I have a question about spheres, because I am having a problem that I don't really know how to solve.  Basically, I have clusters of stars, and I want to know how many stars are within 6 pc of each star.  So, what I have done is taken a sphere at each star position with a radius of 6 pc, and then performed 

sp.quantities.total_quantity(["particle_mass"])

Now I wanted to double-check this number by brute-force, so I calculated the distance between all the particles and then summed the masses of all the particles that were less than 6 pc.  

I am not getting the same values--they are close, but not the same (they seem to mostly scatter up or down by a factor of 2).  

So...to the question!!  Because I am measuring particles in a grid code (Enzo) is there any sort of shifting to the grid position instead of using the particle positions here? Because stars are pretty packed I could imagine that this could cause the differences I am seeing.

I am not sure that the problem is not something silly I am doing, but wanted to check this possibility as well.

Thanks!

Stephanie