ASTR 1102 002 2008 Fall Semester Joel E Tohline Alumni Professor Office 247 Nicholson Hall Slides from Lecture06 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 Apparent magnitudes m Catalog of Stars Data drawn from two textbook appendices Appendix 4 The Nearest Stars Appendix 5 The Visually Brightest Stars Stars of different brightness Intrinsic Brightness Distribution of Stars in our Galaxy 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 unusual in this respect because it does not have a companion star about which it orbits Measuring Stellar Masses Intrinsic Brightness vs Stellar Mass