What was the Star of Bethlehem not is far easier to answer than what was the Star of Bethlehem. Indeed, the second question may be impossible to determine. Many efforts have been made with things to consider offered. But those efforts fall short.
First, we must understand that something happened that caught the attention of the Maji. These learned men saw something. The account given by them when they visited Herod was that the star rose in the east, then came to rest. Had it not come to an apparent rest they could not have ever found the place where Jesus was born. So, their account is likely to have an historical basis.
Second, the star must have remained for a long time. The Maji would have first contacted each other, then assembled a caravan. A caravan would have moved slowly. Certainly, they would have had a guard for the precious items given as gifts to Jesus. Herod understood this when he ordered the deaths of all boys in the early years of their lives, not just newborn babies.
So, we would have had a celestial body moving from the east and apparently standing still. Remember, it would have had to maintain its position relative to the Earth all twenty-four hours of the day. The way an object can appear to stand still is to be in a geosynchronous orbit. But a stable orbit must be about the center of the earth, so only orbits over the equator can be geosynchronous. Bethlehem is above forty degrees latitude. This presents a problem.