What was the Christmas Star? Over the years many astronomical explanations have been suggested.
Before we take a look at some astronomical suggestions, we'll begin at the start - the original account in St. Matthew's Gospel.
Christmas Star in St. Matthew's Gospel
It's interesting to note what was originally included in the story and what has come down to us as interpretation or embellishment. For example, there is no mention of there being three kings, only 'Magi' (wise men, magicians or possibly astrologers) who left three gifts. The Greek word generally translated 'star' (αστερα - astera/astra from which we get 'astronomy') can also mean planet or could refer to other objects such as a comet. There is no mention that the star is particularly bright, and nor does it seem to have significance for anyone other than the Magi. There have been at least six purely astronomical explanations for the Star of Bethlehem.
Nova or supernova explosion
The idea that the Magi saw a nova or supernova explosion was hinted at by Kepler and has had many supporters since then. However, there is no western record of such an event and the Chinese records only have one possible mention of a nova or supernova over the potential time for Jesus' birth. There is also no known supernova remnant, which we would expect to find if there had been a supernova at the birth of Jesus.
This explanation has its origins even further back in time, dating to early Christian theologian Origen in AD 248. Again Chinese records can be invoked but give no good support apart from the potential 5 BC nova/comet. One advantage of the comet theory is that comets move across the sky. It had been argued that this fits the interpretation of the Gospel that the star moved as it directed the Magi. Though this same argument could be applied to an object moving with the stars if the journey of the Magi took some months. Most classical depictions of the nativity show the 'star' as a comet.
Conjunction of Jupiter and Saturn
Kepler is also associated with the idea that the close conjunctions of Jupiter and Saturn were the event associated with the 'star'. In fact there were three conjunctions, when the two planets were close to one another in the sky, but none of these were close enough that they'd appear as one object. For this reason most analysts reject this theory, though such an event could have been of religious or astrological significance.
Stationary Point of Jupiter
Jupiter, in its apparent path across the sky, is generally seen to move from east to west across the starry background. Due to the relative movements of the Earth and the planets this motion appears to slow and then stop as the planet reaches what is called a stationary point. The planet then appears to move from east to west for some days before again stopping and resuming its west to east movement. At the possible time of the birth of Christ one of the stationary points could have occurred when Jupiter was directly overhead at Bethlehem at the same time of night for several nights. The disadvantage of this explanation lies in the lack of any rarity in the phenomenon as it would happen every year.