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Announcements

Announcements. Pick up graded homework Lecture slides on web site have been updated Take the test W, Th, or F before class Physics seminar today, 1:00, SL 121: The Utah Science Center, Joe Andrade. Properties of Stars. 25 October 2006. Today:.

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Announcements

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  1. Announcements • Pick up graded homework • Lecture slides on web site have been updated • Take the test W, Th, or F before class • Physics seminar today, 1:00, SL 121: The Utah Science Center, Joe Andrade

  2. Properties of Stars 25 October 2006

  3. Today: • More properties of stars: brightness, temperature, size • A survey of the stars in our neighborhood

  4. Brightness of Stars • True brightness (or “luminosity”) is a star’s actual rate of energy output, measured (for example) in watts. The sun’s luminosity is about 4 x 1026 watts. • Apparent brightness is determined by the “intensity” of starlight striking a detector. It is measured (for example) in watts per square meter. The sun’s apparent brightness from earth’s location is about 1400 watts per square meter. True brightness Formula: Apparent brightness = 4π(distance)2

  5. Brightness of Stars True brightness Formula: Apparent brightness = 4π(distance)2

  6. How far away are the stars? • A clue: Compare brightness of our sun to brightness of stars . . . It’s the difference between night and day!

  7. Magnitude system for brightness • Smaller numbers imply brighter stars. • “Apparent magnitude” is a measure of apparent brightness. Antares has mag. 1; Polaris has mag. 2; naked eye limit is about 6. Sirius has mag. –1.5. • “Absolute magnitude” is a measure of true brightness. It’s what the apparent magnitude would be if the star were 33 light-years away. Sun’s absolute magnitude is about 5. • The formulas that relate magnitudes to brightnesses (in watts or W/m2) are complicated and not so important.

  8. Names of Stars • Brightest stars have actual names, like Sirius, Canopus, Betelgeuse, Rigel, Vega, Polaris. • Prominent stars in each constellation have Greek-letter designations, like Alpha Centauri, Epsilon Eridani, Tau Ceti. • For dimmer stars, must resort to various numbering schemes.

  9. The Nearest Stars

  10. The Brightest Stars(as viewed from earth)

  11. Sirius, Canopus, Orion

  12. Star colors and temperatures

  13. Calculating sizes of stars Luminosity = (constant) x (4πR2) x (temperature)4 Suppose two stars (A and B) have the same temperature, but A is 100 times more luminous than B. How do their sizes compare? A must have 100 times as much surface area, but this means its radius (or diameter) is only 10 times greater. A B

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