chapter15 - Chapter 15 Surveying the Stars 15.1 Properties...

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Unformatted text preview: Chapter 15 Surveying the Stars 15.1 Properties of Stars • Our goals for learning • How do we measure stellar luminosities? • How do we measure stellar temperatures? • How do we measure stellar masses? How do we measure stellar luminosities? The brightness of a star depends on both distance and luminosity Luminosity: Amount of power a star radiates (energy per second = Watts) Apparent brightness: Amount of starlight that reaches Earth (energy per second per square meter) Thought Question These two stars have about the same luminosity -- which one appears brighter? A. Alpha Centauri B. The Sun Thought Question These two stars have about the same luminosity -- which one appears brighter? A. Alpha Centauri B. The Sun Luminosity passing through each sphere is the same Area of sphere: 4 (radius) π 2 Divide luminosity by area to get brightness The relationship between apparent brightness and luminosity depends on distance: Luminosity Brightness = 4 (distance) π 2 We can determine a star’s luminosity if we can measure its distance and apparent brightness: Luminosity = 4 (distance) π 2 x (Brightness) Thought Question How would the apparent brightness of Alpha Centauri change if it were three times farther away? A. It would be only 1/3 as bright B. It would be only 1/6 as bright C. It would be only 1/9 as bright D. It would be three times brighter Thought Question How would the apparent brightness of Alpha Centauri change if it were three times farther away? A. It would be only 1/3 as bright B. It would be only 1/6 as bright C. It would be only 1/9 as bright D. It would be three times brighter So how far are these stars? Parallax is the apparent shift in position of a nearby object against a background of more distant objects Apparent positions of nearest stars shift by about an arcsecond as Earth orbits Sun Parallax angle depends on distance Parallax is measured by comparing snapshots taken at different times and measuring the shift in angle to star Parallax and Distance p = parallax angle d (in parsecs) = 1 p (in arcseconds) d (in light - years) = 3.26 ´ 1 p (in arcseconds) Most luminous stars: 10 6 L Sun Least luminous stars: 10-4 L Sun (L Sun is luminosity of Sun) The Magnitude Scale m = apparent magnitude , M = absolute magnitude apparent brightness of Star 1 apparent brightness of Star 2 = (100 1/ 5 ) m 1- m 2 luminosity of Star 1 luminosity of Star 2 = (100 1/ 5 ) M 1- M 2 How do we measure stellar temperatures? Every object emits thermal radiation with a spectrum that depends on its temperature An object of fixed size grows more luminous as its temperature rises Properties of Thermal Radiation...
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chapter15 - Chapter 15 Surveying the Stars 15.1 Properties...

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