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    florx's Avatar
    florx Posts: 1, Reputation: 1
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    #1

    Mar 18, 2010, 10:05 PM
    [PreCal] Need help with Magnitude!!
    The magnitude of M of a star is a measure of its brightness B given by the formula M = -2.5 log (B/B0), where B0 is a constant, the brightness of an "average star."

    a) If a star's brightness increases by a factor of 10, by how much does its magnitude increase or decrease?

    I got the answer from the back of the book and it says "decreasing by -2.5." How did they get this answer? Please help me with this problem.

    Thank you so much!
    KISS's Avatar
    KISS Posts: 12,510, Reputation: 839
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    #2

    Mar 19, 2010, 03:18 AM

    Increasing by a factor of 10 is a ratio of B0/B of 10/1 or 10.

    LOG(10) = 1; Base 10 logarithm. The exponent of 10^x.

    Thus M = -2.5*LOG(10) = -2.5
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