Algorithm to find 100 closest stars to the origin

Question

First let me phrase the proper question:

Q: There is a file containing more than a million points (x,y) each of which represents a star. There is a planet earth

Answer 1

It's a famous question and there has been lot's of solution for that: http://en.wikipedia.org/wiki/K-nearest_neighbor_algorithm

if you did not find it useful, there are some other resources such as Rurk's computational geometry book.