Double star

Lore & Background

Binary stars are important to stellar astronomers as knowledge of their motions allows direct calculation of stellar mass and other stellar parameters. The only (possible) case of 'binary star' whose two components are separately visible to the naked eye is the case of Mizar and Alcor (though actually a multiple-star system), but it is not known for certain whether Mizar and Alcor are gravitationally bound. Since the beginning of the 1780s, both professional and amateur double star observers have telescopically measured the distances and angles between double stars to determine the relative motions of the pairs. If the relative motion of a pair determines a curved arc of an orbit, or if the relative motion is small compared to the common proper motion of both stars, it may be concluded that the pair is in mutual orbit as a binary star. Otherwise, the pair is optical. Multiple stars are also studied in this way, although the dynamics of multiple stellar systems are more complex than those of binary stars. The following are three types of paired stars: Optical doubles are unrelated stars that appear close together through chance alignment with Earth. Visual binaries are gravitationally bound stars that are separately visible with a telescope. Non-visual binaries are stars whose binary status was deduced through more esoteric means, such as occultation (eclipsing binaries), spectroscopy (spectroscopic binaries), or anomalies in proper motion (astrometric binaries). Improvements in telescopes can shift previously non-visual binaries into visual binaries, as happened with Polaris A in 2006. It is only the inability to telescopically observe two separate stars that distinguishes non-visual and visual binaries.

Reader's Guide

Observation of visual double stars by visual measurement will yield the separation, or angular distance, between the two component stars in the sky and the position angle. 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°. These measurements are called measures. 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. Plotting the measures in the plane will produce an ellipse. This is the apparent orbit, the projection of the orbit of the two stars onto the celestial sphere; the true orbit can be computed from it. Although it is expected that the majority of catalogued visual doubles are visual binaries, orbits have been computed for only a few thousand of the over 100,000 known visual double stars. Confirmation of a visual double star as a binary star can be achieved by observing the relative motion of the components. If the motion is part of an orbit, or if the stars have similar radial velocities or the difference in their proper motions is small compared to their common proper motion, the pair is probably physical. 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.

Did You Know?

• Mizar, in Ursa Major, was observed to be double by Benedetto Castelli and Galileo.
• The bright southern star Acrux, in the Southern Cross, was discovered to be double by Fontenay in 1685.
• At least 1 in 18 stars brighter than 9.0 magnitude in the northern half of the sky are known to be double stars visible with a 36 in telescope.
• The Washington Double Star Catalog contains over 100,000 entries, each of which gives measures for the separation of two components.