It is no easy job to mark off a piece of sky and count the stars you can see. Even with all their skills and equipment, the astronomers find it just as hard to count the stars. In fact, this job cannot be done with perfect accuracy, for certain dark patches of the starry sky are actually clouds of gas hiding goodness knows how many stars behind them. Astronomers, however, can estimate the stars in our Galaxy. After long and patient detective work they have formed a picture of our great cartwheel of stars and know where the star populations are thin and where they are dense. They can count the stars in telescope pictures of this or that part of the heavens and estimate the number of stars in the entire Galaxy. The estimate, which is 100 billion stars, may be accurate to within a billion or so.