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 10 26 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

5. Brightness of Stars Formula: Apparent brightness = True 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/m 2 ) 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 Name Distance (light years) Apparent magnitude Luminosity (compared to sun) Sun–26.71 Alpha Centauri A4.401.5 Alpha Centauri B4.41.40.44 Alpha Centauri C4.3110.00006 Barnard’s star5.99.50.00042 Wolf 3597.613.50.00002 HD 057358.37.50.0055 Luyten 726-8A8.412.50.00006 Luyten 726-8B8.413.00.00004 Sirius A8.6–1.421.8 Sirius B8.68.30.003 Ross 1549.410.50.00048 Ross 24810.312.30.00011

10. The Brightest Stars (as viewed from earth) Name Distance (light years) Apparent magnitude Luminosity (compared to sun) Sun–26.71 Sirius A8.6-1.421.8 Canopus310-0.614,000 Arcturus37-0.1110 Alpha Centauri A4.40.01.5 Vega250.048 Capella420.1130 Rigel7700.240,000 Procyon11.40.47.0 Betelgeuse4300.59400 Achernar1440.51070 Hadar (Beta Cen)5250.612,000

11. Sirius, Canopus, Orion

12. Star colors and temperatures

13. Calculating sizes of stars 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. Luminosity = (constant) x (4πR 2 ) x (temperature) 4 A B