04 Apr Binary Star Systems
Binary Star Systems for the Humanities Student
Some of the interesting observations being done at SALT include the observations of a close binary system. Are you unfamiliar with the term? A binary system is a system of two objects in space (usually stars, but they can also be brown dwarfs, planets, galaxies, or asteroids) which are so close to one another, that their gravitational interaction causes them to orbit about a common centre of mass.
A binary star that has an orbital period of less than 30 years implies that the two system components are less than about 10 AU apart. Because of this proximity, most close binaries are spectroscopic binaries and/or eclipsing binaries. Sometimes, the only evidence of a binary star comes from the Doppler effect on its emitted light. Often in these cases, the binary consists of a pair of stars where the spectral lines in the light emitted from each star shifts first toward the blue, then toward the red, as each moves first closer to us, and then away from us, during its motion about their common centre of mass, thus we know that the two stars are orbiting one another.
In some spectroscopic binaries, spectral lines from both stars are visible and the lines are alternately double and single. Such a system is known as a double-lined spectroscopic binary (often denoted “SB2”). In other systems, the spectrum of only one of the stars is seen and the lines in the spectrum shift periodically towards the blue, then towards red and back again. Such stars are known as single-lined spectroscopic binaries (“SB1”).
The orbit of a spectroscopic binary is determined by making a long series of observations of the radial velocity of one or both components of the system. The observations are plotted against time, and from the resulting curve a period is determined.
Binary stars that are both visual and spectroscopic binaries are quite rare, and are a precious source of valuable information when found, hence the excitement of being able to observe such systems at SALT. Spectroscopic binary stars move fast in their orbits because they are close together, usually too close to be detected as visual binaries. Binaries that are both visual and spectroscopic thus must be relatively close to Earth.
Mass transfer will occur at some stage in most close binaries, an event which profoundly affects the evolution of the component stars. The evolution of close binaries depends on the initial masses of the two stars and their separation. When the more-massive star evolves into a red giant first, material will spill through the inner point onto its companion, thereby affecting its companion’s evolution. Mass transfer can also alter the separation and orbital period of the binary star.
Binary stars are often detected optically, in which case they are called visual binaries. Many visual binaries have long orbital periods of several centuries or millennia and therefore have orbits which are uncertain or poorly known. They may also be detected by indirect techniques, such as spectroscopy, which is done at SALT.
Often a system of binary stars are too close (or too far away) to be resolved into an optical pair. However, a spectrum of such an object will display the spectral fingerprints of two different stellar types (if the stars are different in spectral type).
Of course, the problem with this method is that since faint, cool stars are more common than brighter stars, the odds are that the companion is too faint to be detected in a spectrum.