Binary Star

Introduction

Binary Star, two stars that are bound to each other by gravity and orbit about a common center of mass. Binary star systems are quite common and the pairing of stars appears to be random in most cases.
Astronomers estimate that approximately one-fourth of the visible stars belong to a binary system. The time it takes for one star to orbit the other can range from hours to centuries depending on the distance between the two stars and their masses. Some binary pairs, called interacting binary systems, are so close that they exchange material. Binary stars are very useful to astronomers because they are the only stars of which astronomers can directly determine mass.

Classification of Binary Systems

Astronomers classify binary systems into one or more of four categories depending on how the two stars of the system are detected. The easiest binary system to detect is one in which both stars can be seen, either with the unaided eye or with the aid of a telescope. This type is called a visual binary system. The star Albireo in the constellation Cygnus is an example of a visual binary system.

Sometimes astronomers can only see one star, but they infer the presence of a second star because the visible star undergoes a slight wobbling motion that follows a regular pattern consistent with the presence of a large mass nearby. Such pairs are known as astrometric binary systems.

Astronomers often detect binary stars by examining the spectrum of light emitted by a star (see Spectroscopy). Repeated, cyclical Doppler shifts in a star’s spectrum indicates that the star alternately moves away or toward the earth. The only known mechanism that could cause such a phenomenon is the presence of a companion star, which the primary is orbiting. Another spectroscopic technique for detecting binary systems is to analyze a star’s spectrum for the presence of elements that the star, by reason of its evolutionary stage, should not have. Binary systems detected either way are known as spectroscopic binary stars. The star Dubhe in the constellation Ursa Major (better known as one of the stars of the Big Dipper) is a spectroscopic binary star system.

The fourth type of binary star system is known as an eclipsing binary. In an eclipsing binary, the two stars orbit each other in a plane that is closely aligned with the line of sight from the earth to the system. In this case, the stars alternately pass in front of each other, thus partially or fully blocking each other’s light output to the earth. To an observer on the earth, the brightness of such a system would vary in a regular and characteristic way. The most spectacular eclipsing binary system is Algol, a bright binary star in the constellation Perseus. Every 69 hours, Algol B passes in front of its brighter companion Algol A, and Algol’s brightness drops by a factor of three for about 9 hours, then returns to its original brightness.

Astronomers occasionally refer to optical doubles, but these are not true binary star systems. Optical double stars are usually separated by vast distances and do not move in orbit about a common center of mass. They only appear in the same region of the sky because by chance they are on almost identical lines of sight to the earth. Mizar, one of the central stars in the constellation Ursa Major, is an example of an optical double.

Interacting Binary Systems

The separation between the stars in a binary star system determines the gravitational force between them, which in turn determines the orbital period of the stars and whether there are secondary interactions between them. Distances comparable to the diameter of the earth’s solar system separate the stars of many binary systems. At such large distances, the mutual gravitational influence of the two stars causes nothing more than stable orbits of the stars around a common center of mass. However, many binary pairs are separated by much smaller distances, sometimes comparable to the sizes of the stars. Because the gravitational force between the stars is proportional to the inverse of the square of the distance separating them, closely separated stars exert very strong gravitational forces on each other. These intense gravitational forces can lead to a variety of effects.

Some close binary pairs—for example the W Ursa Majoris system—are composed of otherwise ordinary stars that are so close that their mutual gravitational pull distorts the shape of one or both stars from a spherical shape into an egg shape. The strong mutual gravitational influence in other close binary pairs can lead to the exchange of material from one star to another. For example, in the Beta Lyrae system, material from the outer envelope of one star flows to the other and forms a disk around it. Such a system, in which material flows from one star to the other, is known as an interacting binary system. In some interacting binaries, a stream of material flows from one star onto the surface of the other, where it erupts in a thermonuclear explosion. Such a system is classified as a dwarf nova when the material flow and resulting explosion is steady, or recurrent nova in cases where the flow of material, and therefore the explosions, are temporarily interrupted by the explosions.

In an interacting binary system consisting of a normal star and a collapsed star—for example a white dwarf, a neutron star, or a black hole—the gravitational field at the surface of the collapsed star is so strong that the flow of material onto its surface causes bursts of highly energetic X rays. An interacting binary system that emits X rays is called an X-ray binary star or X-ray burster. Interacting binary stars such as the W Ursa Majoris-type variables, dwarf or recurrent novas, and X-ray bursters can be studied with a variety of ground-based and space-born telescopes sensitive to radiations across the entire electromagnetic spectrum.

Importance of Binary Stars to Astronomy

The motions of visual binary systems are the most direct and easiest to determine, but astronomers have also developed techniques for deducing the motions of astrometric binaries, spectroscopic binaries, and eclipsing binaries.

Binary star systems are very important to astronomers because they offer unique opportunities to determine important information that cannot be obtained from single stars. In particular, with knowledge of the separation between the two stars of a binary system and the time they take to orbit each other, astronomers can determine the individual masses of the stars. Astronomers have found a strong correlation between the mass of a star and its luminosity, or light output. Therefore, once a star’s mass is known, astronomers can calculate its intrinsic luminosity and use this information to estimate its distance from the earth (see Magnitude).

Knowledge of the distance to a binary system allows astronomers to estimate the distance to other stars in its vicinity. This has been crucial to the development of stellar evolution theory, which has advanced primarily due to the study of star clusters. Star clusters are close groups of stars that appear to have evolved from the same material at about the same time. Knowing the distance to a star cluster allows astronomers to calculate the intrinsic luminosities of the stars in the cluster. From the mass-luminosity relationship, the masses of the stars in the cluster can then be inferred. Using information on the masses of the stars and information on their evolutionary states gained through spectroscopy, astronomers can determine the effect of mass on the evolution of the stars.

• Tweet
• Share
• Share
• Share
• Share

Sign up here with your email