Energy is supplied by conversion of hydrogen to helium.
Use proportions . The radius of a one solar mass black hole is 3 km. Explain why Shapley was successful in noticing the sun's off-center position in our galaxy, while Kapteyn and Herschel had erroneously thought the sun to be at the galaxy's center. The distance to M31 was first estimated in the 1920's by observing the periods and apparent magnitudes of the Cepheid variables in it.