Hey I have this question and I am wrestling with it all day and I am completely lost.

The V magnitude of a star is 15.1, B-V =1.6, and absolute magnitude Mv= 1.3 The extinction in the direction of the star in the visual band is av = 1 mag/kpc. What is the intrinsic colour?

Can somebody help me?

Edit: Ok, apparently I have to calculate first the distance and after that the excess colour and then I can calculate the intristic colour.

I guess I need this formula for the distance: V = Mv + 5log(r/10pc) + Av But if i fill this in and get the R I get a different r and I dont know what the next step if to go to the excess colour.


closed as off-topic by Kyle Kanos, Waffle's Crazy Peanut, John Rennie, Qmechanic Jan 9 '14 at 15:06

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  • $\begingroup$ What didn't you understand here? $\endgroup$ – Torsten Hĕrculĕ Cärlemän Jan 9 '14 at 12:06
  • $\begingroup$ How I can go from these values tot the intrinstic colour. I have no idea what formulas I need to use $\endgroup$ – user1200276 Jan 9 '14 at 12:51

A star's color has almost nothing to do with its visual magnitude. The color is a function of size, age, etc. per the OBAFGKMNS classification scale (roughly). The visual magnitude is a measure of the light intensity (in the visual spectrum) which reaches us on Earth, so it depends not only on the star's color but its overall output and distance.
Further, the perceived color is redshifted in proportion to the speed at which the star is receding from us (which in general is a function of distance).


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