Aspects of this topic are discussed in the following places at Britannica.
...only from binary systems and only if the scale of the orbits of the stars around each other is known. Binary stars are divided into three categories, depending on the mode of observation employed: visual binaries, spectroscopic binaries, and eclipsing binaries.
If the images of the two components of a binary star system can be separated by telescope, it is called a visual binary. Stars whose components are too close to each other to be distinguished visually can sometimes be identified as binaries by spectroscopic observation; as the members of these spectroscopic binaries move alternately toward the Earth and away from it, a Doppler effect of...
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