Visual Binaries

Transcript: In a binary star the brighter or more massive star is usually called A, and the fainter or less massive star is called B. Castor A and Castor B is an example. The most common type of binary is a visual binary. Visual binaries can be resolved by a telescope. The separate images can be tracked over a period of months or years with images that show the orbit directly. The space motion is derived from a combination of the motion on the plane of the sky, the transverse velocity, with the radial motion measured by the Doppler Effect. Together these give the three dimensional space velocity. Most visual binaries have separations of about ten to twenty astronomical units. Over a hundred thousand are known. At a distance beyond about twenty parsec, at these typical separations visual binaries become unresolved, and sometimes one member of a visual binary is at the limit of a telescope’s observation and so the second motion must be inferred from the first.