Mark Kidger of the European Space Agency confesses to a certain obsession at this time of year -- one that has nothing to do with the big bang or dark energy.
"It's probably the oldest mystery in astronomy," the British scientist said.
Kidger is speaking of the "Star of Bethlehem," the heavenly sign in the Gospel of St. Matthew that guided the wise men in their search for the baby Jesus.
Although it is no longer a staple of planetarium shows, the star and its story remain as fascinating for astronomers and historians on this Christmas Day as they have for almost two millenniums.
Modern technology and scholarship have opened many new windows on the event -- and a bibliography on the topic runs into the hundreds of titles. Kidger has been hooked on the subject since he was 17.
"It's a fabulous story," he said, and "at the moment, it's almost impossible to prove one or the other of the rival theories is right. You can have your opinion, but nobody can say they're definitively right on it."
Was it a comet? A supernova? A spectacular convergence of bright planets? Can we run our celestial computers backward and rediscover it? Or was it a myth or a miracle, inaccessible to science?
"It may be asking too much of astronomy and history to provide an actual event," said Owen Gingerich, a retired professor of astronomy and the history of science at the Harvard-Smithsonian Center for Astrophysics. "This could simply be an interpretive myth to make a point. ... I think we just do not know."
That hasn't stopped generations of researchers from seeking an answer. Here's where everyone begins:
Wise men from East
The story of the Star of Bethlehem resides in just one source, the Gospel of St. Matthew. It says "wise men" traveled to Jerusalem from "the East" to inquire about "he that is born king of the Jews." They told King Herod, "We have seen his star in the east and we have come to worship him."
Historians believe the "wise men" were learned astrologers, perhaps Jews from Babylon in what is now Iraq, or Zoroastrians from Persia -- now Iran. The Gospel says they arrived in Judea during the reign of King Herod, who died in 4 B.C.
The Gospel suggests that Herod hadn't seen the star himself. But his priests, citing prophesy, directed the wise men toward Bethlehem. And the star, "which they saw in the east went before them, till it came and stood over where the young child was."
In A.D. 248, the Egyptian scholar Origen suggested it was a comet or a new star, not a planet. The Greek words Matthew used suggest that the star appeared suddenly in the east, at the first light of dawn.
"It could be a comet that had come around the sun and become visible, or a nova [an exploding star] which suddenly flared up," Kidger said.
The German astronomer and astrologer Johannes Kepler argued in favor of a supernova after he witnessed one in 1604. That idea prevailed for centuries.
But "modern astronomy can show quite clearly that it wasn't a supernova," Kidger said. Such explosions leave tell-tale remnants of glowing gas that can be seen and dated. "And there is nothing there that is 2,000 years old."
A planetary conjunction? Modern computers enable astronomers to determine precisely where all the planets visible to the naked eye were during that period in the last decade B.C.
A big break came in 1968, when Roger W. Sinnot noted an extremely close conjunction of the bright planets Venus and Jupiter on June 17, in the year 2 B.C. Writing in Sky & Telescope magazine, Sinnot said the conjunction was so tight that "the fusion of the two planets would have been a rare and awe-inspiring event."
Hundreds of Christmas planetarium shows soon took up the idea -- and audiences loved it.
In The Star of Bethlehem in History, Biblical researcher Ernest Martin says the conjunction in the western sky -- over Judea as seen from Babylon -- would have impressed the Magi, too, especially in the wake of an astonishing series of planetary conjunctions in 3 and 2 B.C.
"Perhaps these unusual relationships were interpreted by the Magi as indicating the birth of the Jewish Messiah into the world," Martin said.
Martin dismissed the inconvenient fact that Herod was already dead in 3 B.C. Instead, he cited early Christian scholars who place the birth of Jesus between 3 and 2 B.C. "This is a powerful witness that deserves emphasis," he wrote.
James Kiefer, author of The Infancy Narratives in Matthew and Luke, accepts that Herod died in 4 B.C., so he looked for an earlier celestial event that might have caught the Magi's attention.
Jupiter and Saturn
He found a rare "triple conjunction" in 7 B.C. that brought Jupiter and Saturn into close proximity three times, on May 29, Oct. 3 and Dec. 4.
Jupiter, of course, was king of the Roman gods. Kiefer cites the Roman historian Tacitus referring to Saturn as the protector of the Jewish people. The conjunctions occurred in the constellation Pisces, the zodiacal sign assigned by astrologers to Syria and Palestine.
Michael Molnar's book, The Star of Bethlehem: The Legacy of the Magi, argues that two lunar occultations of Jupiter in Aries in 6 B.C. might have primed astrologers to watch for a royal birth in Judea.
Gingerich likes Molnar's analysis. He suggests that "there could well have been a visitation of astrologers from the east that set everything ajar in Jerusalem, and could very well have been remembered and incorporated into the biblical account without necessarily being actually the timing for Jesus' birth."
R.M. Jenkins, writing in the Journal of the British Astronomical Association in 2004, goes further.
"The Gospel was written in an age of oral history and myth, and the story is not supported by any other primary source," he said. The evidence suggests that the story "was made up: a story carefully crafted by Matthew to persuade predominantly Jewish readers that Jesus was the long-awaited Messiah."
Jenkins notes that biblical scholars believe the Gospel of Matthew was penned long after the events -- probably between A.D. 80 and 100 -- by someone living after St. Matthew, who was a disciple and contemporary of Jesus. And he might have drawn on subsequent events that he had witnessed, to lend weight to his story.
Among the possibilities was Halley's comet, which made a spectacular and well-reported appearance in A.D. 66. That year, a procession of Magi accompanied the Armenian King Tiridates through Jewish and Christian communities in Syria, then on to Rome to pay homage to the Emperor Nero.
"Nobody can be sure which of these [explanations] if any is correct," Jenkins said, but only this comet fits the Gospel story so well.
Kidger contends that the Jupiter-Venus conjunctions in 3 and 2 B.C. were too late. The triple conjunction of Saturn and Jupiter in 7 B.C. "seems a little early" to fit easily with other calculations for when Jesus was born (about 5 B.C.), he said. But it was "something astrologers would have been very interested in. There's a lot of symbolism about it."
He suspects that the triple conjunction in 7 B.C. might have primed astrologers to watch for another celestial sign of a royal birth in Judea.
For that role he proposes a bright new star -- a "nova," smaller than a supernova -- recorded by Chinese astronomers in March and April of 5 B.C.
"That would have been reasonably striking, visible in the dawn sky," Kidger said. Better still, if you assume an eight-week camel ride from Persia to Jerusalem, the new star would have appeared to drift westward each night, leading the Magi toward Judea.
From Jerusalem, Kidger said, "the star would have been exactly due south at dawn. Bethlehem is due south of Jerusalem, so they would have seen the star 'going before them.'"
If this theory (proposed in 1977 by English astronomer F. Richard Stephenson) is correct, Kidger said, advances in technology in the coming decades should enable scientists to detect the dim, expanding cloud of hydrogen that would mark the location of the Chinese nova, and the Christmas star.
"That would be quite a fabulous story," he said.