Spectroscopic binary star
Spectroscopic binary stars are star systems that consist of two suns that circle closely around each other. They are so close that the resolution of even the largest telescope is insufficient to distinguish them directly. This is usually only possible indirectly through spectroscopy , as the name suggests.
Such double stars can be detected by a periodic Doppler effect in the common line spectrum or sometimes by other spectral anomalies. For stars that are not too weak, both radial velocities can be determined, from which the mass ratio follows. The sum of the masses and thus also the two individual masses are obtained from the orbits . These data and measurement methods, which are essential for astrophysics, are also possible with other (especially the many telescopic) double stars.
Shift of the spectral lines
The binary stars treated here reveal themselves through their periodic Doppler effect in the spectrum or through other spectral anomalies. The orbital movements cause a regular shift of the spectral lines , the often common spectral lines split up ( two-spectral system ): when one star moves away from us, the Doppler effect pushes its spectral lines towards red ( redshift ), while at the same time the spectral lines of the other star, the moving towards us, being shifted towards blue. The period of the shift can be used to determine the orbital time , together with the magnitude of the shift, the orbital speed and the distance between the stars. The orbit inclination towards the line of sight must also be taken into account.
If the brightness is similar, the two color bands overlap to form a mixed spectral type. However, if the difference in brightness is greater than a magnitude , the spectrum of the main star outshines that of the companion, and the line shift can only be determined to one side. In such one- spectrum systems the orbital inclination and the mass ratio of the two stars remain unknown.
The highest measured radial velocities of very narrow systems are 1500 km / s. Less than 1 km / s are hardly detectable; they occur in extremely long orbital systems or when the orbit plane is perpendicular to the line of sight.
Properties of the components
Many spectroscopic binary stars belong to the group of giant stars ; the first spectroscopic discovery was the bright star Mizar in the drawbar of the Big Dipper in 1889 , which even represents a quadruple system (2 pairs of stars). His distant companion Alkor (the so-called eye examiner ) is also a spectroscopic double star.
Because the spectral lines are particularly easy to measure in bright stars, Spica (main star in Virgo ), Algol in Perseus and Beta Aurigae were revealed as close pairs of stars in 1889 , as well as thousands of weaker stars in the following years.
With stars that are not too weak, interferometry can enable additional measurements and statements about the components of the binary star.
Typical cycle times and spectral classes
While telescopic double stars need several years to centuries to orbit one another, the period of spectroscopic systems is a few hours to weeks. The extreme values are 1 to 5 years (for distant stars whose angular distance is too small for telescopic discovery) or 80 minutes for WZ Sagittae . This is at the same time a cover-changer with such a close distance that matter overflows into a compact companion and turns it into a recurring nova .
All spectral classes are represented in spectroscopic binary stars. Particularly close components usually belong to the spectral classes O, B, A and F, while giant stars of classes G, K and M often occur in longer orbital times.
See also
literature
- H. Zimmermann, A. Weigert: Lexicon of Astronomy , p.55-57. Spektrum-Verlag, Heidelberg-Berlin 1999
- Wulff Dieter Heintz: Double stars . Series The Scientific Pocket Book Volume 30, 200p., Goldmann-Verlag, Munich 1971