Double star

[5] It is only the inability to telescopically observe two separate stars that distinguishes non-visual and visual binaries.

[1] Since that time, the search has been carried out thoroughly and the entire sky has been examined for double stars down to a limiting apparent magnitude of about 9.0.

[9] The unrelated categories of optical doubles and true binaries are lumped together for historical and practical reasons.

The position angle specifies the direction in which the stars are separated and is defined as the bearing from the brighter component to the fainter, where north is 0°.

In the measures of a visual binary, the position angle will change progressively and the separation between the two stars will oscillate between maximum and minimum values.

When observed over a short period of time, the components of both optical doubles and long-period visual binaries will appear to be moving in straight lines; for this reason, it can be difficult to distinguish between these two possibilities.

Superscripts are also used to distinguish more distant, physically unrelated, pairs of stars with the same Bayer designation, such as α1,2 Capricorni, ξ1,2 Centauri, and ξ1,2 Sagittarii.

[19] Visual doubles are also designated by an abbreviation for the name of their discoverer followed by a catalogue number unique to that observer.

For example, the pair α Centauri AB was discovered by Father Richaud in 1689, and so is designated RHD 1.