830s would result in major advances in astronomy. Although in the first stage of its history, astronomical photography was primarily the preserve of amateurs and a few isolated professionals, these pioneers were important innovators in the development of the new technology. For example, much of the terminology of photography, including the word itself, was introduced by the astronomer John Herschel. Technical improvements such as the invention of dry plates and increases in photosensitivity brought photography by the 1880s into the scientific mainstream. Faint images invisible by ordinary optical methods could be captured by long photographic exposures. In 1887 the French Academy of Sciences initiated a large international project, the Carte du Ciel, to produce a comprehensive photographic map of the heavens. Photography transformed astrometry, the study of the positions of stars, and photometry, the study of the brightness of stars. The spectra of stars were also photographed and analyzed by Sechhi, who used spectral-line characteristics to classify stars according to their color and surface temperature. At Harvard College Observatory the modern classification system (O-B-A-F-G-K-M-R-N-S, from blue to red, hotter to cooler) was developed to classify stellar spectra, a photographic project that would culminate in the publication in 1924 of over 200,000 stellar spectra. The increasing use of the reflecting telescope coincided with the spread of photography, permitting ever more faint and distant objects to be observed and analyzed.

0 0

Post a comment