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Homework Help: Luminosity to magnitude

  1. Nov 17, 2011 #1
    I am trying to find out how fast a star's magnitude changes over time, and have two columns of data.
    The first is the time since the stay left the main sequence, and the second is Log (L/L_sun).

    I am unsure as to how to chance the luminosity values into magnitudes, which means I cant answer the question.

    I thought it might be m=2.5 log L but I dont know if this is correct or not.

    If anyone can help it would be greatly appreciated

    Thanks :D
  2. jcsd
  3. Nov 17, 2011 #2
    Depends whether you want apparent or absolute magnitude.
    For apparent the formula would be
    [itex]m_1 - m_2 = -2.5 \log{\frac{L_1}{L_2}}[/itex]
  4. Nov 17, 2011 #3
    But i'm not given actual luminosities, i've been given Log (L/L_sun)
    so for example one of my values is 4.4799, how would i turn this into a magnitude?

    And yes, I was looking for an apparent magnitude, forgot to mention it in my first post
  5. Nov 17, 2011 #4
    [itex]m_1 = m_{sun} - 2.5\log{\frac{L}{L_{sun}}}[/itex]
    where [itex]m_{sun} = –26.74[/itex]

    So for the value you gave you'd get
    [itex]m_1 = -26.74 - 2.5*4.4799[/itex]
    which gives a pretty high magnitude as the lower the magnitude, the brighter the object.
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