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H-R Diagram

H-R Diagram. Absolute Bolometric Magnitude. The ABM value includes ALL types of EMR a star emits. The ABM is the absolute magnitude the star would have if we could see all wavelengths. We generally compare the ABM value with that of the Sun, which is a 4.7. Luminosity, Radius, & Temperature.

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H-R Diagram

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  1. H-R Diagram

  2. Absolute Bolometric Magnitude • The ABM value includes ALL types of EMR a star emits. • The ABM is the absolute magnitude the star would have if we could see all wavelengths. • We generally compare the ABM value with that of the Sun, which is a 4.7

  3. Luminosity, Radius, & Temperature • Luminosity is affected by surface area and temperature. • A hot star with a small surface area may not be very luminous… think about a candle. • Remember E = T4 and the surface area for a sphere is 4πr2.

  4. 0 The Size (Radius) of a Star We already know: flux increases with surface temperature (~ T4) But brightness also increases with size: Star B will be brighter than star A. A B Absolute brightness is proportional to radius squared, L ~ R2. Quantitatively: L = 4 p R2s T4 Surface flux due to a blackbody spectrum Surface area of the star

  5. 0 Example: Star Radii Polaris has just about the same spectral type (and thus surface temperature) as our sun, but it is 10,000 times brighter than our sun. Thus, Polaris is 100 times larger than the sun. This causes its luminosity to be 1002 = 10,000 times more than our sun’s.

  6. 0 Organizing the Family of Stars: The Hertzsprung-Russell Diagram We know: Stars have different temperatures, different luminosities, and different sizes. To bring some order into that zoo of different types of stars: organize them in a diagram of Luminosity Temperature (or spectral type) versus Absolute mag. Hertzsprung-Russell Diagram Luminosity or Temperature Spectral type: O B A F G K M

  7. 0 The Hertzsprung-Russell Diagram Most stars are found along the Main Sequence

  8. 0 The Hertzsprung-Russell Diagram (2) Same temperature, but much brighter than MS stars Stars spend most of their active life time on the Main Sequence (MS).

  9. 0 The Brightest Stars The open star cluster M39 The brightest stars are either blue (=> unusually hot) or red (=> unusually cold).

  10. 0 The Radii of Stars in the Hertzsprung-Russell Diagram Betelgeuse Rigel 10,000 times the sun’s radius Polaris 100 times the sun’s radius Sun As large as the sun

  11. 0 The Relative Sizes of Stars in the Hertzsprung-Russell Diagram

  12. 0 Luminosity Classes Ia Bright Supergiants Ia Ib Ib Supergiants II II Bright Giants III III Giants IV Subgiants IV V V Main-Sequence Stars

  13. 0 Example: Luminosity Classes • Our Sun: G2 star on the Main Sequence: • G2V • Polaris: G2 star with Supergiant luminosity: G2Ib

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