f10_nov09 - Ast 307 - Nov. 9, 2010 ch. 17 & 19: Lives of...

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Ast 307 - Nov. 9, 2010 The Hertzsprung-Russell Diagram M V Spectral type bright faint hot cool A very useful diagram for understanding stars Plot the two main observable properties of stars: • Temperature (x) vs. Luminosity (y), or Spectral Type (x) vs. Absolute Magnitude (y) Stars tend to group into certain areas The Hertzsprung-Russell Diagram Stars can be plotted on a graph of luminosity vs. temperature. If you do this, the majority of stars fall along a diagonal track which is called the “Main Sequence”; 90% of stars lie on the Main Sequence. Stellar Luminosity Classes • How can two stars have the same temperature, but vastly different luminosities? • The luminosity of a star depends on 2 things: • surface area (varies as radius squared) • (thermal) radiation emitted per unit surface area • L = 4 ! R 2 " T 4 • The stars have different sizes!! • Conclusion: stars in the upper right corner of the H-R Diagram must be very large.
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In some cases you can tell the luminosity class directly from the widths of spectral lines. Masses of Stars • Mass is the most important property of any star. • The mass of a star can only be measured directly by observing its gravitational effect on another object • We do this by observing two stars which orbit each another: a binary star system. Measuring Masses from Binary Stars • We already know how to use orbits to determine the masses of the objects involved: P 2 = 4 " 2 G ( M 1 + M 2 ) a 3 OR M 1 + M 2 = 4 2 G # $ % % % % ' ( ( ( ( a 3 2
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This note was uploaded on 04/30/2011 for the course AST 317 taught by Professor Dinerstein during the Fall '10 term at University of Texas at Austin.

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f10_nov09 - Ast 307 - Nov. 9, 2010 ch. 17 & 19: Lives of...

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