Astronomical spectroscopy

Astronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other hot celestial objects. Spectroscopy can be used to derive many properties of distant stars and galaxies, such as their chemical composition, temperature, density, mass, distance, luminosity, and relative motion using Doppler shift measurements.

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.^{[citation needed]} 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

Letter	Wavelength (nm)	Chemical origin	Colour range
A	759.37	atmospheric O2	dark red
B	686.72	atmospheric O2	red
C	656.28	hydrogen alpha	red
D1	589.59	neutral sodium	red orange
D2	589.00	neutral sodium	yellow
E	526.96	neutral iron	green
F	486.13	hydrogen beta	cyan
G	431.42	CH molecule	blue
H	396.85	ionised calcium	dark violet
K	393.37	ionised calcium	dark violet

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. Doppler shift studies of the spectral lines 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). The hypothetical nebulium that was invoked to account for certain bright lines in gaseous nebulae were shown by Ira Sprague Bowen in 1927 as due to doubly ionized oxygen at extremely low density. As Henry Norris Russell put it, "Nebulium has vanished into thin air." 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 1920s 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 red. 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 observed was a normal galactic spectrum, but highly redshifted. According to Hubble's Law, this implied that the quasar must be extremely distant in space and time, and therefore highly luminous. Quasars are now thought to be galaxies forming in the early years of our universe, with their extreme energy output powered by super-massive black holes.

Planets and asteroids

Planets and asteroids shine only by the reflected light of their parent star. 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 from gaseous atoms and molecules excited to fluorescence by sunlight and/or chemical reactions. Nearby comets can even be seen in X-ray as solar wind ions flying to the coma are neutralized. The cometary X-ray spectra therefore reflect the state of the solar wind rather than that of the comet. 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).

Mechanisms

Stars emit light at all frequencies in the core; molecules, atoms, and ions that may be present near the surface of the star will absorb strongly at certain characteristic frequencies. Thus the observed spectrum will contain relatively dark bands at the characteristic frequencies of constituents of the outer layers. Absorption spectra of nebulae and planetary atmospheres arise simply when molecules absorb light of from the parent star and re-emit it in random directions and at different frequencies.

Amateur Spectroscopy

It is not hard to view spectra of astronomical objects. A simple, homemade DVD spectrograph can be used to view solar spectra. Note that looking at the Sun directly or by reflection can result in temporary or permanent loss of vision. Proper equipment must be used, and children should be supervised.

Recently, there has been a resurgence in amateur astronomical spectroscopy. It is possible to use a digital camera or telescope to easily view spectra of astronomical objects.

Inexpensive diffraction gratings like the Paton Hawksley Star Analyser or the Rainbow Optics Star Spectroscope can be used to split the starlight. Software like RSpec can be used to plot the curves.

An excellent introduction to amateur spectroscopy is Astronomical Spectroscopy for Amateurs, published in 2011.

Also, Sky & Telescope Magazine has produced an excellent video interview that explains how to get started.

See also

• Gunn-Peterson trough
• Lyman-alpha forest
• Photometry
• Spectrometer
• Emission spectrum
• Atomic and molecular astrophysics

External links

• Libraries of stellar spectra - D. Montes, UCM
• Spectroscopy by Amateur Astronomers
• The Star Analyser 100 User Gallery