Historical Introduction to Spectroscopy
Before looking in detail at how spectra are formed and what they can tell us about stars and other celestial objects it is worth briefly discussing the rise of spectroscopy in astronomy.
Isaac Newton showed that a glass prism could be used to split sunlight into a spectrum in 1666. Further studies by William Wollaston in 1802 revealed some black lines on the component colours of the solar spectrum. More detailed observations by Joseph von Fraunhofer resulted in 574 of these lines being mapped by 1815. These lines were named "Fraunhofer lines" in his honour. The image below shows a solar spectrum with Fraunhofer lines.
Two key questions arise from studying these lines - what do they represent and how are they formed? The solutions to these questions were to take some time. Leon Focault matched the lines produced by a sodium lamp with some of the dark lines in a solar spectrum in 1849. In 1857 Gustav Kirchoff and Robert Bunsen identified sodium in a solar spectrum. They found that a luminous solid or highly compressed hot gas could produce a continuous spectrum whilst a diffuse hot gas produced a spectrum with narrow bright lines on a black background.
As spectroscopes were coupled to telescopes additional chemicals were identified in the spectra of stars and nebulae. Sir Norman Lockyer and Jules Janssen independently discovered the element helium in solar spectra before it was isolated in a laboratory on Earth in 1895. The use of spectroscopy, coupled with the spread of photography for astronomical purposes gave rise to the science of astrophysics from astronomy in the second half of the nineteenth century.
Three general types of spectra were now known, a continuous spectrum, showing all the component colours of the rainbow, and two types of line spectra, the first, dark-line spectra like the solar spectrum and those from stars and the second, bright-line spectra as emitted from gas discharge tubes and some nebulae. The Swiss school teacher, Johann Balmer in 1885 developed an empirical formula that determined the wavelengths of the four visible lines in hydrogen's spectrum. Five years later the Swedish physicist, Johannes Rydberg expanded Balmer's formula to apply to some other elements. The Danish physicist, Niels Bohr, finally provided an explanation as to how spectral lines formed in the 1920s. His work relied on quantum physics and the concept of energy shells or orbits for electrons.