AST 341 Lecture Notes - Lecture 8: Main Sequence, Ob River, Hipparcos
22 Dec 2016
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Recalling the de nition of e ective temperature: l = 4 r2 t 4 e , we have a relation among l, te, and r. Therefore, once two of these variables are known, the other one can be determined. Then, there are only 3 independent variables: m , l, and te. R was eliminated because the radius would have been very di cult to measure. Its angular size on the sky is very small, i. e. the angular diameter of the sun at a distance of 1 pc is. If we compare this with the resolution of hubble, 0. 1 arcsec, we see that the radius of a star cannot be directly measured even for very nearby stars. The radius must therefore be inferred from direct measurements of the e ective temperature and the bolometric luminosity. Let"s then see how these two quantities are measured. The e ective temperature of a single star can be determined without knowledge of the distance.
