Stars are the visual masterpieces of the sky that have captivated humanity for untold centuries. Many believe that the alignment of these celestial bodies can bring good and bad luck or that their positions can assist in predicting the future. Sailors have used the celestial star map as a guide for as long as humanity has struck out across the ocean. What makes these astronomical bodies so amazing? The fact is that they produce such an amazing visual display and are the most abundant feature in our sky. No stagnant show, however, is quite as captivating as a binary star.
A binary star is a system of two stars that orbit a common center mass. Within the coupling, the brightest star is known as the primary star and the other star is the secondary or companion star. The term double star can be used synonymously with the term binary star, but more often than not the term refers to optical double stars. The term binary star is not to be confused with an optical double star, which is a coupling of stars that appear visually close together but share no physical connection. The double star may be defined as optical if the stars being measured have significantly different proper motions, which are the angular changes of a star in relationship to the Sun, or radial velocities, which is the velocity of an object either toward or away from the observer. Another way that the optical double star is defined depends on whether the measured parallaxes of the individual stars are significantly different distances from Earth.
Binary stars were first theorized in 1767 by John Michell, but the first observation and cataloguing of double stars began in 1779. The term binary star was first used in context by Sir William Herschel when he stated, “If, on the contrary, two stars should really be situated very near each other, and at the same time so far insulated as not to be materially affected by the attractions of neighboring stars, they will then compose a separate system, and remain united by the bond of their own mutual gravitation towards each other. This should be called a real double star; and any two stars that are thus mutually connected, for the binary sidereal system which we are to now consider.” The first orbit of a binary star was not computed until 1827 when the orbit of Xi Ursae Majoris was calculated by Felix Savary. The Washington Double Star Catalogue is a database of known binary and optical double stars containing more than 100,000 couplings. Only several thousand of these pairs have their orbits calculated.
There are four categories of binary stars: visual binaries, spectroscopic binaries, eclipsing binaries, and astrometric binaries. These categories of binary stars are defined by the way in which the coupling is observed. The observations, however, are not mutually exclusive as several binary stars fall within more than one category. A visual binary is a pair of stars in which the angular separation of the bodies is enough for each individual star to be observed by a telescope. Within each visual binary, the brightness of the primary star plays a key role in the identification of the secondary star. If the primary star is extremely brighter than the secondary, the light pollution emitted by the primary will make the secondary unobservable.
The second type of binary star is the spectroscopic binary. Sometimes the only evidence of this pairing comes from the Doppler Effect, or the change in frequency of the light wave as the source moves, on its emitted light. In these cases, the binary pair emits light beginning in the blue spectrum of light which shifts into the red spectrum as the stars revolve around the center mass. Commonly, the separation between these types of stars is extremely small and the orbital velocity is high. The vast majority of these star couplings cannot be detected with a telescope.
Eclipsing Binary Star
The next category of coupling is called the eclipsing binary. This star pair is categorized due to the fact that the orbital plane of the stars parallels the observation point so nearly that the stars eclipse one another as they revolve around the center mass. Eclipsing binaries are variable stars due to the fact that the light emitted is expressed by an almost constant emission with a significant noticeable change in intensity as the stars eclipse one another. If one star in the pairing is significantly smaller than the other, the smaller star will be eclipsed totally by the larger, but as the smaller star eclipses the larger star an annular eclipse occurs.
The final category of binary star is called an astrometric binary. This binary was first discovered when astronomers noted stars that seemingly orbited around empty space. These are stars that are relatively nearby to the Earth and seem to wobble around a point in space with no visible companion star. Mathematicians use the properties of know binaries to calculate the mass of the missing companion star which might be too dim to be seen or simply out of the observer’s vantage point.
Mass transfer in a binary star
A mass transfer can occur within binary stars as the main sequence star increases in mass, it may at some point exceed its Roche lobe and the companion star may begin to absorb the mass of the other star. The mass may be absorbed by direct impact or through an accretion disc, which is a circumstellar disk formed by diffused material in orbital motion around a central body. When this occurs, the accretion disc often becomes the brightest observable point of the binary, sometimes even becoming the only observable point due to light pollution caused by the disc itself.
Binary stars are an amazing phenomenon to be observed in the heavens. One thing remains certain about star observation, it will continue to captivate humanity and motivate science for many generations to come, whether or not the stars can be used to predict the future or bring us luck.