Astronomical Spectroscopy - Stars

Stars

Astronomical spectroscopy began with Isaac Newton's initial observations of the light of the Sun, dispersed by a prism. He saw a rainbow of colour, and may even have seen absorption lines. These dark bands which appear throughout the solar spectrum were first described in detail by Joseph von Fraunhofer. Most stellar spectra share these dominant features of the Sun's spectrum: emission at all wavelengths across the electromagnetic spectrum (a continuum) with many discrete absorption lines at certain wavelengths, resulting from a deficiency of observed photons at those particular wavelengths.

Fraunhofer's original (1817) designations of absorption lines in the solar spectrum

The absorption lines in stellar spectra can be used to determine the chemical composition of the star. Each element is responsible for a different set of absorption lines in the spectrum, at wavelengths which can be measured extremely accurately by laboratory experiments. Then, an absorption line at the given wavelength in a stellar spectrum shows that the element must be present. Of particular importance are the absorption lines of hydrogen (which is found in the atmosphere of nearly every star); the hydrogen lines within the visual spectrum are known as Balmer lines.

In 1868, Sir Norman Lockyer observed strong yellow lines in the solar spectrum which had never been seen in laboratory experiments. He deduced that they must be due to an unknown element, which he called helium, from the Greek helios (sun). Helium wasn't conclusively detected on earth until 25 years later.

Also in the 1860s, emission lines (particularly a green line) were observed in the coronal spectrum during solar eclipses that did not correspond to any known spectral lines. Again it was proposed that these were due to an unknown element, provisionally named coronium. It was not until the 1930s that it was discovered that these lines were due to highly ionised iron and nickel, the high ionisation being due to the extreme temperature of the solar corona.

In conjunction with atomic physics and models of stellar evolution, stellar spectroscopy is today used to determine a multitude of properties of stars: their distance, age, luminosity and rate of mass loss can all be estimated from spectral studies, and Doppler shift studies can uncover the presence of hidden companions such as black holes and exoplanets.