Editing Visual binary (section)

===Spectroscopic parallax===
Spectroscopic parallax is another commonly used method for determining the distance to a binary system. No parallax is measured, the word is simply used to place emphasis on the fact that the distance is being estimated. In this method, the luminosity of a star is estimated from its spectrum. It is important to note that the spectra from distant stars of a given type are assumed to be the same as the spectra of nearby stars of the same type. The star is then assigned a position on the Hertzsprung-Russel diagram based on where it is in its life-cycle. The star's luminosity can be estimated by comparison of the spectrum of a nearby star. The distance is then determined via the following inverse square law:

:<math> b = \frac{L}{4\pi d^2} </math>

where <math>b</math> is the apparent brightness and <math>L</math> is the luminosity.

Using the Sun as a reference we can write

:<math> \frac{L}{L_{\odot}} = \bigg(\frac{d^{2}_{\odot}}{b}\bigg)\bigg(\frac{d^{2}}{b_{\odot}}\bigg) </math>

where the subscript <math>\odot</math> represents a parameter associated with the Sun.

Rearranging for <math>d^2</math> gives an estimate for the distance.<ref>[http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=35616&fbodylongid=1667 European Space Agency, ''Stellar distances'']</ref>

:<math> d^2 = \bigg(\frac{L}{L_{\odot}}\bigg)\bigg(\frac{b_{\odot}}{b}\bigg) </math>