AST 10G Lecture Notes - Lecture 7: Cosmic Distance Ladder, Cosmic Dust, Electromagnetic Spectrum
Document Summary
We see brief moment in star"s life. Stars all form in great clouds of gas & dust, ~chemical composition: hydrogen, helium (by mass), ~2% elements heavier than helium. Most fundamental properties of stars: luminosity, surface temperature, & mass. Brightness of star depends on distance & how much light it actually emits. Must distinguish star"s brightness & actual amount of light. Apparent brightness-how bright star looks in sky, define by amount of power (energy/sec) reaching us per unit area. Luminosity-total amount of power that a star emits into space: Talk about luminosity: how bright stars are in an absolute sense, regardless of distance. Apparent brightness or another light source obeys inverse square law with distance. Important formula for apparent brightness, luminosity & distance of any light source. Units for apparent brightness are watt per square meter. Surface area of sphere is 4pi x distance^2. Measure a. b. by detector that records how much energy strikes its light sensitive surface each sec.