The Star's Origins in Matthew:
In the Gospel of Matthew, wise men from a foreign land appear in Herod's court sometime after Jesus' birth. Their intention is to pay homage to "king of the Jews" who they know was born "For we observed his star at its rising and have come to pay him homage" (Matthew 1:2 New Oxford Annotated Bible). They then proceed to Bethlehem "and there, ahead of them, went the star that they had seen at its rising, until it stopped over the place where the child was" (Matthew 1:9). Christian tradition has held that there were three wise men, though Matthew does not specify this, only that three gifts were given.
Other References to the Christmas Star:
Matthew - the only cannonical Gospel to reference it - offers few details, though the Gospel is quite clear that Herod's priests were unable to notice it, even after notified of it by the wise men (else they would have found the infant Jesus' house).
The apocryphal gospel of James, brother of Jesus (according to tradition) and the writings of first-century bishop Ignatius (who said the star "outshone all the celestial lights, and to which the Sun and Moon did obeisance") do describe the star as being quite dazzling.
Overall, these references provide some connection to Messianic prophecies in the Old Testament books.
Traveling to Bethlehem:
From the quotes in Matthew, it is therefore possible to assume that the star was not itself particularly distinct from other stars. The wise men, or "magi," were presumably astrologers. The gospel indicates that the wise men became aware of Jesus' birth when the star appeared and set out for Judea at that time. The journey from their nation to Judea would have anywhere between one and three weeks (assuming they were Babylonian). So the options are they they either arrived after Jesus birth, or the star appeared some time prior to Jesus' birth, thus giving them time to arrive.
Assuming that the Star of Bethlehem existed, the question remains of what caused it physically.
Given a more "dazzling" Star, it might have been a supernova. However, such a supernova would still leave radiation remnants and none have been detected which fall within the dates needed to be the Star of Bethlehem. It is not ruled out completely, but there is no evidence to particularly support the theory.
Astronomer David Hughes of England's Sheffield University has reviewed historical documents and found other references to a star "standing over" a location, both of which were actually comets.
Possible Comet Candidate:
In 248 A.D., the Christian writer Origen first proposed the idea that the Bethlehem Star might have been a comet. He suggested the sui-hsing
("broom star") noted by Chinese astronomers in 5 B.C., which approximately matches the accurate date for Jesus' birth (generally calculated at around 4 B.C.).
The comet was visible for 70 days, which provided plenty of time for the wise men to travel from their homelands to Herod's court and then on to Bethlehem while it was still in the sky.
The Planet Hypothesis:
Another hypothesis is that the Star of Bethlehem may have been Jupiter. Sometimes, planets go through a stationary point in the travel through the sky, appearing to make small loops against the rest of the stars while they go through their retrograde motion. When Jupiter appears stationary during such a cycle, it does so for about a week.
Records suggest that Babylonian astronomers watched closely for such motion and attributed great significance to it. At the time, those looking at the heavens did not yet realize that planets differed from stars. Some other planets are also possible candidates.
Of course, when Matthew says that the wise men observed the star "rising," he may not have meant the literal rising of the star in the heavens, but rather an astrological alignment of stars perceived through the lens of the mathematical and geometric formulas used by the magi in interpreting such things.
In this case, the things which the magi looked for would likely not be something which modern-day astronomers particularly care about, so any physics analysis would lead nowhere.
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