Most stars in our galaxy are not single, like the sun, but are under the gravitational influence of neighboring stars, whose close proximity probably indicates that they share a common origin. Two prominent “double” stars, which can be seen with the naked eye, are the pair Alcor and Mizar, in the constellation Ursa Major (the Big Dipper). A telescope reveals that Mizar (which was the first component of a double star to be discovered) is itself part of a double system of stars called Mizar A and Mizar B. Spectroscopic observation further shows that Mizar’s two visual components are also themselves binary stars, making Mizar a .quadruple star. There is evidence that Alcor varies in its velocity in space, which suggests that it too is a binary star. There are many multiple stars, but they seem stable only when part of a hierarchical network of double stars, in which each pair acts as a component in an even larger set of binary systems.
Identifying double stars
Nearby binary stars in our galaxy are close enough to observe visually, but often the individual stars are not far enough apart to be seen as separate units. They can, however, be detected spectroscopically, even in other galaxies. When a double-star system consists of two different kinds of stars—for example, one hot star and one cool one—it produces an overlapping composite spectrum that quite clearly shows the existence of two stellar objects. Even if the pair are similar in nature, they can still be identified as two separate stars because each spectrum exhibits a Doppler shift as the stars orbit around each other. The shift indicates whether the star is receding from us in its orbit, or approaching.
If two closely associated stars orbit in the same plane as the earth’s orbit, the doublet changes in brightness because one star periodically cuts off the light from the other one as they rotate around each other. Such a combination is called an eclipsing binary star.
Stellar masses and sizes
It is difficult to determine accurately the mass of a solitary star, but if two stars are physically associated (as they are in a binary system), their individual masses can be deduced by studying their orbital motions. Most visual binaries, however, take many years to complete their orbits—many have not yet completed one orbit since their original discovery. For this reason, eclipsing binaries, with periods of only a few days, are much more useful for astronomers in estimating the masses of stars. Also, the Doppler shift in their spectra shows the speed at which the stars are traveling. From a knowledge of the period and velocity of a binary star, the size of the orbit can be calculated. Their individual masses are then determined by taking into account the gravitational effects that are necessary to produce such an orbit. And by observing how long the light of one star is eclipsed by the other (and knowing the speed of each), the sizes of the two stars can also be calculated. But if the two eclipsing binary stars are very close together, their mutual gravitational attraction is extremely strong, with the result that many stars of this type interact by pulling material from each other. This makes calculations of individual masses imprecise.
Variable binary stars
Not only does the light from eclipsing binaries vary when they eclipse, but their brightness often also varies when they are close together and interact. This characteristic makes it difficult for astronomers to differentiate variable binaries from intrinsically variable stars— whose brightness varies because of internal factors. Some binary stars (such as the prototype W Ursae Majoris) may be so close to each other that they are in contact, and the binary system consists of two stellar cores orbiting round each other and surrounded by a common envelope. The orbits of some stars (typified by Beta Lyrae) are so close that one becomes distorted by its companion, making it a semi-detached binary star. Wolf-Rayet stars are often part of a close binary system that consists of a massive luminous main-sequence star and a less massive, but even more luminous companion. The intense luminosity of the small star expels the outer part of its atmosphere in the form of a dense stellar wind. This wind produces a complex but characteristic spectrum, which may be the result of the interchange of mass between the two stars.
Telescopes launched above the earth’s atmosphere have detected many X-ray sources associated with binary systems. If one member of a pair is a supergiant star and the other is a compact object—either a neutron star or a black hole—X rays may be emitted.
n spite of the difficulties, accurate values for stellar masses have been obtained for stars on the main sequence (the main band of stars on the Hertzsprung-Russell diagram). If these values are plotted on a graph against their magnitudes, they lie on a straight line that demonstrates the direct relationship between stellar mass and luminosity. Other graphs indicate that mass, luminosity, and surface temperature of main-sequence stars are all related: the more massive a star, the brighter it is.