According to the Bible, when Jesus was born three Magi saw a star in the East that 'signaled the birth of a new king'. But just what was it, from an astronomical point of view, that the Magi could actually have seen?

Fred Grosse, a professor of physics and astronomy at Susquehanna University in Selinsgrove, Pa., says there are several popular theories that may answer this question.

“Astronomical objects or events which would be of interest to serious stargazers of the time include comets and meteors, nova or supernova, and auroras,” Grosse says. But the favorite candidate hypothesis for the star of Bethlehem, he explains, is a planetary conjunction.

Supernova, planetary conjunction or Venus?

A conjunction happens when two celestial objects appear to pass very near to each other from our perspective on Earth. Often, these conjunctions look like one large object rather than separate ones.

“In 6 BCE, Jupiter and Saturn passed each other three times, in May, September, and December – a triple conjunction,” Grosse says. Since the actual year of Jesus’ birth is tough to pin down, an event in 6 BCE remains a good candidate to explain what the Magi saw.

"Because this conjunction only happens once every 140 years, it would have been a significant event to astrologers from Babylon.

“A conjecture is that they saw the first passing from their homes, left for Jerusalem, and got there in time for the second or third passing to guide them to Bethlehem,” he says. “(Astronomer Johannes) Kepler knew of this conjunction, and since his time astronomers have connected the triple conjunction with the Star of Bethlehem.”

Though this is the dominant explanation for the star in the east, it’s not the only one.

“Nova and Supernova are new objects which appear in the sky and then dim to oblivion,” Grosse explains. And they can put on the kind of show that first century astrologers would have noticed.

“A supernova in 1006, the brightest ever, was as bright as the sun," he says. "It was visible to the naked eye for more than two years.”

According to Chinese records, there were supernova – or “guest stars” as the Chinese described them – that appeared in the sky right around the time when Jesus may have been born, in 4 and 5 BCE.

But the fact that there’s a possible scientific explanation to what the Magi saw, doesn’t mean to Grosse that 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.”