That’s… complicated. The common image many people have of the Christmas Star comes from many different sources. The Christian Biblical story of the “Star of Bethlehem” actually doesn’t say much about what the “star” looked like, but centuries of art and Christmas card images have turned it into a huge blazing beacon in the heavens. This event will not look like that. (See “What will it look like?” above.) The Star of Bethlehem actually wasn’t something that everyone saw – it was only the Magi, the “wise men from the East” who saw it. That alone tells us it probably wasn’t as simple as a bright light in the sky.
There are some theories that suggest that the “wise men from the east” were astrologers, and so they would have been excited about things like planetary conjunctions, things that were seen as significant but not immediately noticeable to the casual viewer. Things like planetary conjunctions would have been highly significant to the Magi, especially if they were rare events or repeated events. If we run with this hypothesis, there was a conjunction of Jupiter and Saturn in 7 B.C. – actually, there were three of them, in an even-rarer triple conjunction. This actually times out fairly well to match the Biblical account, since we know that the 8th-century monk who did the math to calculate the year 1 A.D made some errors and was off by a few years – the Nativity story probably actually occurred a few years before 1 A.D. in our current calendar. For example, King Herod, who was alive during the Nativity story, actually died around 4 B.C., so we know the story had to take place before then.
If this idea is correct, the wise men saw the triple conjunction in 7 B.C., interpreted it to mean there would be a royal birth in Judea, and traveled to the land of King Herod. King Herod had no idea what they were talking about – his court astrologers had not seen a “star” that they thought was important. (Neither did Chinese astronomers of the day, who took meticulous observations of any new objects like comets, new stars, and the like.) The wise men leave Herod and travel to Bethlehem, arriving sometime in 6 B.C. or perhaps a bit later.
None of this is certain, of course – there are other possible explanations for the Star of Bethlehem, like a conjunction of Venus and Jupiter in 2 B.C. that would have been the brightest star in the sky. And given the nature of the Nativity story, there isn’t enough evidence to even say whether the Star was an astronomical object, a divine inspiration, an interpretation by the wise men, or an idea added in later revisions of the Bible throughout the ages. And really, not everyone feels the Star of Bethlehem needs an explanation in the first place. As I said, it’s complicated.
So, because it’s possible that the Star of Bethlehem was inspired by a rare triple conjunction of planets in 7 B.C., and because this year we are seeing a single conjunction of the same planets Jupiter and Saturn, which happens to occur in December, the Great Planetary Conjunction of 2020 has been dubbed the Christmas Star. And so many people are expecting the Christmas Card version of the event: a huge light in the sky.