ASTR 1346 1st Edition Lecture 18 Outline of Last Lecture I Hubble s Galaxy Classification A Ellipticals B Lenticulars C Irregulars II Distribution of Galaxies A Group B Cluster C Supercluster D Voids Filaments and Walls Outline of Current Lecture I Cosmic Distance Ladder A Radar Ranging B Stellar Parallax C Spectroscopic Parallax D Variable Stars 1 Cepheid Variables 2 RR Lyrae Current Lecture l A Radar Ranging sends out pulses of radio frequency radiation and record the radio waves when they bounce back This system of measurement is only valid within the solar system and can only go up to 1 AU The equation for this is distance speed x time B Stellar Parallax measures distances of planets It measures the apparent displacement of a foreground object relative to distant background as the observer s position changes The baseline is the diameter of Earth And is limited for planets in our solar system This equation is D 1 P C Spectroscopic Parallax is used to measure main sequence stars luminosity can be read directly from HR Diagram This is the process of using the stellar spectrum to infer distances by first determining temperature and then luminosity This These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute measurement is limited only to nearby main sequence stars and the equation is apparent brightness luminosity distance squared D Variable Stars are stars that expand and contract These are stars that vary in brightness with time 1 Cepheid Variables are Pulsating Supergiant Stars These are high mass stars in young star clusters that pulsate between 1 100 days Can measure up to 75 million light years 2 RR Lyrae are Pulsating Horizontal Stars These are low mass stars in old star clusters that pulsate every few hours to 1 day These are limited to only our Galaxy