Professor Fred Grosse at Susquehanna University, Selinsgrove, Penn., believes that the Magi witnessed a planetary conjunction.
A conjunction happens when two celestial objects appear to pass very close to each other and, from our perspective on Earth, look like one large object. In 6 BCE, Jupiter and Saturn passed each other three times, in May, September, and December. It's hard to pin down the actual year of Jesus' birth but the event in 6 BCE is a good candidate to explain what the Magi saw. Other theories include Novas and Supernovas.
Either way, according to Grosse, "The fact that there's a scientific explanation doesn't mean the event loses its transcendence. The symbolism is apparent. A small clear light, on a cold dark night, in a sometimes cold and dark world, leads the wise to the message of Jesus. The message tells us to love each passenger who journeys with us on this small, fragile, planet Earth."