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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: • More properties of stars: brightness, temperature, size • A survey of the stars in our neighborhood 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. Formula: Apparent brightness = True brightness 4π(distance)2 Brightness of Stars Formula: Apparent brightness = True brightness 4π(distance)2 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! 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. Names of Stars • Brightest stars have actual names, like Sirius, Canopus, Betelgeuse, Rigel, Vega, Polaris. • Prominent stars in each constellation have Greekletter designations, like Alpha Centauri, Epsilon Eridani, Tau Ceti. • For dimmer stars, must resort to various numbering schemes. The Nearest Stars Name Distance (light years) Sun Apparent Luminosity magnitude (compared to sun) –26.7 1 Alpha Centauri A 4.4 0 1.5 Alpha Centauri B 4.4 1.4 0.44 Alpha Centauri C 4.3 11 0.00006 Barnard’s star 5.9 9.5 0.00042 Wolf 359 7.6 13.5 0.00002 HD 05735 8.3 7.5 0.0055 Luyten 726-8A 8.4 12.5 0.00006 Luyten 726-8B 8.4 13.0 0.00004 Sirius A 8.6 –1.4 21.8 Sirius B 8.6 8.3 0.003 Ross 154 9.4 10.5 0.00048 Ross 248 10.3 12.3 0.00011 The Brightest Stars (as viewed from earth) Name Distance (light years) Sun Apparent Luminosity magnitude (compared to sun) –26.7 1 Sirius A 8.6 -1.4 21.8 Canopus 310 -0.6 14,000 Arcturus 37 -0.1 110 Alpha Centauri A 4.4 0.0 1.5 Vega 25 0.0 48 Capella 42 0.1 130 Rigel 770 0.2 40,000 Procyon 11.4 0.4 7.0 Betelgeuse 430 0.5 9400 Achernar 144 0.5 1070 Hadar (Beta Cen) 525 0.6 12,000 Star colors and temperatures 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