In some spectroscopic binaries the spectra of both stars are visible and the lines are alternately double and single. Such stars are known as doubleline binaries. In others, the spectrum of only one of the stars is seen and the lines in the spectrum move periodically from blue to red and back again.
Determining the orbit of a spectroscopic binary is done by making a long series of observations of the radial velocity of one or more component of the binary. The observations are plotted against time, and from the resulting curve a period is determined. If the orbit is circular, then the curve will be a sine curve. If the orbit is elliptical, the shape of the curve will depend on the eccentricity of the ellipse and the orientation of the major axis with reference to the line of sight.
It is impossible to determine individually the semimajor axis a and the inclination of the orbit plane i. However, the product of the semimajor axis and the sine of the inclination (i.e., asin i) may be determined directly in linear units (eg, kilometers). If either a or i can be determined by other means, as in the case of eclipsing binaries, a complete solution for the orbit can be found.
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