Observation Binary Stars

a view of the famed binary Albireo, in constellation Cygnus, the Swan. picture NASA

"Binary stars" are stars which have one or more companion star. Two or more bodies are orbiting around each other. Observing binary stars is an interesting activity as most pairs are different in color. Such observations may lead to build a fine observation database. On another hand, professional astronomers still welcome observations made by amateurs as the field is enormous (about 78,000 binaries have to be checked) and professional astronomers few on the job. Binary systems are used in professional astronomy to estimate stellar masses (via determination of orbits, periods in a binary). Despite the fact that CCD cameras are now used, visual observation is still the best way for binaries with a substantial difference of brightness and for pairs of close and faint stars

Extrapolating data about stars in the vicinity of the Sun allows to say that almost half of all stars (about 46 percent) are part of a double star system. 8 percent are part of a triple star system, as 1 percent of a quadruple star system. 45 percent of stars should be single stars. The idea now however is to think that further studies of single stars tend to see that 85 percent of them have companions -some of them faint, and that on another hand these widespread stellar companionships do not prevent the existence of planetary systems too around those multiple systems. Astronomers generally believe that widely spaced twin, or binary, stars grow from two separate clouds, while the closer-knit binary stars start out from one cloud. But how this latter process works has not been clear. Asymmetrically shaped densities around proto-stars leads to a unevenly feeding of the disk, which fragmentates and trigger a binary star formation

A more general term which may be used for binary stars is "multiple-star systems". "Binary star system" or "binary" may be better used about two-star associations strictly. As far as physical features of star systems are concerned, stars in a binary system may be separated by a large fraction of a light-year or be almost touching; stars orbits are elliptical orbits; they occur around a common center of mass (the larger the star's mass, the smaller the orbit; such a technical pecularity translates few into the diagrams well showing that one element is orbiting around the other). Generally, the shorter the period of revolution, the smaller the orbit's eccentricity, as the equal the brightness of the two components, the same spectral type they are (when the brightness is different, the fainter star is blue if the brighter is a giant star, redder if it's a main sequence star). Multiple star systems may vary as far as their components are concerned: an additional star e.g. may orbit one pair of binaries; two pairs of binaries may orbit each other; some systems may even comprise more than four stars: a distant binary e.g. may orbit over a long period of time around two pairs orbiting each other. Wide binary stars are separated by as much as one light-year in their orbits, farther apart than some stellar nurseries are wide. Recent studies are showing how widest binary systems likely begin as three stars, not just two. Most stars are born in small, compact systems with two or more stars at the center of a cloud core. When more than two stars share a small space, they gravitationally pull on each other in a chaotic dance. The least massive star often is kicked to the outskirts of the cloud core while the remaining stars grow larger and closer by feeding on the dense gas at the center of the cloud core. If the force of the kick is not forceful enough, the runt star will not escape, but instead begin a very wide orbit of the other two, creating a wide binary. However, sometimes astronomers find only two stars in a wide binary meaning that the early binary system turned one through the mergers of its stars and that another star came to form a new binary. This can happen if there is enough gas in the cloud core to provide resistance to the first binary motion, leading to a merger. The wide binary nearest to Earth is Alpha Centauri. The star itself is a close binary. Alpha Centauri has a small companion, Proxima Centauri, which orbits at a distance of about one-quarter of a light-year. All three stars were born close together several billion years ago, before a powerful dynamic kick sent Proxima out into its wide path, where it has been orbiting ever since

Binary or multiple star systems are sorted according to the way they have been found such. In turn, the way they are found is varying according to system's features (separation of the components, distance from Earth, or how the orbital plane is oriented to our line of sight). The mean distance between the components of a binary system, in Astronomical Units (AU -92,960,116 miles -149,597,870 kilometers), is 3 to 50 (that is 279 million miles to 4.6 billion miles -450 million to 7.5 billion km), not much that is

• Visual Binaries: these stars has been found binary or multiple as they are separated enough to be visually observed as such. Their orbits' semimajor axes are varying between a few astronomical units (AU) to thousands of AU; their periods of revolution are varying from several years to many thousand years (thus they are called too "long-period visual binaries"). Astrometric binaries are visual binaries secondary star of which is too faint to be observed; their existence may be induced from main star proper motion wobbles only
• Spectroscopic Binaries: these binary or multiple stars are too close to each other to be visually observed. They are found binary due to that the both stars' or one's only, spectral lines in the system composite spectrum, are periodically Doppler-shifted. Semimajor axes vary between a fraction of an AU to several tens of AU. Orbital periods vary from hours to several years (thus such binaries are called too "short-period spectroscopic binaries"). Most of time brightest star spectral lines only are seen shifted as secondary star is too faint to be observed. For information, overcontact binaries are extreme stars on the verge of merging and thought to play a major role in the evolution of galaxies as they are the main producers of elements such as oxygen. Some double may have one component drawing material of the other as mostly such double are sharing material down to their interior
• Eclipsing Binaries: these binary or multiple stars are identified due to one or more stars eclipsed each other, yielding luminosity drops in the system. Eclipse features vary according to components' respective sizes and luminosity. Spectroscopy may be used to further characterization of these systems

As far as observation is concerned, two main values are used about binaries: separation, position angle (P.A.). Catalogues are also giving R.A., declination, magnitudes for stars of the binary system and date at which pair was last observed. Main star of the system is said the "primary", as second star the "secondary". All binaries always have the primary having the largest magnitude. The secondary is always fainter (it then may be named "companion", too). Separation is the apparent separation between the primary and the secondary (in arcseconds (")). Position angle gives the location of the secondary relative to the primary. P.A. is measured in degrees, from North of the primary, counterclockwise (90° is celestial East of the primary, 180° South and 270° is celestial West).

two useful references about binary amateur astronomy are to be found; one at the U. S. Naval Observatory (USNO) under the form of the Washington Double Star Catalog which is the definite reference about double and multiple stars; another at The Belmont Society section for double stars, where a simple but efficent page lists the colors of main northern hemisphere double and mutliple systems