read this it's the US government answer .
http://www1.va.gov/opa/feature/celebrate...
The flag of the United States is one of the oldest national standards in the world. General George Washington first raised the Continental Army flag in 1776, a red-and-white striped flag with the British Union Jack where we now have stars.
Several flag designs with 13 stripes were used in 1776 and 1777, until Congress established an official design on June 14, 1777 — now observed as Flag Day. The act stated, “That the Flag of the thirteen United States be thirteen stripes, alternate red and white, that the union be thirteen stars, white in a blue field, representing a new constellation.” Washington explained it this way: “We take the stars from heaven, the red from our mother country, separating it by white stripes, thus showing that we have separated from her, and the white stripes shall go down to posterity representing liberty.”