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ch15 - Ch15-most fundamental properties of stars luminosity...

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Ch15 -most fundamental properties of stars: luminosity, surface temp, and mass -brightness depends both on its distance and how much light it actually emits. -brightness: 1. Apparent brightness (flux) = brightness of a star as it appears to our eyes – amount of starlight reaching Earth 2. luminosity= the total amount of pwr that a star emits into space (regardless of distance) -inverse square law for light: apparent brightness of a star or any other light source regarding the distance – if we viewed the sun from twice Earth’s distance, it would appear dimmer by a factor of 2^=4. -the amount of light received per unit area decreases w/ increasing distance by the square of the distance - apparent brightness = luminosity / (4π * distance^2) -standard units of luminosity are watts, apparent brightness – watts per square meter -total luminosity and total apparent brightness – only if we could detect photons across the entire electromagnetic spectrum -stellar parallax = the small annul shifts in a star’s apparent position caused by Earth’s motion around the Sun -the more distant stars have smaller parallax angles. -parsec (pc) = the distance to an object w/ a parallax angle of 1 arcsecond d (in parsecs) – 1/ p (in arcseconds) d (in light yrs) = 3.26 * 1/p(in arcseconds) -Stars have a wide range of luminosities, w/ our Sun somewhere in the middle. -Dim stars are far more common than bright stars. Our Sun is still brighter than the vast majority of stars in our galaxy even though it is roughly in the middle. -Magnitude system = classified stars according to how bright they look to human eyes in order -Apparent magnitudes – compare how bright diff. stars appear in the sky -A larger apparent magnitude means a dimmer apparent brightness -Absolute magnitudes = the apparent magnitude a star would have if it were at a distance of 10 parsecs from Earth -Only surface temp, not interior temp., is directly measurable. -We determine surface temp from either the star’s color or its spectrum.
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