Simply astronomical – the Square Kilometre Array
Box 2 | Spectroscopy – a vital tool
Spectroscopy is a vital part of an astronomer's toolbox. By studying the spectrum of an object, astronomers can tell whether it is a star, galaxy, quasar, or some other object. The spectrum also provides information on other characteristics such as whether a star is hot or cool, old or young, and whether it is moving towards us or away.
Astronomers study three types of spectra. The first is the so-called black body spectrum where the emission from an object such as a star varies smoothly across a continuous range of wavelengths. The region or wavelength emitting the most energy provides information on the temperature of the object (eg, the core of a star).
A second type is known as an absorption spectrum where dark lines appear against a continuous background spectrum. Absorption spectra are produced when light from a distant object passes through a cloud of cool gas in interstellar space. Atoms and ions in the cloud absorb the light at characteristic wavelengths and produce the dark lines. The wavelengths of these lines provide information on the chemical composition of the cloud.
In contrast, an emission spectrum consists of bright lines appearing on a dark background, corresponding to a number of characteristic wavelengths. These spectra can be produced, for example, in clouds of interstellar gas heated by nearby stars and can also be used to determine the chemistry of the cloud.
The three types of spectra are not only used to study visible light, but all parts of the electromagnetic spectrum. An important emission line in radio astronomy is from atomic hydrogen at a wavelength of 21 centimetres. Future studies of 21 centimetre emission are expected to shed light on the early evolution of the universe.
Boxes
Box 1. What is a radio telescope?
Box 3. Is Einstein still right?
Related sites
Spectroscopy: unlocking the secret in starlight (CSIRO, Australia)
Spectra and what scientists can learn from them (NASA, USA)
Wavebands – photons from space (CSIRO, Australia)
External sites are not endorsed by the Australian Academy of Science.
Posted April 2009.