Was there a Star of Bethlehem? If so, was it a really a star, or was it some other celestial event that lured the Magi to Bethlehem and so frightened King Herod that he ordered young children killed to eliminate a newborn prince? If not, why does the Bible tell us a star existed, and why have scientists over the centuries persisted in trying to identify its origin?

At the brink of the third Christian millennium, two astronomers have revealed in tell-all books--the scientific, not the gossipy, kind--divergent theories about the heavenly manifestation recorded in the Book of Matthew. Each scientist believes he has identified the Star of Bethlehem, and one thinks it still hangs in the night sky.

Michael R. Molnar, in "The Star of Bethlehem: The Legacy of the Magi" (Rutgers, $25), goes where no astronomer has gone by examining astrological manuscripts of the period. He believes an occultation, or blockage, of Jupiter by the moon on April 17, 6 B.C., was a sign that a "divine and immortal person" had been born.

That spring, Jupiter appeared as a "morning star," a bright planet in the eastern sky before sunrise, Molnar explained in an interview. Over several months, it moved across the heavens until it seemed to "stop" over Bethlehem. The Magi, he said, who were healers and magicians as well as astronomers and mathematicians, would have followed Jupiter on its months-long journey--arriving in Judea about December.

Mark Kidger, in "The Star of Bethlehem: An Astronomer's View" (Princeton, $22.95), argues that nobody has solved the mystery of the star because other theorists have looked for a single phenomenon. No event alone could have created such attention or excitement, he said--not a comet, not a meteor shower, not a supernova, not even the rare double or triple passing of one planet by another.

Kidger believes he has found a series of occurrences that tipped the Magi to the Messiah's birth: a triple passing of Jupiter and Saturn in 7 B.C.; the "massing" or coming together of Mars, Jupiter and Saturn in 6 B.C.; two occultations of Jupiter in 6 B.C.; and a nova in 5 B.C. The nova was recorded by Chinese and Korean astronomers and would have been visible throughout the East. This was the Star of Bethlehem, according to Kidger's theory.

Both Molnar and Kidger used computer programs to reconstruct the position of the planets and constellations near and around 4 B.C. and 7 B.C., the period historians typically give in dating the birth of Jesus.

Most biblical scholars date Jesus's birth before 4 B.C because of a 6th-century blunder by Dionysius Exiguus, Dennis the Little. In establishing a Christian liturgical calendar, the monk failed to factor in a four-year period during which Caesar Augustus ruled under his given name, Octavian. He also, for the purists out there, jumped from 1 B.C. to 1 A.D., thus losing another year--and causing the birth date of Jesus to be off by at least five years.

Scholars later realized that, according to the Dionysian calendar, King Herod would have died in 4 B.C., four years before Jesus was born. But it was too late to correct the error.

L. Michael White, professor of classics and director or religious studies at the University of Texas at Austin, has read neither book but is ready to play spoiler to both Molnar's and Kidger's theories. There is a fundamental problem, he said, with trying to prove there was a Star of Bethlehem: The story of the visit of the Magi is "all made up."

"There's nothing historical about it, including the star," said White. "My guess is that no single natural phenomenon will ever respond precisely to this story."

Molnar anticipated such criticism in his book. He relates a common argument that the Book of Matthew, the only Gospel that speaks of the star and the Magi, was not based on factual reports, but simply recasts Hebrew prophecies as if they had occurred. Molnar said extensive research into astrological texts and horoscopes persuaded him that his theory supports the "intriguing story" in Matthew.

"The details in the account of the Magi's visit," he writes, "strongly suggest an attempt, albeit a confusing one, to pass along details of an important celestial manifestation--one that was noted or even anticipated by people watching for celestial omens about the advent of the Messiah."

Kidger, 39, contacted at his office at the Institute of Astrophysics in the Canary Islands, said he understands White's skepticism. "I can cope with that--it is a valid theory," he said. "I am just limiting myself to, 'If [the star] did exist, it was probably this.' "

Molnar, 54, who has worked on several projects for the National Aeronautics and Space Administration and has taught at Rutgers University and the University of Toledo, said he has no doubt that astrologers believed a "super king" would be born in Judea.

A collector of ancient coins, Molnar began his search for the Star of Bethlehem five years ago after buying a 1st-century Roman coin. On one side was a portrait of Jupiter; on the other, the zodiacal sign of Aries the Ram, looking back over its shoulder at a star. From texts of astrologers, who were also astronomers, Molnar learned that Aries was the sign of Judea in Herod's time.

He also found evidence that astrologers of the time were looking for the precise conditions he discovered on April 17, 6 B.C. Jupiter, which was blocked by the moon, was in the constellation Aries, as were the sun and Saturn, and the moon was "nearby."

Molnar said his astronomy colleagues, including Kidger, have tried since the Renaissance to locate the Star of Bethlehem in Pisces the Fish because of the fish's association with Christianity. But the correct location of the star would have been in Aries, he said.

Last week, Molnar presented his ideas to a crowd of more than 200 people at Harvard University, many of them astronomers and astrophysicists. The response was "overwhelmingly positive," said Owen Gingerich, an astronomer at the Harvard-Smithsonian Center for Astrophysics and a longtime supporter of Molnar's theory. He called Molnar's book "the best thing [on the star] that's come down the pike in many decades."

Molnar declined to state whether he has any religious affiliation or whether his research has confirmed his belief in Matthew's account. He said he has attempted to remain "religiously neutral."

Kidger, an agnostic who attended church sporadically while growing up in England, said that 22 years of research into the Star of Bethlehem hasn't made him "a Christian in any sense." But "it does make you think," he said.

"If my explanation is the right one, it really is quite astonishing that that star appeared exactly at the right moment."

Even more remarkable, he said, is that someone might prove the existence of the nova of 5 B.C. A nova is an old or dying star whose surface suddenly explodes and produces a luminosity a million times brighter than before. This burst of energy would have sent out a huge hydrogen cloud, which, though faint today, eventually could be detected with advanced instrumentation--if the observer knows where to look.

For anyone out there who wants to take up the search, Kidger has identified a candidate, a "not dying but quite old star" called DO Aquilae. "I tried to outline a way that if the theory is correct, one day it will be proven."

Explanations Abound About the Source of Star of Bethlehem

For centuries, theologians and astronomers have speculated on the Star of Bethlehem, trying to identify a celestial event that could have signaled the birth of a Messiah. Here are some of the possibilities:

* Venus--The planet Venus has been the most popular choice over the years as the Star of Bethlehem, often appearing at Christmastime as a luminous evening or morning star. However, because Venus was one of five planets followed regularly by ancient astrologers-astronomers, it would not have appeared extraordinary even in its most beautiful manifestations.

* Halley's comet--Many medieval theologians believed the Bethlehem star was a comet, a notion Florentine artist Giotto di Bondone depicted in a 14th-century fresco. His model probably was Halley's, which appeared in 1301. Halley's did streak across the sky two millennia ago, but in 12 B.C.--five years earlier than most historians allow for Jesus's birth. A comet is not a likely choice for the star because astrologers would have viewed it as an omen of death and destruction, not a sign of divine birth.

* Conjunction--Many astronomers since the Renaissance have proposed that the Bethlehem sign was not a star but an alignment of two planets, called a conjunction. A triple conjunction, when planets such as Jupiter and Saturn pass close to one another three times in a year, is very rare and therefore most notable. Though viewed best through a telescope, some conjunctions would have been visible through the naked eyes of the ancients.

* A nova or supernova--A nova is not a new star, as the name implies, but an old or dying star that suddenly becomes a million times brighter because of a surface explosion, then fades within weeks back to its normal luminosity. A supernova is much more intense, at least a billion times as brilliant, and lasts up to a year--probably too long for the Star of Bethlehem.