# Flux & apparent magnitude

1. ### Schmetterling

31
How to calculate the visual magnitude of a star having only the flux of such star?
The equation involves 2 magnitudes and 2 fluxes, corresponding to 2 objects a and b:

ma-mb= -2.5log10$\left( \frac{F_a}{F_b}\right)$

but I just have Fa and I have to obtain ma.

Is it valid to take ma=-2.5log10 Fa?

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14,658
4. ### Schmetterling

31
Ok, but in that table the values for "m" don't appear, only the fluxes :-s

5. ### Simon Bridge

14,658
... yes they do: right at the top of the 4th and 5th columns where it says "m=0". But you can use any reference values that make sense in terms of your coursework.
Specifically: you should not get your reference-values off wikipedia. You must have access to a library or a text book or something like that?
In a pinch - go to wikipedias primary source.