'Astronomical spectroscopy' is the technique of
spectroscopy used in
astronomy. As spectroscopy is described in its own article, this article focuses on its use in astronomy. The object of study is the
spectrum of
electromagnetic radiation, including visible light, which
radiates from
stars and other celestial objects. Spectroscopy can be used to derive many properties of distant stars and galaxies, such as their chemical composition and also their motion, via the
Doppler shift.
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 two dominant features of the Sun's spectrum: emission at all wavelengths across the optical spectrum (the
continuum) with many discrete
absorption lines superimposed on top.
Fraunhofer's original (1817) designations of absorption lines in the solar spectrum
Fraunhofer and
Angelo Secchi were among the pioneers of spectroscopy of the Sun and other stars. Secchi is particularly noted for classifying stars into
spectral types, based on the number and strength of the absorption lines in their spectra. Later the origin of the spectral types was found to be related to the temperature of the surface of the star: particular absorption lines can be observed only for a certain range of temperatures; because only in that range are the involved atomic
energy levels populated.
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.
Nebulae
In the early days of telescopic astronomy, the word
nebula was used to describe any fuzzy patch of light that didn't look like a star. Many of these, such as the
Andromeda Nebula, had spectra that looked in many ways a lot like stellar spectra, and these turned out to be
galaxies. Others, such as the
Cat's Eye Nebula, had very different spectra. When
William Huggins looked at the Cat's Eye, he found no continuous spectrum like that seen in the Sun, but just a few strong
emission lines. These lines did not correspond to any known elements on earth, and so just as
helium had been identified in the Sun, astronomers suggested that the lines were due to a new element, 'nebulium' (occasionally ''nebulum'' or ''nephelium''). In fact, it was discovered by
Ira Sprague Bowen in 1927 that the lines were due to
doubly ionized oxygen, a very familiar element. But nebulae are typically extremely
rarefied, much less dense than the hardest vacuum ever produced on earth. In these conditions, atoms behave quite differently and lines can form which are suppressed at normal densities. These lines are known as
forbidden lines, and are the strongest lines in most nebular spectra.
Galaxies
The spectra of
galaxies look somewhat similar to stellar spectra, as they consist of the light from millions of stars combined. Galactic spectroscopy has led to many fundamental discoveries.
Edwin Hubble discovered in the 1920's that, apart from the nearest ones (those in what is known as the
Local Group), all galaxies are receding from the Earth. The further away a galaxy, the faster it is receding (see
Hubble's Law). This was the first indication that the universe originated from a single point, in a
Big Bang.
Doppler shift studies of
clusters of galaxies by
Fritz Zwicky found that most galaxies were moving much faster than seemed to be possible, from what was known about the mass of the cluster. Zwicky hypothesised that there must be a great deal of non-luminous matter in the galaxy clusters, which became known as
dark matter.
Quasars
In the 1950s, some strong
radio sources were found to be associated with very dim objects that seemed to be very blue. These were named ''Quasi-stellar radio sources'', or
quasars. When the first spectrum of one of these objects was taken, it was something of a mystery, with absorption lines at wavelengths where none were expected. It was soon realised that what was being seen was a normal galactic spectrum, but highly
redshifted. According to
Hubble's Law, this implied that the quasar must be extremely distant, and therefore highly luminous. Quasars are now thought to be galaxies forming, with their extreme energy output being powered by super-massive
black holes.
Planets and asteroids
Planets and
asteroids shine only by reflecting the Sun's light. The reflected light contains absorption bands due to
minerals in the rocks present for rocky bodies, or due to the elements and molecules present in the atmospheres of the
Gas giants. Asteroids can be classified into three main types, according to their spectra: the C-types are made of carbonaceous material, S-types consist mainly of
silicates, and M-types are 'metallic'. C- and S-type asteroids are the most common.
Comets
The spectra of
comets consist of a reflected solar spectrum from the dusty clouds surrounding the comet, as well as emission lines formed when the
solar wind collides with gases surrounding the comet. Analysis of the composition of comets has shown that they are made of pristine material from the earliest days of the solar system. Many
organic chemicals are known to exist in comets, and it has been suggested that cometary impacts provided the Earth with much of the water for its
oceans and the chemicals necessary for the formation of
life. It has even been suggested that life may have been brought to earth from
interstellar space by comets (the
Panspermia theory).
See also
★
Gunn-Peterson trough
★
Lyman-alpha forest
★
Photometry
★
Spectrometer
★
Emission spectrum
External links
★
The Science of Spectroscopy - supported by NASA. Spectroscopy education wiki and films - introduction to light, its uses in NASA, space science, astronomy, medicine & health, environmental research, and consumer products.
★
Libraries of stellar spectra - D. Montes, UCM
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