How can I calculate the visual magnitude of a star with only its flux data?

[Schmetterling]

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

but I just have F_a and I have to obtain m_a.

Is it valid to take m_a=-2.5log₁₀ F_a?

 

[Simon Bridge]

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

 

[Schmetterling]

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

 

[Simon Bridge]

Ok, but in that table the values for "m" don't appear

... 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 textbook or something like that?
In a pinch - go to wikipedias primary source.

 

[pmacias]

I would first like to clarify that the equation provided is known as the magnitude difference equation and it is used to calculate the difference in visual magnitudes between two objects. To calculate the visual magnitude of a star, we need to have both the flux of the star in question and the flux of a reference star with a known visual magnitude.

Since you have mentioned that you only have the flux of the star, it is not possible to calculate its visual magnitude using the magnitude difference equation. This is because the equation requires the flux of a reference star for comparison.

However, if you have the flux of a reference star with a known visual magnitude, then you can use the magnitude difference equation to calculate the visual magnitude of the star in question. In this case, the equation would be rearranged to solve for ma, as shown below:

ma = mb - 2.5log10 (Fa/Fb)

Where:
ma = visual magnitude of the star in question
mb = visual magnitude of the reference star
Fa = flux of the star in question
Fb = flux of the reference star

Therefore, it is not valid to simply take ma = -2.5log10 Fa as this does not take into account the flux of the reference star. It is important to have both the flux data of the star in question and a reference star to accurately calculate its visual magnitude.