# Luminosity of a star

by kelseymclean
Tags: luminosity, magnitude, telescopes
 P: 6 The question is A main sequence star is barely visible at a distance of 20 pc with a certain telescope. The star subsequently ascends the red giant branch, during which time its surface temperature drops by a factor of 3 and its radius increases by a factor of 50. a) Determine the how luminous the star is now compared to its original luminosity, that is, determine LRED GIANT/LORIGINAL b) Determine how far away this star could be seen now by the same telescope. So far, I have, L=R^2 * T^4. and that Radius is proportional to $$\sqrt{luminosity}$$/temperature. and Apparent magnitude - absolute magnitude = 5 log(base 10) (Distance/ 10 pc). I am stuck at determining the radius and luminosity of the star before it ascends. Any hints/ help would be greatly appreciated.