Visual binary
A visual binary is a gravitationally bound system that can be resolved into two stars. These stars are estimated, via Kepler's 3rd law, to have periods ranging from a number of years to thousands of years. A visual binary consists of two stars, usually of a different brightness. Because of this, the brighter star is called the primary and the fainter one is called the companion. If the primary is too bright, relative to the companion, this can cause a glare making it difficult to resolve the two components. However, it is possible to resolve the system if observations of the brighter star show it to wobble about a centre of mass. In general, a visual binary can be resolved into two stars with a telescope if their centres are separated by a value greater than or equal to one arcsecond, but with modern professional telescopes, interferometry, or space-based equipment, stars can be resolved at closer distances.
For a visual binary system, measurements taken need to specify, in arc-seconds, the apparent angular separation on the sky and the position angle – which is the angle measured eastward from North in degrees – of the companion star relative to the primary star. Taken over a period of time, the apparent relative orbit of the visual binary system will appear on the celestial sphere. The study of visual binaries reveal useful stellar characteristics: Masses, densities, surface temperatures, luminosity, and rotation rates.
In order to work out the masses of the components of a visual binary system, the distance to the system must first be determined, since from this astronomers can estimate the period of revolution and the separation between the two stars. The trigonometric parallax provides a direct method of calculating a stars mass. This won't apply to the visual binary systems, but it does form the basis of an indirect method called the dynamical parallax.
In order to use this method of calculating distance, two measurements are made of a star, one each at opposite sides of the Earth's orbit about the Sun. The star's position relative to the more distant background stars will appear displaced. The distance, is found from the following equation,
Where is the parallax, measured in units of arc-seconds.
...
Wikipedia