|
A binary star is defined as: a stellar system which is composed of two stars orbiting one another about their common center of mass. The two stars are held together by the force of their mutual gravity.
The term "double" star was created by the Greek astronomer Ptolemy to describe pairs of stars appearing quite close together, yet discernible to the naked eye, such as v Sagittarrii, two fifth magnitude stars. Sir William Herschel first used the term "binary" star in his 1802 paper "On the Construction of the Universe", and explained that there is a decided difference between a "binary star", and what Ptolemy called a "double star": "... if a certain star should be situated at any, perhaps immense, distance behind another, and but little deviating from the line in which we see the first, we should have the appearance of a double star. But these stars being totally unconnected would not form a binary system. 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 neighboring stars, they will then compose a separate system, and remain united by the bond of their mutual gravitation toward each other. This should be called a real double star [e.g. a binary star]". Scientists have discovered three different types of binary star systems: visual binaries, spectroscopic binaries, and eclipsing binaries. Today, there are over 23,000 known binary stars , and it is believed that 50% of all stars belong to binary or multiple star systems.
For these stars, the presence of a close-by companion can have a profound influence on their evolution. Stars can exchange material, especially during the stages when one of them swells up into a giant or supergiant, or has a strong wind. When this happens and the companion stars are sufficiently close, material can flow from one star to another, decreasing the mass of the one and increasing the mass of the other. Such mass transfer can be especially dramatic when the recipient is a stellar remnant such as a white dwarf or neutron star.
A visual binary is defined as "A binary star in which two components are telescopically resolved."
There are two types of visual binaries: optical double stars, in which the two stars in question actually lie far apart from each other, but because of their alignment in space, appear to be close to one another; and physical double stars, in which the two stars are actually close to one another and orbit around a barycenter, creating a system. Optical doubles are stars that appear, from our earthly observations, to be double stars, but in actuality, are quite a long distance apart and are not attracted by gravity. In 1782, William Herschel began searching for optical binaries in hopes of discovering a measurable parallax, by comparing a close star to the more distant star in an optical double. As it turned out, Herschel had much better luck with visual binaries, and in 1804, concluded that a pair of stars known as Castor were orbiting one another. This event marked the very first time observational evidence clearly showed two objects in orbit around each other. By observing the motion of the fainter of the pair of stars around the brighter, it is possible to determine the mass of the two stars. In order to determine the mass of the visual binary stars in a system, it is necessary to take down observations on the angular separation of the stars and the position angle over an extended period of time. This can be a daunting task, however, because the orbital period of a visual binary ranges anywhere from a few years to thousands, and for most systems it is necessary to take observations for over half the orbital period, which is often decades, to obtain accurate results. Only a small percentage of binary stars are visual binaries, gravitationally associated stars that orbit each other around a common point, or barycenter. They can be seen optically through a telescope, but in order to do so, the stars must usually be separated by fairly wide distances, with long orbital periods.
A spectroscopic binary star is a binary star in which the binary nature is periodic variations in radial velocity, showing orbital motion. Basically, they look like single stars, but are in fact double stars. It is a binary with parts that are so close together that astronomers know it is a binary only by its spectrum. When the components orbit each other in a plane almost parallel to the line-of-sight from the Earth, they move periodically toward and away from the Earth. Because of the Doppler Effect, their line-of-sight, or radial, velocities can be measured from the Doppler shifts of the spectral lines. This was first theorized by John Goodricke in 1782, when he said that the star Algol, or beta Persei, was an eclipsing binary, rather than a single variable star. He based this on the observations he made of the periodic changes in the brightness of Algol. It was proved in 1889 when H.C. Vogel and J. Scheiner discovered periodic Doppler shifts in the lines of Algol's spectrum. The spectrum emitted by the binary stars can be used to determine the radial velocity of the orbit of the two stars. A spectroscopic binary will alternate between blue and red shifted spectral lines, as the stars approach towards and recede from Earth. If the star appears head on, no Doppler shifts will be present in the spectrum.(Wavelength) A single-line spectroscopic binary is when the light from the Doppler shift appears as one line, indicating that we are seeing the light from only star. Depending on the positions of the two stars in their orbits, and if the light from both stars is visible, the Doppler shifts will alternate, split and merge.
Abell's Exploration of the Universe defines an eclipsing binary as "a binary star in which the plane of revolution of the two stars is nearly edge on to our line of sight, so that the light of one star is periodically diminished by the other passing in front of it." Very often the two stars are orbiting around each other in close rangee, and one is much smaller than the other. In an eclipsing binary, the two stars do just as their name suggests, and eclipse each other periodically, making them appear fainter in the celestial sphere. This period during the eclipse is referred to as the minimum. When the lower magnitude star is eclipsed, primary minimum occurs; when the higher magnitude star is eclipsed, secondary minimum occurs. Because eclipsing binary stars change in brightness like this, they are also included in the class of variable stars.
|
STARS