Captain Kirk and three wise men lead a trip through the heavens

The Star of Bethlehem has confounded astronomers and historians for at least four centuries. According to the Bible, it guided the three wise men to the place where Jesus was born. Having set out on King Herod's command, the magi - who were Babylonians or perhaps Persians - journeyed until the star "stopped rising" in the sky. At that point, they entered the house where Mary was staying, knelt down in homage to Jesus, opened their treasure chests and offered him gold, frankincense and myrrh.

But details of the Star of Bethlehem are scanty, to say the least. It is mentioned only in Matthew, and even then gets just a couple of lines. If it played such an important role in the Nativity story, why is more not said about it in the Bible? More worryingly, might the star have never existed? Even if it did, was it really a star or some other astronomical phenomenon?

The Star of Bethlehem is just one of 12 mysteries explored by Mark Kidger in Astronomical Enigmas . Kidger - a professional astronomer based in the Canary Islands - pours cold water on various theories about the star, including the possibility that it was Venus, Uranus, a meteor or a near-Earth asteroid. He also doubts that it was some form of ball lightning, or an unusually southerly appearance of the northern lights.

The star was probably not Halley's comet, which was observed by the Chinese in 12BC, at least five years before Jesus's probable birth. Moreover, Kidger thinks it unlikely that the star was a supernova - a spectacular explosion that accompanies a dying star. After all, Chinese chronicles show that the supernova closest to the date of the Nativity occurred in AD185.

In Kidger's view, what first alerted the magi to the great events that were to take place in Judea was a "conjunction" of Jupiter and Saturn in September of 7BC, when these two giant planets rose together in the east just as the Sun was setting in the west. A second warning of the impending birth of the king of the Jews occurred in the February of 6BC, when Jupiter, Saturn as well as Mars gathered in the constellation of Pisces.

But the decisive event for the magi, Kidger argues, was a nova that blazed out of the dawn sky in March of 5BC. A nova is the thermonuclear explosion that occurs when a dead collapsed star sucks material from a larger dying star. Normally seen as a sudden million-fold brightening of a previously faint star, "this was the celestial signal that told (the magi) to make their plans and prepare for their journey", he writes.

Having concluded that the Star of Bethlehem was the nova of 5BC, Kidger then backtracks and says we will probably never know which of the celestial events that took place around Jesus's birth was the star. This disappointing about-turn is not altogether surprising, given that the book seeks to explore astronomical enigmas rather than known facts. Still, it makes an unsatisfactory ending to the discussion.

At least the Star of Bethlehem is a tightly defined mystery. The problem with the other 11 enigmas in the book is that none is really an enigma, but rather an entire field of research in its own right. Is there life on Mars? What are the benefits of manned space travel? Will we all be killed by an asteroid crashing into Earth? How and why was Stonehenge built? Can we use the Moon's natural resources for human benefit?

What we end up with, then, is a sprawling book that tries to cover too much ground. Kidger, who gives many popular lectures, is obviously enthusiastic about his subject matter. But as the cosmologist and author John Barrow recently pointed out in an interview with Physics World magazine, too many popular-science writers make the mistake of trying to write about everything they know in a single volume. Far better, in Barrow's eyes, to give your book a tight brief. Indeed, Kidger has already written a book that focuses solely on the Star of Bethlehem.

It is, then, hard to summarise the rest of his latest effort. Among the many facts that caught my eye is that the six Apollo missions brought more than 380kg of Moon rock back to Earth. Astronauts travelling on an atomic-powered spacecraft accelerating at just one-tenth of the Earth's gravity would reach Mars within a month. And did you know that a stony asteroid 300m in diameter hitting our planet at 15m per second would explode with a force equivalent to 60,000 Hiroshima bombs?

Kidger is in places a bit too starry eyed for my taste, seeming to claim at one point that the internet revolution was due to the Apollo missions. I also found some chapters repetitive and rambling; do we really need a page-long digression about whether Star Trek is science fiction or science fantasy? Another quibble is that the author sometimes defines technical terms several pages after he first mention them, while leaving others unexplained altogether.

One particularly timely chapter asks whether Pluto, which is five times smaller than our own Moon, deserves to be called a planet. In August 2005, US astronomers claimed to have found a tenth planet after discovering an icy body, three times bigger than Pluto, in the distant reaches of the solar system. The finding caused members of the International Astronomical Union to suggest that Pluto be downgraded to a mere "trans-Neptunian" planet.

In fact, Pluto's status has been in dispute ever since it was discovered in 1930 by Clyde Tombaugh, who patiently searched through images of 45 million stars - a feat that Kidger calls "the greatest in the history of observational astronomy".

So does Kidger think Pluto should be a planet? Despite admitting a strong affection for this "small, remote and mysterious" body, he tamely concludes that "Pluto is Pluto" and whether it is an impostor is "for you to decide".

Matin Durrani is acting editor, Physics World .