A star's surface sends out energy in form of electromagnetic radiation and has an emissivity close to 1. Find the solar radius of Rigel that emits energy at a rate of 2,7 * 10^32W and has a surface temperature of 11000K. You may assume the star is spherical. Attempt at solution: Formula for the heat current in radiation: H = AeσT^4. Solve this with respect to A gives surface area: A = H / (eσT^4) = 2.7 * 10^32 / (1 * (5,67 * 10^-8) * 11000^4) = 3,25 * 10^23 m^2. Formula for the area of a sphere: A = 4πr^2. Solve this with respect to r gives radius: r = (A / 4π)^0,5 = (3,25 * 10^23 / 4π)^0,5 = 1,61 * 10^11 m. The radius of the sun: 6,96 * 10^8 m. Solar radius = (1,61 * 10^11) / (6,96 * 10^8) = 231,06Rsun. The Rigel radius is 231 times the radius of the sun. However, Wikipedia says 78. I've also seen 63, 98 etc., but 231 sounds a bit high. What have I done wrong?