Editing Binary star (section)

====Visual binaries====
{{main|Visual binary}}
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]], or even high-powered [[binoculars]]. The [[angular resolution]] of the telescope is an important factor in the detection of visual binaries, and as better angular resolutions are applied to binary star observations, an increasing number of visual binaries will be detected. The relative brightness of the two stars is also an important factor, as glare from a bright star may make it difficult to detect the presence of a fainter component.

The brighter star of a visual binary is the ''primary'' star, and the dimmer is considered the ''secondary.'' In some publications (especially older ones), a faint secondary is called the ''[[comes]]'' (plural ''comites''; companion). If the stars are the same brightness, the discoverer designation for the primary is customarily accepted.<ref name=aitken41>{{cite book |title=The Binary Stars |author-link=Robert Grant Aitken |author=Aitken, R.G. |location=New York |publisher=Dover |year=1964 |page=41}}</ref>

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 coordinate system|polar coordinate]]s with the primary star at the origin, and the most probable [[ellipse]] is drawn through these points such that the [[Kepler's laws of planetary motion|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 respect to the primary on the plane of the sky. From this projected ellipse the complete elements of the orbit may be computed, where the [[semi-major axis]] can only be expressed in angular units unless the [[Parallax|stellar parallax]], and hence the distance, of the system is known.<ref name = "csep10"/>