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: m_{a}-m_{b}= -2.5log_{10}[itex]\left( \frac{F_a}{F_b}\right)[/itex] but I just have F_{a} and I have to obtain m_{a}. Is it valid to take m_{a}=-2.5log_{10} F_{a}?
the "b" indexed values would have to be some reference value that depends on the scale you are using. http://en.wikipedia.org/wiki/Apparent_magnitude#Standard_Reference_Values
... 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.