astr2 - Stellar Characteristics 14/09/2009 12:20:00...

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Unformatted text preview: Stellar Characteristics 14/09/2009 12:20:00 Parallax Measurements Parallax is the apparent displacement of an object because of a change in the observers point of view Ex: alternate looking with one eye at a time. Notice the apparent shift against the background. Your brain knows how far apart your eyes are and measures the angle of the apparent shift. The closer an object is to you the ______ the apparent shift? Larger Distance calculation D=1/p D= distance to object o Parsecs 1 pc = 3.26 lightyears = 3.09 times 10^13 km = 206.265 au sun earth (1 au = 1 arcsec), arrow pointing up from sun is distance = 1 pc light year is a measure of distance p = parallax angle (1/2 of apparent angular shift) o arcseconds 1 degree of the sky is 2 full moons side by side o 1 degree = 60 arcmin, 1 arcmin = 60 arcseconds Stellar motions Space velocity o Magnitude and direction of movement Vt = 4.74ud o U = arcsec/yr (1/360 of a degree) o D = parsec Vr = c(lambda lambda-nott)/lambda-nott o If received is shorter than emitted it is blue shifted, opposite is red shifted o If V is positive value then star is moving away from us, red shifted V = (vt^2+vr^2)^.5 or squareroot of (vt^2+vr^2) o The real motion, one of interest o Apparent Brightness How bright it appears to you, b Brightness varies with distance o Farther you are, the dimmer it gets Luminosity is how much energy the star actually emits into space, L Inverse square law Sa 4pir^2 Brightness is 1/r^2 B = L/4pid^2 So if you double the distance from the star, how does the apparent brightness change? Brightness decreases by a factor of 4 Luminosity We can compare all stars to our Sun, so that we can calculate luminosity 4 solar luminosity means it pumps out 4 times the luminosity of the sun L = 4 pi d^2 b L-knot = 4 pi d-nott^2 b-knot this is for the sun Dividing star by the sun we get: o L/L=knot = (d/d-knot)^2 times (b/b-knot) o L = 4 L-knot o D-knot is always 1 AU Luminosity Function Characterization of stellar populations Count stars within a certain distance Plot numbers of stars at different luminosities o Literally, form earth you count stars, then categorize by brightness/luminosity, then you plot it o o shows that there are more little, dim stars than big, bright stars; radius doesnt matter you still get the same plot o sun is an average star o for every magnitude there is a luminosity Magnitude Scale Hipparchus scale...
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astr2 - Stellar Characteristics 14/09/2009 12:20:00...

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