The problem is that the timing does not line up well with the reign of Herod the Great, who died in 4 BCE and is explicitly mentioned in Matthew. If we assume that the Gospel chronology is roughly correct, then Halley’s Comet—the only plausible candidate for the comet—appeared too early to be relevant to Jesus’ birth.
More importantly, Matthew describes the star as something the magi saw “at its rising” and something that later “stood over” a particular house. Comets do not behave this way—their motion is smooth and predictable.
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Interestingly, for ancient readers, comets were usually thought to be predictors of disaster rather than good fortune. The Jewish historian Josephus wrote of a comet that passed over Jerusalem before its destruction in the first Jewish War. And Suetonius notes that the appearance of a comet prompted a superstitious Nero to have several important figures in his empire executed. If the star was a comet, writes Raymond Brown, author of The Birth of the Messiah, it is difficult to imagine why the magi would have followed such a bad omen to Bethlehem.
The supernova theory
Another dramatic possibility is that the magi witnessed a supernova—an exploding star bright enough to be visible in daytime—or a less intense nova, which still produces a sudden and striking increase in brightness.
Chinese astronomers, who kept careful records dating back to 1000 BCE, recorded a possible tailed comet in 5 BCE and a possible nova or supernova in 4 BCE. These are promising dates: they fall within a plausible timeframe for the birth of Jesus and the final years of Herod.
(The little town of Bethlehem has a surprising history)
Once again, however, the fit is imperfect. A supernova or nova would be visible to everyone, not just astronomers. But Matthew gives no indication that Herod or the residents of Jerusalem had noticed an unusual light in the sky; Herod seems surprised when the magi arrive and must ask them privately about the timing of the star’s appearance. Moreover, just as with comets, supernovae also do not “stand over” a single home.
Where the nova hypothesis succeeds is in its dramatic imagery. The early 2nd-century Christian bishop Ignatius of Antioch described its light as “beyond description” and exceeding all other stars—a celestial sign worthy of a king’s birth.
The planetary conjunction theory
The most popular modern explanation is that the star was actually a conjunction of two or more planets in the night sky. The theory dates to the 8th-century astronomer Mash’allah, whose astrological history was based on an earlier Babylonian theory that important events and power shifts are predicted by the conjunctions of planets in the sky. He claimed that three important events—the great flood, the birth of Jesus, and the birth of the Prophet Muhammad—were all anticipated by conjunctions of Saturn and Jupiter.
Grant Mathews, a professor of theoretical physics at the University of Notre Dame, studied planetary alignments in the rough time frame for the birth of Jesus (8-4 BCE). He noticed that on April 17 in 6 BCE there was a special alignment of the sun, moon, Jupiter, Saturn, and the vernal equinox in the Aries constellation, while Venus aligned in adjacent Pisces. This specific alignment of the planets (with Mercury and Mars in Taurus) was so unusual that it will not occur again until 16,213 CE.