AS102 Study Guide - Final Guide: Star System
Document Summary
Stars are classified according to their luminosity and their surface temperature. A star"s luminosity is the total amount of power it radiates into space. Recall that our own star, the sun, has a luminosity of 3. 8 x 1026 watts. So, every second, the sun radiates 3. 8 x 1026 joules of energy into space in all directions in the form of light. Luminosity cannot be measured directly because its brightness depends on its distance from us as well as its true luminosity. So, instead we measure a star"s apparent brightness, realizing that luminosity and apparent brightness are related by apparent brightness = luminosity/4 r2. This formula is true because apparent brightness of a star follows an inverse-square law (every time the distance from a light doubles its intensity decreases by a factor of four). If we were very close to a star we could measure its luminosity directly. Re-arranging this result we have the following, luminosity =
