Over the years, one popular idea is that the Star of Bethlehem was a comet, a celestial object with a long tail that passes through the solar system for a short time.  This theory has been around since at least about A.D. 250, with the Christian writer Origen, who wrote:

The star that was seen in the east we consider to have been a new star, unlike any of the other well-known planetary bodies, either those in the firmament above or those among the lower orbs, but partaking of the nature of those celestial bodies which appear at times, such as comets, or those meteors which resemble beams of wood, or beards, or wine jars, or any of those other names by which the Greeks are accustomed to describe their varying appearances.

One popular notion that made the rounds for a while was that the Star of Bethlehem was an appearance of Halley's Comet.  It was the  astronomer Edmund Halley who, in A.D. 1715, discovered that a number of the famous comets of history were actually reappearances of the same comet.   One such reappearance was in A.D. 1305.  This reappearance might have been observed by the Italian artist Giotto di Bondone.  That very year, Giotto painted a famous fresco entitled  Adoration of the Magi, which included a very comet-like depiction of the Star of Bethlehem.  However, the arithmetic of the cycle of Comet Halley indicates that it would have revisited the Earth in A.D. 12, much too late to have been within the lifetime of King Herod. 

Another comet theory is explained by astronomer Colin Humphreys who considers a number of historical sources and scientific data, including Chinese observations of comets from the period, concluding that the Star of Bethlehem might indeed have been a comet.   

So What Was The Star of Bethlehem Anyway?

In my opinion, all the above theories are in the category of "maybe, maybe not."  They all have their compelling arguments and yet none fit all the available facts of science, history and Scripture.  However, as far as I'm concerned, all rationalistic, naturalistic theories to locate a celestial object as the Star of Bethlehem suffer from one major problem: the Star of Bethlehem as described in Scripture does not behave like a natural celestial object.  The text of Matthew 2:9 states:    

When they had heard the king, they departed; and, lo, the star, which they saw in the east, went before them, till it came and stood over where the young child was.

So after their audience with Herod in Jerusalem, the star "went before" the wise men, and "stood over" the place where Jesus was.  The distance from Jerusalem and Bethlehem is about six miles, maybe a three or four hour walk.   So the wise men could have reached Bethlehem the same night. 

However, natural celestial bodies rise in the east, reach their highest at the meridian, and set toward the west.  However, Bethlehem is nearly due south of Jerusalem.  Any natural star would pass to their left or right as the wise men headed south from Jerusalem, and would not have "went before" as Scripture indicates.

Also, for a star to have "stood over" a place, it would have to pass through the zenith, otherwise it would appear off to the north or south.  There are no visible supernova remnants that pass through the zenith at the latitude of Bethlehem, and neither Jupiter nor Regulus pass overhead at Bethlehem's latitude.  A comet could have passed overhead at the latitude of Bethlehem, but there's still a Scriptural problem that every natural celestial body cannot overcome....