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 few 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.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 star's mass. This will not apply to the visual binary systems, but it does form the basis of an indirect method called the dynamical parallax.The two stars orbiting each other, as well as their centre of mass, must obey Kepler's laws. This means that the orbit is an ellipse with the centre of mass at one of the two foci (Kepler's 1st law) and the orbital motion satisfies the fact that a line joining the star to the centre of mass sweeps out equal areas over equal time intervals (Kepler's 2nd law). The orbital motion must also satisfy Kepler's 3rd law.Binary systems are particularly important here – because they are orbiting each other, their gravitational interaction can be studied by observing parameters of their orbit around each other and the centre of mass. Before applying Kepler's 3rd Law, the inclination of the orbit of the visual binary must be taken into account. Relative to an observer on Earth, the orbital plane will usually be tilted. If it is at 0° the planes will be seen to coincide and if at 90° they will be seen edge on. Due to this inclination, the elliptical true orbit will project an elliptical apparent orbit onto the plane of the sky. Kepler's 3rd law still holds but with a constant of proportionality that changes with respect to the elliptical apparent orbit.The inclination of the orbit can be determined by measuring the separation between the primary star and the apparent focus. Once this information is known the true eccentricity and the true semi-major axis can be calculated since the apparent orbit will be shorter than the true orbit, assuming an inclination greater than 0°, and this effect can be corrected for using simple geometryIn order to find the luminosity of the stars, the rate of flow of radiant energy, otherwise known as radiant flux, must be observed. When the observed luminosities and masses are graphed, the mass-luminosity relation is obtained. This relationship was found by Arthur Eddington in 1924.Generally speaking, there are three classes of binary systems. These can be determined by considering the colours of the two components.