Double and Mutiple Stars

A surprising number of stars have turned out to be members of binary or multiple systems. Some of these are visible in small telescopes, and a few make for very pretty views due to contrast or position. Double stars are usually bright compared to deep sky objects, and thus are easier to find and less affected by light pollution. The challenge lies in resolving or splitting the components. The larger the aperture and the better the optics, the better your chances will be. Use fairly high magnification to reduce glare and put space between the stars. Many double stars are showcases in even the smallest, cheapest telescopes.

What is a double or binary star ? It's a pair of stars in orbit around a common centre of gravity under their mutual gravitational attraction. The individual stars in the system are called components: component A is the primary or the brightest and component B is the secondary or the faintest (in general). There are several types of binaries according to the techique of detection used.

Visual Double Stars have two components that can be resolved visually through a telescope. In more recent times, observers used photometric techniques to get high precision measurements of the global magnitude and colours of these systems.

Eclipsing Double stars occur when the orbit is aligned with respect to Earth, one component may occult the other one. These "eclipses" are seen as periodic perturbations (drops) on the light curve (luminosity as a function of time, e.g. HIC 31173). These double stars are called eclipsing binaries

Spectroscopic Double Star Systems consist of components that are too close to each other to be resolved as separate stars, are studied by spectroscopy (study of the stellar atmospheres, radial velocity measurements via the Doppler effect).The study of their spectra shows periodic shifts due to the orbital motion around a mean velocity which is the motion in the radial direction of the system as a whole.

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