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ASTR 1102-002 2008 Fall Semester

ASTR 1102-002 2008 Fall Semester. Joel E. Tohline, Alumni Professor Office: 247 Nicholson Hall [Slides from Lecture06]. Gustav’s Effect on this Course.

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ASTR 1102-002 2008 Fall Semester

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  1. ASTR 1102-0022008 Fall Semester Joel E. Tohline, Alumni Professor Office: 247 Nicholson Hall [Slides from Lecture06]

  2. 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!

  3. Chapter 17: The Nature of Stars

  4. 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

  5. Apparent magnitudes (m)

  6. Catalog of Stars Data drawn from two textbook appendices: Appendix 4 = “The Nearest Stars” Appendix 5 = “The Visually Brightest Stars”

  7. Stars of different brightness

  8. Intrinsic Brightness Distributionof Stars in our Galaxy

  9. 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

  10. 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”

  11. 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”

  12. The Sun’s Continuous Spectrum (Textbook Figure 5-12)

  13. 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”

  14. Color-Temperature Relationship

  15. 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.

  16. Color Filters: U, B, V

  17. 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

  18. Intrinsic Brightness vs. Color

  19. Hertzsprung-Russell (H-R) diagram

  20. 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

  21. 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.

  22. Measuring Stellar Masses

  23. Intrinsic Brightness vs. Stellar Mass

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