Optical Double Stars
Not all stars which appear close together in the sky are physically related. They could actually be at different distances from us, but simply appear along the same line of sight. Such stars are called optical double stars. A good example is Mizar (ζ UMa or Zeta Ursae Majoris) and Alcor (80 UMa) in the handle of the Big Dipper. Mizar and Alcor are physically unrelated stars at distances of 78 and 81 light years from us, respectively, but appear less than 1/4 degree apart in our sky. The ancient Greeks used the ability to resolve Mizar and Alcor as a test of visual acuity. Mizar and Alcor, an optical double star in the Big Dipper. Optical double stars do not have much physical significance.Visual Binary Stars
Visual binary star systems are those in which in which the components are physically bound to each other by gravity, and can both be seen visually. The brightest component in the system is labelled with the suffix “A”, the second-brightest as “B”, and so on. Systems with two, three, four, five, or six components have been identified. Visual binary systems tend to be relatively close to us, so that the individual stars can be resolved. Their components are also widely separated physically, by distances of tens to a few hundred AUs. The stars in such systems are gravitationally bound to each other but otherwise do not interact, as do other close binaries, where one star may draw off material from the surface of the other. Less than 1,000 visual binary systems have been detected. The apparent orbit of Alpha Centauri A and B. The closest star system to us, Alpha Centauri, at 1.338 parsecs’ distance, is a visual binary. α Cen A and α Cen B are separated by an average distance of about 23 AU - slightly greater than the distance between Uranus and the Sun. They orbit each other with a period of about 80 years. A third component, α Cen C, is also called Proxima Centauri because it is currently the closest star to us, at a distance of 4.22 light years (1.295 parsecs). For many years after its discovery in 1915, Proxima was thought to be a third member of the system at a much greater distance from the other two. Recent observations, however, suggest it may not be gravitationally bound to the system. A small telescope reveals that Mizar or ζ UMa is also a visual binary star, with components ζ UMa A of magnitude 2.2 and ζ UMa B of magnitude 3.9, separated by 14.3”.Spectroscopic Binaries
Binary stars which are too distant, or whose components are too close together to be resolved, will appear as one star visually. However, we can still detect the binary nature of the system by investigating its spectrum. The Doppler effect induced by the component stars’ orbital motion will affect the stars’ spectral lines. Such systems are called spectroscopic binaries. The majority of binary systems have been detected by Doppler shifts in their spectral lines. Spectroscopic binaries are discovered by the doppler shift in their spectral linescaused by the component stars’ orbital motion. As the stars orbit each other, one star (A) may be moving towards us, while the other (B) may be moving away. The spectrum of A will be blue-shifted toward shorter wavelengths (higher frequencies) while B’s spectrum will be red-shifted toward longer wavelengths (lower frequencies). As the stars continue orbiting, A will recede, and its spectral lines will move towards the red end of the spectrum, while B’s will move toward the blue. If the stars are moving across our line of sight, then no Doppler shift occurs, so the lines stay in their mean positions. In spectroscopic binaries, the component stars are often very close, and may in fact exchange material due to tidal interactions. Orbital periods range from a few hours to months, with separations of much less than an AU in many cases. The first spectroscopic binary system discovered was Mizar or ζ Ursae Majoris in 1889. Mizar was already known as a visual binary, but spectroscopic analysis of the brighter component (ζ UMa A) showed that it was also a spectroscopic binary. Subsequent observations revealed that ζ UMa B was also a spectroscopic binary - thus the whole system was comprised of four stars.Eclipsing Binaries
Sometimes, in a binary star system, one star will pass in front of the other, producing what we call an eclipsing binary. Eclipsing binary systems are also variable stars, because the light of the combined system varies in brightness as one component eclipses the other. There are a few thousand eclipsing binary systems known. Most are also spectroscopic binaries. A few are also visual binaries. As with spectroscopic binaries, the two stars in an eclipsing system are physically close and are often distorted by each other. Mass can be transferred from one star to the other. A light curve must be obtained in order to classify a system as an eclipsing binary. This is simply a plot of apparent magnitude over time. Eclipsing binary light curves are characterised by periodic dips in brightness that occur whenever one of the components is eclipsed. Unless the two stars are identical in brightness, one of the eclipses (called the primary eclipse) will result in a greater drop in brightness than the other, secondary eclipse. One period of an eclipsing binary system therefore has two minima. The light curve of the eclipsing binary star system Algol.From top to bottom: normal, secondary eclipse, normal, primary eclipse. The first known and most famous eclipsing binary star system is Algol, or Beta Persei (β Per). Algol was given its name by the Arabs, to whom it was known as the Demon star, possibly due to its changing brightness. Algol varies from magnitude 2.2 to 3.5 over its 2.87-day orbital period.Astrometric Binaries
Sometimes one component of a binary star system is not observable, but can be detected as the other component moves across the sky. Since both stars are orbiting around their mutual center of mass, the visible component will show a perturbation or a “wobble” in its proper motion as it is observed repeatedly over time. Binary systems detected by such astrometric means are called astrometric binaries. The motion of an astrometric binary. Relatively few binaries have been detected astrometrically, primarily due to the need for long-term observations, and the uncertainty in position and proper motion measurements. The best known example of an astrometric binary is Sirius, or Alpha Canis Majoris (α CMa). In 1844, Friedrich Bessell pointed out that Sirius had a wobble in its proper motion. From this he inferred that the visible component (Sirius A) must have an dim, unseen companion (Sirius B). Sirius B was first observed telescopically by Alvan Clark in 1862, and is now known to be a dim white dwarf star. Procyon, or Alpha Canis Minoris (α CMi), was another star first detected as an astrometric binary. It too has a white dwarf companion that can now be observed telescopically.Binary Stars and Stellar Mass
Binary stars are of great importance to astronomers because they provide virtually the only means of directly determining the masses of stars other than our Sun. As a star’s mass determines its life cycle and fate, being able to accurately determine stellar masses is vital to refining our models of stellar evolution. To find the mass of a binary system we need to apply Kepler’s Laws. Adapted for a binary system, they are:- The stars orbit each other in elliptical paths, with the center of mass (or barycenter) as one common focus.
- A line between the stars sweeps out equal areas in equal periods of time.
- The square of the orbital period, T, is directly proportional to the cube of the average distance from the centre of system mass, r: T2 = r3.