# Astronomy, Intensity from Given Flux

jmm5872
A light source has uniform intensity I(with respect to wavelength) within a
circle of diameter 3 arcmin on the sky. The flux of radiation reaching the observer is
10^(−16) erg cm^(−2) s^(−1). Calculate the intensity of the light from this source.

Relevent Equations:

This is where my confusion comes in, I'm not really sure what the relavant equations are. The only equation I have found in the textbook is this:

F = Integral (d$$\omega$$)cos$$\theta$$

(I can't seem to get the latex to work correctly, I hope this makes sense as is).

I also know flux is watts per unit area, but I'm new to this on the astronomy side. I also know an erg is 10^-7 Ws. How would I use the 3 arcmin, would I find the area of a circle with 1.5 arcmin radius? Don't you need the distance to be able to calculate the flux? Any hints to get started would be great.

## Answers and Replies

Mentor
The flux should be the integral over the angular area (sterradians) of the intensity. Since the intensity is claimed to be constant, presumably you can divide the flux by the angular area to obtain the uniform intensity.