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The HR Diagram

The HR Diagram. Astronomy 315 Professor Lee Carkner Lecture 8. Flux and Luminosity. How bright a star looks (W/m 2 ) How bright the star is (W) We measure the flux, but we want to know the luminosity. Inverse Square Law. What determines how bright a star looks?

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The HR Diagram

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  1. The HR Diagram Astronomy 315 Professor Lee Carkner Lecture 8

  2. Flux and Luminosity • How bright a star looks (W/m2) • How bright the star is (W) • We measure the flux, but we want to know the luminosity

  3. Inverse Square Law • What determines how bright a star looks? • A star radiates energy in all directions • The flux is the luminosity divided by the area of that sphere F = L/4pd2 • Flux decreases as the inverse square of the distance

  4. Inverse Square Law

  5. Distance • How do we find distance? • Can’t get to a star or bounce a radar beam off it etc. • Find something you can measure that is related to the distance

  6. Parallax • e.g. thumb in front of your face • Larger the distance the smaller the shift

  7. Parallax Explained p • Half the total shift of the star is called the parallax angle p tan p = ½ B / d d ½ B

  8. Using Parallax • We normally use a simplified version of this equation: p = 1/d • d is in parsecs (1 pc = 3.26 light years = 3.09 X 1013 km) • Can only use parallax to get distances out to 100 pc (1000 pc from space)

  9. Absolute Magnitude • If we know the distance we can find the absolute magnitude (M) • They are related by: • Where m-M is called the distance modulus

  10. Classifying Stars • We can measure: • Use both to get luminosity • Use to get temperature • What results do we get for a large group of stars?

  11. The H-R Diagram • Make a plot of luminosity versus temperature (or absolute magnitude v. spectral type) • Note: L on y-axis, T on x-axis but increasing right to left • What do you see? • Stars concentrated in a diagonal band that rums from high L, high T to low L low T

  12. HR Diagram

  13. Regions on the Diagram • The line that the bulk of stars fall on is called the Main Sequence • Below the main sequence the stars are hot but dim • Above the main sequence we have stars that are bright but cool

  14. Size of Stars • We can relate the temperature and luminosity to the size with the Stefan-Boltzmann law • From the temperature and luminosity we can get the radius (R) • What do we find? • Blue main sequence stars -- • Red main sequence stars -- • Called red dwarfs

  15. Radius of Stars

  16. Luminosity Classes • Luminosity classes are used to specify where a star falls on the HR Diagram • In order of increasing brightness and size: • V -- • IV -- • III -- • II -- • I -- • The luminosity class is given after the spectral type:

  17. Luminosity Classes

  18. Census of Stars • A quick look at an HR diagram makes it seem as if all regions are equally populated • If you take a certain region of space and count all of the stars in it, you find: • Reasonable numbers of medium main sequence stars and white dwarfs

  19. Relative Numbers of Stars

  20. Selection Effect • Most stars are faint • From a casual look at the sky it would seem like most stars are bright • When you select a group of stars to study, the criteria you use to select them affects your answer to your study

  21. Spectroscopic Parallax • If you know the spectral type of a main sequence star you can estimate the luminosity from the HR diagram • Called spectroscopic parallax • Has nothing to do with real parallax

  22. Finding Star Properties

  23. Physics: apparent shift of object from different vantage points Property: Physics: inverse square law Property: Physics: Spectral lines depend on temperature Property: Physics: Stefan-Boltzmann Law Property: Determining Star Properties

  24. Next Time • Read Chapter 17.8 • List 1 due Friday

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