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# class19a - Hertzsprung-Russell Diagram Astronomers have...

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Hertzsprung-Russell Diagram Astronomers have made surveys of the temperatures and luminosities of stars and plot the result on H-R (or Temperature-Luminosity) diagrams. Many stars fall on a diagonal line running from the upper left (hot and luminous) to the lower right (cool and faint). The Sun is one of these stars. But some fall in the upper right (cool and luminous) and some fall toward the bottom of the diagram (faint). What can we say about the stars in the upper right? What can we say about the stars toward the bottom? If all stars had the same size, what pattern would they make on the diagram?

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Masses of Stars The gravitational force of the Sun keeps the planets in orbit around it. The force of the Sun’s gravity is proportional to the mass of the Sun, and so the speeds of the planets as they orbit the Sun depend on the mass of the Sun. Newton’s generalization of Kepler’s 3 rd law says: P 2 = a 3 / M where P is the time to orbit, measured in years, a is the size of the orbit, measured in AU, and M is the sum of the two masses, measured in solar masses.

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class19a - Hertzsprung-Russell Diagram Astronomers have...

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