The pentagram is a fascinating arcane symbol and well known to Neopagans and occultists.
In my own view, the pentagram's origins are in part associated with the passage of the planet Venus through the skies, a view propounded by Dan Brown's The Da Vinci Code, but don't let that put you off. Others, such as this Freemasonry website, dispute it, and I would hazard a guess that's because of the sacred feminine associations of Venus, as the Freemasons are a very masculine association.
I know there's a lot of current interest in this question, and Dan Brown's assertion that a four-year venus cycle informs the Olympics periodicity seems wrong to me. As far as I know, the Venus path is on an eight-year cycle. After that period, Venus, the Sun, Earth and the stars are in the same relative positions (more).
However, I certainly ain't no astronomer and can only go by what I read. If you have any information at all associated with this matter, or anything to do with the pentacle and its origins, I'd be grateful if you'd let me know.
Nick Anthony Fiorenza of Lunar Planner/The Venus Transit has very kindly permitted the use of his animation. I recommend his fascinating site.