Astro spectroscopy
Astrospectroscopy is the name for the wavelength-dependent analysis of the radiation from astronomical objects . In astronomy , almost exclusively electromagnetic waves are examined spectroscopically , i. H. Radio waves , infrared , light , UV , X-ray and gamma radiation . Only gravitational wave detectors and astroparticle physics , which investigate neutrinos, for example, are an exception.
Continuous spectra
With the exception of the short-wave ultraviolet and X-ray ranges, the continuous spectrum of a star obeys Planck's law of radiation almost exactly , so that an effective temperature can be assigned to each star at which the total energy emitted by the star is equal to that of a black body with this temperature. The wavelength of the radiation maximum (which is in the visible light of most stars) is linearly related to the photosphere temperature (Wien's law of displacement , discovered in 1896). This surface temperature or the visible color of the star essentially corresponds to its spectral class . In infrared and radio astronomy , this correlation is also applied to cooler objects such as interstellar dust or gas clouds .
Spectral lines
From the spectrum of lines that objects such as stars , gas nebulae or interstellar gas emit, information is obtained about chemical substances and elements that are present in the respective objects, as well as about their frequency. Since the strength of the spectral lines also change with temperature and pressure , the temperature and acceleration due to gravity , on which the pressure on a star's surface depends, can be determined from the line spectrum .
Conclusions about the tangential velocity and thus the rotation of the star can be drawn from the width of the spectral lines in the light of a star . Because if one edge of the star moves towards the observer due to its own rotation and the opposite edge moves away, the Doppler effect shifts each spectral line to shorter wavelengths ( blue shift ) or to longer wavelengths ( red shift ). Because of the great distance between the stars, one can only observe the light from the entire radiating surface, which means that the spectral lines widen.
In the case of double stars, on the other hand, the Doppler effect enables the orbital speed of both stars to be determined, provided that they have a larger angular distance (visual double stars). A very narrow, spectroscopic double star is revealed by periodic doubling or broadening of the spectral lines. In the case of single stars, the Zeeman effect allows conclusions to be drawn about the prevailing magnetic field .
A very important method is the spectroscopic determination of the radial velocity of stars. Together with their astrometrically detectable proper movement , it results in the spatial movement, from which z. B. the sun apex and the rotation of our Milky Way system can be calculated - see also Oort's rotation formulas .
If one looks at the spectra of the light that is emitted by distant galaxies , one finds that the shift in the spectral lines depends on the distance between the galaxies. The further away a galaxy is, the more the lines are shifted into the red. This effect is called the Hubble effect after its discoverer . From this one concludes that the universe is expanding, and indirectly to its beginning, the so-called Big Bang . For the most distant galaxies, where other distance measurement methods fail, the distance is determined from the redshift.
Astrospectroscopy can also be used for the analysis of exoplanetary atmospheres in order to be able to make statements about habitability and biomarkers .
technology
Before photography was introduced , spectroscopes were used to visually view and measure spectral lines . They usually consisted of a prism and an eyepiece with variable angle for high-resolution solar spectroscopy, or a prism fixed in the eyepiece for star and nebula spectroscopy. Diffraction gratings were also used later (see grating spectroscope ). With photography, these methods increasingly replaced the spectrograph , with which even faint spectra can be measured.
history
Astronomical spectroscopy began with Josef Fraunhofer , who discovered dark lines in the solar spectrum in 1814 but was not yet able to explain them. The interpretation of these Fraunhofer lines was only possible as a result of the experiments by Kirchhoff and Bunsen , who in 1859 determined typical colors for glowing gases.
From the 1860s onwards, inexplicable lines repeatedly led to the postulation of hypothetical elements such as nebulium , which could only later be traced back to transitions of known elements unknown from the laboratory. In 1868, however, the solar spectrum provided the first indications of the then still unknown element helium .
At the turn of the century one could already spectroscopically the large planets and distant galactic emission nebulae . Among other things, the Martian canals discovered in 1877 at the beginning of the 20th century were interpreted by supposed spectra of moss and lichen, which was only refuted by the Mariner space probes in the 1960s .
literature
- Thomas Eversberg, Klaus Vollmann: Spectroscopic Instrumentation - Fundamentals and Guidelines for Astronomers. Springer, Heidelberg 2014, ISBN 3662445344
- John B. Hearnshaw: The analysis of starlight - two centuries of astronomical spectroscopy. Cambridge Univ. Press, New York 2014, ISBN 1-10-703174-5
- James B. Kaler: Stars and their spectra - an introduction to the spectral sequence. Cambridge Univ. Press, Cambridge 1997, ISBN 0-521-30494-6
- Günter D. Roth: History of Astronomy , Kosmos-Verlag, Stuttgart 1987
- J. Bennett, M. Donahue, N. Schneider, M. Voith: Astronomy , Chapter 5 Light and Matter . Textbook, Ed. Harald Lesch, 5th edition (1170 pages), Pearson-Studienverlag, Munich-Boston-Harlow-Sydney-Madrid 2010.
Web links
Individual evidence
- ↑ Lisa Kaltenegger , et al .: Deciphering Spectral Fingerprints of Habitable Exoplanets. Astrobiology, Vol. 10, Issue 1, pp. 89-102, 2010, abstract @ adsabs.harvard.edu, pdf @ arxiv.org, accessed October 16, 2012