The astronomer has to choose 3 star images out of 8.This is a problem of combinations. This can be expressed as 8C3 i.e. combination of 8 objects taken 3 at a time.
[tex]8C3= \frac{8!}{3!*5!}=56 [/tex]
This means, if 8 images are nearby the astronomer will have 56 choices of the three stars.
