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Practice Finding Angular Size in an Astronomy Context

Shown below is an optical image of Betelgeuse. Note that unlike most stars, which appear as point sources, Betelgeuse is big enough and close enough that the Hubble Space Telescope was able to resolve it. Use the ruler on the bottom of the image to estimate the physical size of Betelgeuse, in km. You will need to convert from cm, and it may be easier to make careful measurements if you first print out the page with the image of Betelgeuse and the ruler on it. Using the expression for angular size,

angular size equation

find the angular size of Betelgeuse, in arcseconds. A multiplicative factor of 6.7 x 10-9 must be used so that the answer is in arcseconds (rather than in radians). The distance to Betelgeuse approximatly 130 pc.

optical image of Betelgeuse


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