Binary Stars and Stellar Masses

Ouite often, two stars may appear to be close together in the sky, although they are really at very different distances. Such chance pairs are called optical binary stars. However, many close pairs of stars really are at the same distance and form a physical system in which two stars are orbiting around each other. Less than half of all stars are single stars like the Sun. More than 50 % belong to systems containingtwo or more members. In general, the multiple systems have a hierarchical structure: a star and a binary orbiting around each other in triple systems, two binaries orbiting around each other in quadruple systems. Thus most multiple systems can be described as binaries with several levels.

Binaries are classified on the basis of the method of their discovery. Visual binaries can be seen as two separate components, i. e. the separation between the stars is largerthan about 0.1 arc seconds. The relative position of the components changes over the years as they move in their orbits (Fig. 9.1). In astrometric binary stars only one component is seen, but its variable proper motion shows that a second invisible component must be present. The spectroscopic binary stars are discovered on the basis of their spectra. Either two sets of spectral lines are seen or else the Doppler shift of the observed lines varies periodically, indicating an invisible companion. The fourth class of binaries are the photometric binary stars or eclipsing variables. In these systems the components of the pair regularly pass in front of each other, causing a change in the total apparent magnitude.

Binary stars can also be classified on the basis of their mutual separation. In distant binaries the separation between the components is tens or hundreds of astronomical units and their orbital periods are from tens to thousands of years. In close binaries the separation is from about one AU down to the radius of the stars. The orbital period ranges from a few hours to a fewyears. The components of contact binaries are so close that they are touching each other.

The stars in a binary system move in an elliptical orbit around the centre of mass of the system. In Chap. 6 it was shown that the relative orbit, too, is an ellipse, and thus the observations are often described as if one component remained stationary and the other orbited around it.

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