Lecture_12

# Lecture_12 - Properties of Stars Surveying the Stars Our...

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1 Surveying the Stars Surveying the Stars Properties of Stars Properties of Stars Our Goals for Learning • How luminous are stars? • How hot are stars? • How massive are stars? Not all stars are exactly like the Sun ! Luminosity: Amount of power a star radiates (energy per second) The units of power is: 1 Joule per second = 1 Watt Apparent brightness: Amount of starlight that reaches Earth - the energy per second per square meter Brightness depends on LUMINOSITY DISTANCE Thought Question These two stars have about the same luminosity -- which one appears brighter? A. Alpha Centauri (4.2 lightyears away) B. The Sun _________________ … because it is much closer

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2 Luminosity passing through each sphere is the same – conservation of energy Area of sphere: = 4 π (radius) 2 Divide Luminosity by AREA to get brightness Relating brightness and luminosity The relationship between apparent brightness and luminosity depends on distance: Luminosity Brightness = 4 π (distance) 2 We can determine a star’s luminosity if we can measure its distance and apparent brightness: Luminosity = 4 π (distance) 2 x (Brightness) Brightness range is enormous so astronomers use a logarithmic or MAGNITUDE scale to compress the range . Each factor of 2.5 times in brightness is 1 magnitude step. Five steps =100x. 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 Getting the distance: Reminder: parallax gives distance to all stars within ~320 lightyears d = 1 p When p is in arc seconds, d is in PARSECS. 1pc = 3.26 ly PARALLAX!
3 Every object emits thermal radiation with a spectrum that depends only on its (absolute) temperature (T) How hot are stars? How hot are stars? 1) Hotter objects emit more “light” (energy) at all wavelengths 2) Hotter objects emit light at shorter wavelengths (higher frequencies); T ~ 3,000,000K/Wavelength at peak (nm) Laws of Thermal Radiation UV Infrared Absorption lines in a star’s spectrum correspond to a spectral type that also reveals temperature (through ionization level of atoms involved). (Hottest)

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Lecture_12 - Properties of Stars Surveying the Stars Our...

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