flux ratio of a star

[alexpi]

Hello

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 1 μm

 

Thanks

Alex

[vlaiv]

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).

https://en.wikipedia.org/wiki/Planck's_law

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).

[robin_astro]

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

[George Jones]

From where did this question come?