Astronomical spectroscopy

Astro spectroscopy is the name given to the wavelength-dependent analysis of the radiation of astronomical objects. In astronomy, the electromagnetic waves are almost exclusively examined, i.e. radio waves, infrared, light, UV, X-ray and gamma radiation. Only gravitational wave detectors and astroparticle physics, for example, examines the neutrinos represent an exception

Continuous spectra

The continuous spectrum of a star obeys with the exception of the short-wave ultraviolet and X-ray regions almost exactly the Planck 's radiation law, so that one can assign each star an effective temperature at which the total emitted energy of the star is equal to that of a black body at this temperature. The wavelength of the radiation maximum (which is for most stars in the visible light) depends linearly with the photospheric temperature together ( Wien's displacement law, discovered in 1896 ). This surface temperature and the visible color of the star corresponds essentially to its spectral type. In the infrared and radio astronomy, this correlation is also applied to cooler objects such as interstellar dust and gas clouds.


From the line spectrum which emit objects such as stars, gas, mist, or the interstellar gas, one gains information on chemical substances and elements present in the objects, and on the frequency. Since changing the strength of the spectral lines with temperature and pressure, can be out of the line spectrum of temperature and gravitational acceleration, on which depends the pressure on a star's surface, determine.

From the width of the spectral lines in the light of a star, conclusions can be on the tangential velocity and thus drag the rotation of the star. Because if moves away the one edge of the star due to its own rotation to the observer to and the opposite edge, each spectral line is shifted by the Doppler effect to shorter wavelengths ( blue shift ) or to longer wavelengths ( red shift ) out. As one can observe only the light of the total radiating surface because of the large distance of the stars, thereby broadening the spectral lines.

For double stars, in turn, the Doppler effect to determine the path velocity of the two stars, insofar as they have larger angular distance ( visual double stars ). A very close, spectroscopic binary revealed by periodic doubling or broadening of spectral lines. For single stars of the Zeeman effect allows conclusions to the prevailing magnetic field.

A very important method is the spectroscopic determination of the radial velocity of stars. Along with their astrometric observable proper motion it gives the spatial movement, resulting in eg the Sonnenapex and the rotation of the Milky Way can be calculated - see also Oort rotation formulas.

Looking at the spectra of the light emitted by distant galaxies, it is found that the shift of the spectral lines depends on the distance of the galaxies. The farther away a galaxy is, the more the lines are shifted into the red. This effect is named after its discoverer Hubble effect. It is concluded that expands the universe, and indirectly to its beginning, the so-called Big Bang. In the most distant galaxies, where others fail distance measurement methods, the distance is determined inversely from the red shift.

For the analysis of exoplanetary atmospheres, the Astro spectroscopy are used to make statements about habitability and biomarkers.


Before the introduction of photography spectroscopes were used for visual observation and measurement of the spectral lines. They usually consisted of a prism and a the angle to variable eyepiece for high-resolution solar spectroscopy, or a permanently attached in the eyepiece prism for star and nebula spectroscopy. Later, diffraction gratings were used (see Gitterspektroskop ). With the photography, these methods increasingly replaced the spectrograph, also with the faint spectra are measured.


The astronomical spectroscopy began with Joseph Fraunhofer, who discovered 1814 dark lines in the solar spectrum, but she could not explain. The interpretation of these Fraunhofer lines was achieved only as a result of attempts by Kirchhoff and Bunsen, at the glowing gases have typical colors found in 1859.

From the 1860s, unexplained lines led to the postulation of hypothetical repeated elements as the Nebuliums, which could be attributed to unknown transitions of known elements later on in the lab. However, in 1868 gave the first clues to the solar spectrum then unknown element helium.

Around the turn of the century you could already ray spectra of the major planets and distant galactic emission nebula. Among other things, the 1877 discovered canals of Mars were interpreted early 20th century by fake spectra of mosses and lichens, which was refuted only in the 1960s by the Mariner spacecraft.