![]() ![]() These then give a factor of 1.4 to 1.8 × variation in the resultant distance. Uncertainties in the absolute magnitude of stars of specific spectral and luminosity class range from about 0.7 up to 1.25 magnitudes. In practice this technique is not very precise in determining the distance to an individual star. To find the distance to the star, d, in parsecs. Now knowing m from measurement and inferring M we can use the distance modulus equation:.A main sequence (luminosity class V) star with a colour index of 0.0 (ie A0 V) has an absolute magnitude of +0.9 for example. Once we know its position on the HR diagram we can infer what its absolute magnitude, M should be by either reading off across to the vertical scale of the HR diagram or looking it up from a reference table. ![]() If we also know its luminosity class we can further constrain its position along this line, that is we can distinguish between a red supergiant, giant or main sequence star for example. Knowing either the star's spectral class or colour index allows us to place the star on a vertical line or band along a Hertzsprung-Russell Diagram.If we use B and V filters we can also determine the blue apparent magnitude, B and thus determine the star's colour index, CI = B - V.Using photometry we can measure the apparent magnitude, m, m V or V for the star.If we take a spectrum of a star we can determine:.Most stars are too far away to have their distance measured directly using trigonometric parallax but by utilising spectroscopy and photometry an approximate distance to them can be determined. It is, however, a way to find the distance to stars. The term spectroscopic parallax is a misnomer as it actually has nothing to do with parallax. ![]()
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