Visual Binaries

Binary star systems in which we can distinguish visually the two stars in orbit around each other are called visual binaries. Most star systems are binary, but only a subset of these are visual binaries.

Example: Castor
The adjacent image shows the two components of the Castor binary system, which is in the constellation Gemini. This was the first binary system discovered, by William Herschel in 1790. The numbers denote the years of observation. In this case the stars in the binary have a large separation so the orbital period is obviously very long: since its discovery the system has made only about half of an orbital revolution. In plots such as this, it is common to fix the brighter star in position (the primary) and to plot the position of the other star (secondary) relative to the primary.

The Sirius Binary System

The Sirius binary star system used to illustrate binary star orbits in the main text of this frame consists of a main sequence primary star of spectral class A1 called Sirius A and a white dwarf companion called Sirius B. They are in orbit around their common center of mass with a period of about 50 years. The system is about 8.6 light years from Earth and lies in the constellation Canis Major (the Big Dog). The primary star, Sirius A, is the brightest star in our sky other than the Sun. The companion Sirius B was the first white dwarf discovered.

Orbits for Binary Stars

Visual binaries consist of two stars in orbit around their common center of mass, with the motion of both stars visible from the Earth. Here is an interactive animation illustrating the motion in binary star systems, which is governed by Kepler's laws.
  • Binary star orbit applet

    • As this animation illustrates, binary star systems execute elliptical motion around the common center of mass for the two stars. This motion is approximately described by Kepler's laws, with Newton's modification to include the effect of the center of mass. Unlike the case for the Sun and planets in the Solar System, the effect of the center of mass can be large for binary stars because the two stars can be comparable in mass.

    Tilt of Binary Orbits

    One important consideration for visual binary orbits is that the plane of orbital revolution for such systems is not usually perpendicular to our line of sight. In general, there is some tilt angle i, as illustrated in the adjacent figure.

    Thus, when we see the orbit of a visual binary we do not see the actual orbit but only the projection of that orbit on the celestial sphere. For example, if the orbit looks like an ellipse, that could be because the orbit actually is elliptical, or because the true orbit is a circle but we are seeing it from an angle that makes the circle look flattened and therefore elliptical. In some cases it is possible to determine the angle i by careful measurement in order to deduce the true orbits of the binary system. In other cases we cannot and the angle i remains uncertain.

    Three-Dimensional Binary Orbits
    Unfortunately, only a limited number of binary systems are close enough to the Earth to obtain sufficiently detailed observations to construct the full three-dimensional orbits for the system. In other cases we must use indirect reasoning to try to deduce as much as possible about the true orbits of the binary.