Step 1: The time frame for Jesus' birth
The first thing is to determine the approximate date of Jesus’ birth. Then astronomers can look at the sky phenomena of that period and try to identify the star. It doesn’t work the other way around: Since virtually any year can boast at least one reasonably interesting sky event, the astronomy must follow the history.

Let’s assume, as many historians have, that the most likely time frame for the birth of Jesus was in the years before A.D. 1. Let’s also assume that the Star of Bethlehem could be observed by skywatchers elsewhere in the world, and not just by the Magi who followed the star to Jesus' birthplace. The Magi, who are known as “wise men” or “kings” in the Christmas story, were actually priests who relied on astrology.

These assumptions would rule out some of the prime suspects in the mystery: comets, brightening stars known as novae, and exploding stars known as supernovae. The Chinese, who did a particularly good job of cataloging astronomical phenomena, recorded no such phenomena during the years in question.

Step 2: Was it really a star?
 

Beyond the timing issue, there’s another consideration: A comet or supernova big enough to attract the wise men’s attention would have been widely noticed by royalty and commoners as well. But King Herod and his advisers seemed not to know or care about the star until the astrologers from the east came to visit. 

However, if we suppose that the “star” actually referred to the planets, the situation is less problematic. The movements and groupings of planets in the night sky were of exceeding interest to astrologers and were closely tracked around the world.

Historical records and modern-day computer simulations indicate that there was a rare series of planetary groupings, also known as conjunctions, during the years 3 B.C. and 2 B.C.

Step 3: Retracing the conjunctions
The show started on the morning of June 12 in 3 B.C., when Venus could be sighted very close to Saturn in the eastern sky. Then there was a spectacular pairing of Venus and Jupiter on Aug. 12 in the constellation Leo, which ancient astrologers associated with the destiny of the Jews.

Between September of 3 B.C. and June of 2 B.C., Jupiter passed by the star Regulus in Leo, reversed itself and passed it again, then turned back and passed the star a third time. This was another remarkable event, since astrologers considered Jupiter the kingly planet and regarded Regulus as the “king star.”

The crowning touch came on June 17, when Jupiter seemed to approach so close to Venus that, without binoculars, they would have looked like a single star.

The whole sequence of events could have been enough for at least three astrologers to go to Jerusalem and ask Herod: “Where is he that is born King of the Jews, for we have seen his star in the east and are come to worship him.”