optical double

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Related to optical double: Binary star system

binary star

binary star or binary system, pair of stars that are held together by their mutual gravitational attraction and revolve about their common center of mass. In 1650 Riccioli made the first binary system discovery, that of the middle star in the Big Dipper's handle, Zeta Urase Majoris. True binary stars are distinct from optical doubles—pairs of stars that lie nearly along the same line of sight from the earth but are not physically associated. Binary stars are grouped into three classes. A visual binary is a pair of stars that can be seen by direct telescopic observation to be a distinct pair with shared motion. A spectroscopic binary cannot be seen as two separate stars, even with the most powerful telescopes, but spectral lines from the pair show a periodic Doppler effect that indicates mutual revolution. Some lines indicate motion toward the earth while others indicate motion away; later, as the stars revolve around in their orbit, this pattern reverses. An eclipsing binary has the plane of its orbit lying near the line of sight, and shows a periodic fluctuation in brightness as one star passes in front of the other. The more massive star (A) of a binary is called the primary, and the less massive (B) is called the secondary; e.g., Sirius A and Sirius B are the primary and secondary components of the Sirius system. It seems likely that more than two-thirds of the stars in our galaxy are binary or multiple (a system of more than two stars moving around their mutual center of mass), since many stars within 30 light-years of the sun are binary or multiple. The masses of the components of a spectroscopic binary can be determined from the observed motions and Newton's law of gravitation; binary stars are the only stars outside the solar system for which masses have been directly determined. Binary stars are thus important indicators from which the masses of all similar stars can be deduced. Measurements of the masses of some of the visual binary stars have been used to verify the mass-luminosity relation. Although most binary stars have distance between them, the components of W Ursae Majoris binaries are actually in contact with each other, their mutual gravity distorting their shapes into teardrops. There are binary systems in which one member is a pulsar: PSR 1913+16, for example, has an orbital period of 7 hr 45 min; in this case the other star is also a neutron star. The orbital period decreases as the system loses energy in the form of gravitational waves; used as a clock to measure the effect of the curvature of space-time on the binary's orbit, such a system confirms Einstein's theory of general relativity.

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optical double

1. See double star.
2. See double galaxy.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
References in periodicals archive ?
I've also indicated whether the pair is known or suspected to be a true binary or an optical double; if I've listed an orbital period (P), it's a binary!
Astronomers call the true couples binary stars and the chance alignments optical doubles.
It's an easy, uneven optical double, with the primary about three times as bright as the companion.
Eta Draconis is one of the "backbone" stars of the dragon asterism, and a wide optical double. Its companion is a 5th-magnitude star, HD 148374, 662" away on a nearly direct line to the celestial pole.
6, 7 The M-type star 64 Draconis isn't a true double, but forms a wide optical double with 65 Draconis.
10 The cool, dim, yellow-white G9 giant sun 75 Draconis, in the northernmost reaches of the constellation, makes an easy optical double with its neighbor, HD 196565, a yellow-orange KO star.
"Double stars" include both binaries and optical doubles, pairs that merely look close together on the sky but whose stars are at different distances and physically unrelated.
Yet all of the included pictures show the potential for dozens of other optical doubles (especially the illustration for 16 and 17 Draconis).
"Matt Wedel replies: If there's a standard designation system for optical doubles, it's news to me.
Tau2 and Tau4 Serpentis are wide optical doubles that seem to mirror each other across an imaginary line extending southwest from Tau5 Serpentis.
Visual observers will still be needed, but for the highest scientific value they will need to perform specific observations--watching for an outburst of a rare class of star, making time-critical observations when robotic facilities might be clouded out, measuring the brightness of close optical doubles, and so on.