Since the star of Bethlehem can't be an asteroid, comet, nova or supernova, it has to be Jupiter. And sometimes Jupiter can get quite close to Earth. In 6 BC there was a triple conjunction with Saturn which gives it what is called a retrograde motion over Bethlehem. Furthermore, when it goes behind the moon it is not seen and then reappears pointing to the birth of the Son of God. The magi had their own way of looking at the stars, so it need not be a big star. All that is required is that it has significance to them. And it does since it is a rare triple conjunction. It happened in 7-6 BC and won't happen again until 7541 AD in the occultation of Jupiter with Saturn appearing to merge into a single object.

Praise the Lord for this discernment!

http://en.wikipedia.org/wiki/Great_conjunction