Spectroscopy – Analysing Light from the Stars: 6th September 2019

Dr David Boyd (BAA & NAS)

The science of spectroscopy started with Isaac Newton’s experiments with prisms and light which are carefully documented in his book Optics. Spectra can be created with either a prism or a diffraction grating. The latter can either reflect or transmit light. Although Newton thought light was made of particles, most of his contemporaries believed light was a wave, a view apparently vindicated by Thomas Young’s double slit experiment and by James Clerk Maxwell’s theory of electromagnetism. However Planck and Einstein later argued light could also be considered as bundles of energy called photons. The product of the wavelength and energy of a photon is a constant known as the velocity of light ~300,000 km/s. The next major advance in astronomical spectroscopy was the discovery by Joseph von Fraunhofer that the solar spectrum contains many dark bands which he labelled A to K. Gustav Kirchhoff and Robert Bunsen developed an early spectroscope which they used to identify the spectral signature of several elements put into the flame of a Bunsen burner. They recognised that some of Fraunhofer’s dark lines in the solar spectrum matched the positions of spectral lines of elements that had seen in their laboratory so they deduced that these elements existed in the Sun. Kirchhoff produced his laws which recognised there were three types of spectra: continuous, absorption and emission. William and Margaret Huggins developed the subject of astrophysics using spectroscopy and recognised that all celestial objects were probably composed of the same elements we see here on Earth. They showed that planetary nebulae were composed of gas not stars and measured the velocity of Sirius using the Doppler Effect. Janssen and Lockyer recognised that an emission line seen in the solar chromosphere during a total eclipse did not match any known element and Lockyer suggested it was a new element he called Helium, found 27 years later on Earth. In 1872 Henry Draper took the first photograph of stellar spectra. At Harvard College Observatory objective prism photographic plates were used to develop a classification scheme for stellar spectra. After several false starts this was eventually based on stellar temperature and the OBAFGKM naming system for spectral types was adopted. This in turn became the basis for the famous Hertzsprung-Russell Diagram which classified stars by spectral type and absolute luminosity. An understanding of why absorption or emission lines appeared in spectra at specific wavelengths was finally achieved by Neils Bohr with his quantum theory of the hydrogen atom. Spectral lines occurred at wavelengths which matched the transitions between electron energy levels in atoms. Spectroscopy is now used as an analysis technique across the whole electromagnetic spectrum.

The talk then described how amateur astronomers could record stellar spectra. You need a way to collect light (eg a telescope), a way to disperse light into a spectrum (eg a prism, grating or grism) and a way to record the spectrum (eg a camera). Because the light from a star is spread out in a spectrum it requires long exposures, but with a spectrum you can understand more about the star than simply by measuring its total light output using photometry. The simplest and least expensive entry into spectroscopy is using a transmission diffraction grating (eg Star Analyser SA100). This will produce low resolution spectra which are capable of identifying the spectral type of bright stars. For results which are more scientifically useful a slit spectroscopy must be used. There are several commercial slit spectroscopes on the market and amateurs are also now using 3D printing to construct their own instruments using published designs. There are several free software packages available to analyse amateur spectra. The results can then be submitted to spectroscopic databases where they are accessible to professional astronomers. There is a good introductory book from Francois Cochard: Successfully Starting in Astronomical Spectroscopy.

The talk concluded with several examples of results obtained by the speaker using a LISA spectroscope from Shelyak Instruments on a C11 scope. Methane absorption bands in the spectrum of Neptune illustrated the potential for identifying the components of exoplanet atmospheres using large professional telescopes. A spectrum of comet C2014 Q2 Lovejoy showed the strong emission line of diatomic carbon which gives images of comets a green colour. A repeat of William Huggins’ observation of the Cats Eye planetary nebula NGC 6543 showed the strong emission lines of doubly ionised oxygen [OIII] which at the time were unknown to Huggins. Measurements of the Doppler shift of spectra of a star over time revealed its binary nature and provided measurements of several parameters of its binary orbit. Spectra taken of the type 1a supernova SN2014J in M82 showed an initial expansion velocity of 15,000 km/s reducing to almost zero after 3 months. Finally, a spectrum of the brightest quasar 3C 273 showed the hydrogen beta line was red shifted by 768 angstroms indicating a recession velocity of 47,400 km/s, a measurement consistent with the Hubble Law for the expansion of the universe.

In summary, spectroscopy is an important technique in developing our understanding of the universe, it provides insight into the nature and behaviour of many celestial objects and enables us to see things that are not detectable by other means such as photometry. As an amateur, you can make a scientifically useful contribution by recording and reporting your spectra.

Notes and summary by David Boyd.