Visual binary
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A visual binary star is a binary star for which the angular separation between the two components is great enough to permit them to be observed as a double star in a telescope. The resolving power of the telescope is an important factor in the detection of visual binaries, and as telescopes become larger and more powerful an increasing number of visual binaries will be detected. The brightness of the two stars is also an important factor, as brighter stars are harder to separate due to their glare than dimmer ones are.
The brighter star of a visual binary is considered the "primary" star, and the dimmer is considered the "secondary." The position angle of the secondary with respect to the primary is measured, together with the angular distance between the two stars. The time of observation is also recorded. After a sufficient number of observations are recorded over a period of time, they are plotted in polar coordinates with the primary star at the origin, and the most probable ellipse is drawn through these points such that the Keplerian law of areas is satisfied. This ellipse is known as the "apparent ellipse," and is the projection of the actual elliptical orbit of the secondary with reference to the primary on a plane perpendicular to the line of sight of the observer. From this projected ellipse the complete elements of the orbit may be computed, with the semimajor axis being expressed in angular units unless the stellar parallax, and hence the distance, of the system is known.