ASTR 1102 002 2008 Fall Semester Joel E Tohline Alumni Professor Office 247 Nicholson Hall Slides from Lecture05 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 Chapter 17 The Nature of Stars 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 Apparent magnitudes m Stars of different brightness Catalog of Stars Data drawn from two textbook appendices Appendix 4 The Nearest Stars Appendix 5 The Visually Brightest Stars 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 Determining Intrinsic Brightness Measure apparent magnitude m Measure distance to star using stellar parallax Move star to a standard distance from us 10 parsecs and adjust its magnitude accordingly absolute magnitude M The absolute magnitude of each star in our catalog tells us how bright the star would appear to be if it was 10 parsecs away from us M is therefore a measure of each star s intrinsic brightness Intrinsic Brightness Distribution of Stars in our Galaxy Catalog of Stars Data drawn from two textbook appendices Appendix 4 The Nearest Stars Appendix 5 The Visually Brightest Stars 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 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 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 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 Color Temperature Relationship 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 Color Filters U B V 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 Intrinsic Brightness vs Color Hertzsprung Russell H R diagram 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 Measuring Stellar Masses Astronomers determine the mass of a star by examining how strong the gravitational field is around that star Isaac Newton s law of universal gravitation 4 7 By studying the motion of planets around our Sun astronomers have determined that the Sun has a mass of 2 x 1030 kilograms We cannot measure the mass of individual isolated stars We have an opportunity to measure the mass of a star if it resides in a binary star system Fortunately most stars are in binary systems The Sun is