Ver. 032301AST 1002 Slide Handouts: Topic E: Part 2 Page 1Copyright © 2001 H.L. CohenDept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 0323011Topic ETopic EPart 2. “Stellar Motions”(Web Version: 03-23-01)Dept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 0323012Proper Motions of Stars(Apparent Motions of Stars)Dept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 0323013“Fixed Stars”§ Till now assumed stars on sky• Fixed (relative to each other)• So star patterns don’t change§ Questions:• Do stars move through space relative to Sun?• If yes — effect observable?• If yes — do motions influence parallax?YesYesBig DipperYesDept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 0323014Want to Contrast§ Stellar Parallax:From Earth’s annual orbital motion about Sun§ Proper Motion:From star’s motion in space (relative to Sun)“Both cause positions of stars on skyto change over time”Dept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 0323015Annual ParallaxDue to Earth’s Orbital MotionASunB12Earth MovesStarDistantReferenceStarsParallaxDept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 0323016Proper MotionASunB12DistantReferenceStarsProper MotionStar MovesRelative to SunShift due to star’s motion relative to SunVer. 032301AST 1002 Slide Handouts: Topic E: Part 2 Page 2Copyright © 2001 H.L. CohenDept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 0323017From Earth See CombinationASunB12DistantReferenceStarsCombinedShift12Earth MovesStar MovesRelative to SunandDept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 0323018Measuring Proper Motion§ Should appear as “drift”of star across sky§ Measure by comparing sky photos taken many years apart§ Has any star moved?(If so . . . which one?)19451995Dept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 0323019Proper Motion is “Drift”§ Suppose . . .Measured drift = 5 arc sec§ What is elapsed time?Answer: 1995 – 1945 = 50 years§ What is the proper motion?Answer: 5"/50 yrs = 0.10" / yr1945199519455"5"Dept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 03230110Angular Drift§ Proper motion is angular drift§ Unit is angle / time (e.g., 0.10" / yr)§ Since very small usuallyarc sec per year (or "/yr)§ Recall parallax . . .• Only an angle (arc sec)• Not drift but periodic (annual) shift1945199519455"5"Dept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 03230111Some ResultsLargest proper motion . . .Barnard’s Star: p.m. = 10.4" /yr !Also second closest star to Sun(If count Alpha Centauri system as one star)• p = 0.54" (what is d in pc?)• d = 1.9 pc (what is d in ly?)= 6 lyDept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 03230112Moon• Answer: 1800" ÷ 10.4"/yr or about 173 years(only about two human life times!)• Star not noticeable to naked eye(mag. = +9.6 requires binoculars or telescope)How Fast is 10.4"/yr?1800"How long to move apparent width of Moon?Ver. 032301AST 1002 Slide Handouts: Topic E: Part 2 Page 3Copyright © 2001 H.L. CohenDept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 03230113More Proper Motion Results*Star Proper MotionAlpha Centauri 3.7"/yrSirius 1.3"/yrAbout 400 stars have More than 1"/yrNaked eye stars Less than 0.1"/yr (typical)Betelgeuse 0.03"/yrPolaris 0.007"/yr* See Study Guide Table 9Dept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 03230114Proper MotionMost proper motions very small but§ Can influence parallax measurements(because parallax also small)§ Unlike parallax . . . proper motion measurements aided by waiting!Dept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 03230115Time and Proper MotionConstellation patterns• Essentially unchanged over human generations but . . .will change over tens of thousands of years• Example followsDept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 03230116Example Star GroupsWill compare two regions of sky:• Big Dipper (an “asterism”)• Leo (“The Lion”)Watch proper motions over 200,000 years(100,000 BC to 100,000 AD)Dept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 03230117Proper MotionLeoPresentDept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 03230118Proper MotionLeo100, 000 BCPresent100, 000 ADVer. 032301AST 1002 Slide Handouts: Topic E: Part 2 Page 4Copyright © 2001 H.L. CohenDept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 03230119Proper MotionBig Dipper PresentDept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 03230120Proper MotionBig Dipper100, 000 BCPresent100, 000 ADDept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 03230121Proper Motions Differ§ Proper Motions in Leo & Big Dipper Differ(Two major differences)§ Do you see how?• Difference #1?• Difference #2?Look again ⇒⇒Dept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 03230122How Different?Big DipperLeo100, 000 BCPresent100, 000 AD100, 000 BCPresent100, 000 ADDept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 03230123AnswerBig DipperLeoFive stars1) ?2) ?All ?Dept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 03230124Ursa Major Region§ Many stars in this region have “common proper motions”• Values – similar• Directions– similar§ Conclusion —Stars move alongparallel paths . . .Belong to a “swarm”. . . a star clusterVer. 032301AST 1002 Slide Handouts: Topic E: Part 2 Page 5Copyright © 2001 H.L. CohenDept. Astronomy – University of Florida – Copyright © H.L. Cohen 2001, Ver. 03230125Proper motions make U Ma cluster’s existence known!• Look like a cluster?• Too close to be recognized(Distance to center 25 pc)Recognizing the U Ma ClusterPhoto cred. Anglo-Australian ObservatoryThis looks like a cluster (NGC 3293)• Cluster far (2500 pc!)• Cluster stars stand out from other starsDept. Astronomy – University of Florida – Copyright © H.L.