What is a double star and how do you observe it?

A double star, also known as a binary star, is a system of two stars that orbit around a common center of mass. Double stars can be classified into three types: optical doubles, visual binaries, and spectroscopic binaries. Optical doubles are stars that appear close together in the sky, but are not gravitationally bound. Visual binaries are stars that can be observed as two separate stars through a telescope, while spectroscopic binaries can only be detected by analyzing their spectral lines.

Observing double stars can be a fascinating experience for amateur and professional astronomers alike. The best way to observe a double star is with a telescope, ideally one with a large aperture and high magnification. A refracting telescope, which uses lenses to bend and focus light, is preferred for observing double stars because it provides a clearer and more detailed image. To observe a double star, the observer must first identify the star and then adjust the focus of the telescope until the two stars appear separated. The observer can then record the position of the stars and the angle between them, which can be used to calculate their orbit and other characteristics.