When George Washington took office, there were 13 stars on the flag. The number changed to 15 while he was in office. When two new States were admitted to the Union (Kentucky and Vermont), a resolution was adopted in January of 1794, expanding the flag to 15 stars and 15 stripes. Anyway, the number of stripes was reduced to 13 later to represent the original 13 colonies. The stars continued symbolizing the States, and now there are 50 of them on the flag.

The Second Continental Congress passed the Flag Resolution Act in 1777. It stated that the flag of the United States would have thirteen stripes alternating red and white in color with a union of thirteen white stars in a blue field. The stars were used to symbolically represent a new constellation.

More Info: en.wikipedia.org