ASTR 1102 002 2008 Fall Semester Joel E Tohline Alumni Professor Office 247 Nicholson Hall Slides from Lecture04 University wide Gustav motivated Calendar Modifications University wide Gustav motivated Calendar Modifications Gustav s Effect on this Course Fall Holiday has been cancelled which means our class will meet on Thursday 9 October This makes up for one class day lost to Gustav last week We will hold an additional makeup class on Saturday 20 September This will account for the second class day lost to Gustav last week Date of Exam 1 has been changed to Tuesday 23 September Course Syllabus Course Syllabus Chapter 17 The Nature of Stars Describe a Population of Stars Individual Stars Location in Space Coordinate angular position on the sky Right ascension Declination Distance from Earth use Stellar Parallax Motion through Space Motion across the sky proper motion Motion toward away from us radial velocity use Doppler Effect Google Earth Sky Stellar Parallax 17 1 Understand Figs 17 1 17 2 and eyes thumb illustrations Star A exhibits a stellar parallax that is twice as large as the stellar parallax exhibited by star B Which star is farther from us How much farther away If parallax angle p is measured in arcseconds and distance is measured in parsecs see 1 7 and Fig 1 14 then d 1 p Stellar Parallax 17 1 Understand Figs 17 1 17 2 and eyes thumb illustrations Star A exhibits a stellar parallax that is twice as large as the stellar parallax exhibited by star B Which star is farther from us How much farther away If parallax angle p is measured in arcseconds and distance is measured in parsecs see 1 7 and Fig 1 14 then d 1 p March sky image September sky image Stellar Parallax 17 1 Understand Figs 17 1 17 2 and eyes thumb illustrations Star A exhibits a stellar parallax that is twice as large as the stellar parallax exhibited by star B Which star is farther from us How much farther away If parallax angle p is measured in arcseconds and distance is measured in parsecs see 1 7 and Fig 1 14 then d 1 p Individual Stars Location in Space Coordinate angular position on the sky Right ascension Declination Distance from Earth use Stellar Parallax Motion through Space Motion across the sky proper motion Motion toward away from us radial velocity use Doppler Effect 5 9 Motion Across the Sky proper motion http www psi edu esquerdo jim astfov gif Prominent and Obscured Objects Prominent and Obscured Objects NOTE Transient Events in time also occur NOTE Transient Events in time also occur NOTE Transient Events in time also occur NOTE Transient Events in time also occur NOTE Transient Events in time also occur Individual Stars Location in Space Coordinate angular position on the sky Distance from Earth Motion through Space Motion across the sky proper motion Motion toward away from us radial velocity Intrinsic properties Brightness luminosity magnitude Color surface temperature Mass Age Stars of different brightness Stars of different colors Individual Stars Location in Space Coordinate angular position on the sky Distance from Earth Motion through Space Motion across the sky proper motion Motion toward away from us radial velocity Intrinsic properties Brightness luminosity magnitude Color surface temperature Mass Age Astronomers Magnitude System Ancient Greek astronomers made catalogues of all the visible stars in the sky Name Position on the sky angular coordinates Any observed motion Brightness on the sky hereafter apparent brightness The Greeks defined a magnitude system to quantify the apparent brightness of each star Astronomers Magnitude System The brightest stars were labeled 1st magnitude stars Successively fainter stars were catalogued as 2nd magnitude 3rd magnitude etc Faintest stars visible to the naked eye were catalogued by Greek astronomers as 6th magnitude stars Astronomers continue to use this magnitude system extending it to much fainter objects that are visible through telescopes but were not bright enough to be seen by Greek astronomers The Sun can also be put on this magnitude system Stars of different brightness Apparent brightness due to Each star s intrinsic brightness Each star s distance from us A star of a given intrinsic brightness will appear to get fainter and fainter if you move it farther and farther away from us Concept of Apparent Brightness 10 stars that are identical in every respect all having for example the same intrinsic brightness will appear to have different brightness in the night sky if they are all at different distances from us Apparent brightness varies as the inverse square of the distance Move a star twice as far away it becomes 4 times fainter Move a star 3 times farther away it becomes 9 times fainter Move a star 10 times farther away it becomes 100 times fainter Move a star to half its original distance it becomes 4 times brighter Move a star to 1 10th its original distance it becomes 100 times brighter Apparent Brightness varies with Distance Individual Stars Location in Space Coordinate angular position on the sky Distance from Earth Motion through Space Motion across the sky proper motion Motion toward away from us radial velocity Intrinsic properties Brightness luminosity magnitude Color surface temperature Mass Age Color Temperature Relationship More About Continuous Spectra from Hot Dense Gases or Solids Kirchhoff s 1st Law Hot dense gas produces a continuous spectrum a complete rainbow of colors A plot of light intensity versus wavelength always has the same general appearance blackbody function Very little light at very short wavelengths Very little light at very long wavelengths Intensity of light peaks at some intermediate wavelength But the color that marks the brightest intensity varies with gas temperature Hot objects are bluer Cold objects are redder The Sun s Continuous Spectrum Textbook Figure 5 12 Wien s Law for Blackbody Spectra As the textbook points out 5 4 there is a mathematical equation that shows precisely how the wavelength color of maximum intensity varies with gas temperature