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Unformatted text preview: The Family of Stars Chapter 8: We already know how to determine a stars surface temperature chemical composition surface density In this chapter, we will learn how we can determine its distance luminosity radius mass and how all the different types of stars make up the big family of stars. d = __ p 1 d in parsec (pc) p in arc seconds 81 Distances to Stars 81 Distances to Stars Parallax is the apparent change in the position of an object due to a change in the location of the observer. Because the stars are so distant, their parallaxes are very small, usually measured in seconds of arc. The quantity that astronomers call stellar parallax (p) is half the total shift of the star. Parallax & Distance Example : The nearest star, Centauri, has a parallax of p = 0.76 arc seconds d = 1/p = 1/0.76 = 1.3, therefore d = 1.3 pc which equals 4.3 LY 1 parsec (pc) is the distance to an imaginary star that has a parallax of 1 arc second. (1 pc = 206,265 AU = 3.26 ly). Due to the effects of Earths atmosphere, telescopes on earth cannot measure parallaxes smaller than about 0.02 seconds of arc . A parallax of 0.02 corresponds to a distance of 50 pc. The formula is d = 1/p where d, the distance to the star, is in parsecs and p, the parallax angle, is in arc seconds. 82 Intrinsic Brightness 82 Intrinsic Brightness Remember the apparent visual magnitude brightness scale (chapter 2)? Apparent visual magnitude, m V , is a brightness scale that simply measures how bright a star appears to us, here on earth. No account is made for the stars being different distances from us. When we refer to the intrinsic brightness of a star, we mean a measure of the total amount of light the star emits. An intrinsically very bright star will appear faint if it is far enough away. To find the intrinsic brightness of a star, we must take into account how far away the star is from us. Brightness and Distance Lets say you have a 100 W light bulb. The amount of light energy from the light bulb reaching your eyes depends on how far away you are from the light bulb. The amount of light energy reaching your eyes follows an inverse square law. Ex: If you move twice as far away you only get the light energy, three times as far away, 1/9 the light energy, four times as far away 1/16 the light energy, etc. If you know the apparent magnitude of a star and its distance from Earth, you can use the inverse square law to correct for distance and learn the intrinsic brightness of the star. Absolute Visual Magnitude If you know the distance to a star, you can use the inverse square law to calculate the brightness the star would have at some standard distance. (10 pc is the standard distance used.) The absolute visual magnitude (M V ) of a star is the is the apparent visual magnitude the star would have if were 10 pc (parsecs) away....
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 Fall '11
 Biegel

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