AST201H1 Chapter Notes - Chapter 15: Inverse-Square Law, Apparent Magnitude, Thermal Radiation

• School: UTSG
• Department: Astronomy & Astrophysics
• Course: AST201H1
• Professor: Michael Reid

Document Summary

Luminosity is the total amount of power (energy per second) the star radiates into space. Apparent brightness is the amount of starlight reaching earth (energy per second per square meter). The amount of light received per unit area decreases with increasing distance by the square of the distance an inverse square law. Inverse square law for light: apparent brightness = luminosity / 4 distance^2. Because the standard units of luminosity are watts, the units of apparent brightness are watts per square meter. When we measure the apparent brightness in visible light, we can calculate only the star"s visible-light luminosity. When we observe a star with a spaceborne x-ray telescope, we measure only the apparent brightness in x rays and can calculate only the star"s x-ray luminosity. We will use the terms total luminosity and total apparent brightness to describe the luminosity and apparent brightness we would measure if we could detect photons across the entire electromagnetic spectrum.