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I have an issue to calculate flux ratio of a star at 1 μm and  0.55μm.

The informations I have are:

Apparent magnitude : 7 mag for 0.55μm

the star is type A

magnitude is  given by : m=-2,5 log (f) -56

I think to substract the magnitude but I don't have information of the magnitude at μm




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Hi Alex, and welcome to SGL.

I'm not sure what the question is?

To calculate ratio of photon flux at 1um and 0.55um - you don't really need magnitude of the star. Stellar class, or particularly it's temperature is enough (if you do black body approximation).


You plug in numbers for 1um and 0.55um and take ratio of the two. That will give you energy flux ratio. If you want photon flux ratio - you need to account for energy of photon at particular frequency (0.55um photons will be more energetic than those at 1um).

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Take care though.  The spectra of most stars are not black bodies and the effective temperature Teff normally quoted for a star is the temperature of a black body with the same total luminosity  which for A stars is very different from a best fit Planck curve to the visible part of the spectrum eg from Wikipedia where Teff is 9500K but the best fit curve is 15000K


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