In 44 BC, following the death of Julius Caesar, a comet appeared in the heavens over Rome and was accepted to be a manifestation of the divine soul of Julius Caesar. It was thought to herald a new age of peace and prosperity. Augustus used this event to emphasize his familial association with the deified Julius.
What does it have to do with the Nativity? It's a popular culture when there wasn't much by way of media.
If you think people have a problem with "In God we Trust" on our money imagine how people in Palastine using Roman coins felt.
Obviously if Caesar gets a star then so should the Messiah right?
In fact like Christians today they tended to borrow validity anywhere they could, including the Magi that brought gifts. Magi was the title given to the priest class of the religion of Zoroastrianism or Mazdayasna. So if they brought gifts then the Jesus religion is better. It's as if Mormons claimed the Pope came and blessed the Mormon Temple in Utah, which of course he would never do, but what a propaganda coup to tell such a lie and get away with it.
There's very good reason to believe the very same comet was claimed to be the Christmas Star it didn't matter that it was anachronistic, there's a lot of scrambled history in the Gospel fan fiction.