Spectroscopy

Spectroscopy is the practice of obtaining, and the study of, the Spectrum of an astronomical object. Astronomical spectroscopy is the key technique by which the physical properties of astronomical bodies are revealed. Spectra are obtained with a Spectrograph, and recordings are made of the distribution of light relative to wavelength for the object concerned.

Current Spectroscopic instruments allow the precise measurement of the following: the flux distribution (energy per unit area per unit time per unit wavelength) of the continuum; the fluxes, wavelengths and shapes of Emission lines; and the wavelengths, strengths (the amounts of energy extracted from the continuum) and the detailed profiles (exact distribution of radiation with wavelength) of Absorption lines.

Reduction of spectrographic data to actual radiative fluxes requires calibration of the sensitivity of the detector, and also requires calibration with standard astronomical sources to correct for wavelength-dependent absorption by the Earth’s atmosphere (if the telescope is ground based) and the telescope/spectrograph optics. The determination of precise wavelengths for radial velocities requires observation of wavelength standards. In older spectrographs, these were iron arcs set within the spectrograph. The iron spectrum was displayed photographically on either side of the spectrum of the source. In modern spectrographs, the standards are gas emission tubes whose spectra are digitally compared with those of the source. The data then allow accurate physical analysis of astronomical sources. For example, the energy distribution of the continuum reveals the nature of the source, whether thermal (produced by a hot gas) or non-thermal (e.g. synchrotron emission from a supernova remnant). Emission line fluxes and wavelengths of nebulae, galaxies, quasars and a variety of other sources allow the determination of temperatures, densities, velocities (both radial velocities and those related to internal motions) and chemical compositions. Absorption line strengths and profiles allow the deduction of temperatures, densities, surface gravities, magnetic fields, rotation speeds and chemical compositions of stars.

These principles extend to all wavelength domains. Gamma-ray, X-ray and ultraviolet spectra are observed by space-borne telescopes and spectrographs. Infrared as well as visual spectra are observed with both ground-based and orbiting spectrographs. Radio telescopes are also fitted with spectrographs that use electronic techniques for the observation of emission, absorption and continuous spectra from a great variety of radio sources, from molecular clouds to galaxies. Spectroscopy has proved to be an invaluable method for us to understand the Physics of numerous astronomical bodies and promises to uncover many more mysteries in the future.

Questions to Ponder

• What are the different types of Spectroscopy?
• How has Spectroscopy aided in our Astronomical pursuits?
• What are the principal components of a Spectrograph?