Dynamical parallax is a method for determining the distance to a visual binary star. By comparing the absolute magnitude to its apparent brightness, the star's distance is calculated. This diagram correlates the spectral class of the star with its absolute magnitude. From the analysis of a star's spectrum, its position on the Hertzsprung-Russell diagram is determined.
Spectroscopic parallax is the most widely used technique for determining the distances of stars that are too distant for their stellar parallaxes to be measured. The technique of stellar parallax is useful for stars within 100 parsecs. Its parallax of 0.3 places it at a distance of 3.3 parsecs or about 11 light-years. The first stellar parallax was measured in 1838 by Friedrich Bessel for the star 61 Cygni. A star's distance d in parsecs is the reciprocal of its parallax p (or d = 1/ p).
A unit of stellar measurement is the parsec it is the distance at which a star would have a parallax of one second of arc and is equivalent to 206,265 times the distance from the earth to the sun, or about 3.3 light-years. If a star's parallax can be measured, it then determines the distance to the star. A shift in the angular position of a star will be greatest when observed at intervals of six months (see accompanying diagram) this makes the parallax equal to the value of one half of the semiannual displacement of the star. Formally, the parallax of a star is the angle at the star that is subtended by the mean distance between the earth and the sun. Trigonometric parallax is the apparent displacement of a nearby star against the background of more distant stars resulting from the motion of the earth in its orbit around the sun. In astronomy the term is used for several techniques for determining distance. Parallax (pârˈəlăks), any alteration in the relative apparent positions of objects produced by a shift in the position of the observer.
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