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Math Help - Stellar Magnitutde Scale and log

  1. #1
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    Stellar Magnitutde Scale and log

    How bright a star appears can depend on how much light the star actually emits and how far away it is. The stellar magnitude scale can be adjusted to account for distance as follows:

    M_2 - M_1 = log (b_1/b_2)
    Here, M refers to a star's absolute magnitude, that is, how brightly it appears from a standard distance of 10 parsecs (or 32.6 light-years). The absolute brightness of Sirius is 1.4 and the absolute brightness of Betelgeuse is -8.1.

    a) Which of these two stars is brighter, in absolute terms, and by how much?

    Ok, so I am not sure what I should do here.
    But what I did try was subbing in 10 for m_2 and subbing in 1.4 for M_1. and then solving for (b_1/b_2). I ended up getting 10^8.6 for (b_1/b_2) which is for Sirius.

    Then I did the same for Betelgeuse. subbing in 10 for M_2 and -8.1 for m_1 and solving for (b_1/b_2). I ended up getting 10^18.1 = (b_1/b_2)

    Then the question asks: Which of these two stars is brighter, i absolute terms, and by how much?

    My answer is that Betelgeuse is brighter than Sirius by 10^18.1- 10^8.6 = ?

    Did I do any of that right? Please provide an explanation. Thank you!
    Last edited by skeske1234; May 16th 2009 at 10:02 AM.
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by skeske1234 View Post
    How bright a star appears can depend on how much light the star actually emits and how far away it is. The stellar magnitude scale can be adjusted to account for distance as follows:

    M_2 - M_1 = log (b_1/b_2)
    Here, M refers to a star's absolute magnitude, that is, how brightly it appears from a standard distance of 10 parsecs (or 32.6 light-years). The absolute brightness of Sirius is 1.4 and the absolute brightness of Betelgeuse is -8.1.

    a) Which of these two stars is brighter, in absolute terms, and by how much?

    Ok, so I am not sure what I should do here.
    But what I did try was subbing in 10 for m_2 and subbing in 1.4 for M_1. and then solving for (b_1/b_2). I ended up getting 10^8.6 for (b_1/b_2) which is for Sirius.

    Then I did the same for Betelgeuse. subbing in 10 for M_2 and -8.1 for m_1 and solving for (b_1/b_2). I ended up getting 10^18.1 = (b_1/b_2)

    Then the question asks: Which of these two stars is brighter, i absolute terms, and by how much?

    My answer is that Betelgeuse is brighter than Sirius by 10^18.1- 10^8.6 = ?

    Did I do any of that right? Please provide an explanation. Thank you!
    First thing that is wrong is the difference in magnitudes is:

    M_2 - M_1 = 2.5 log_{10} (b_1/b_2)

    CB
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