A light to guide us

Centuries after the star of Bethlehem appeared, the mystery of another bright light in the heavens still puzzles astronomers

An unusually bright star is a particularly Christmassy thing. And, 400 years ago, just such a star was lighting up the night sky. It blazed brightly in the constellation of Ophiuchus, the serpent holder, and all who saw it were mystified.

An unusually bright star is a particularly Christmassy thing. And, 400 years ago, just such a star was lighting up the night sky. It blazed brightly in the constellation of Ophiuchus, the serpent holder, and all who saw it were mystified.

Four centuries later, everyone is still mystified. We now know that "Kepler's star" was a supernova; the blazing funeral pyre of a star that blew itself apart in an orgy of cataclysmic destruction. It was the last supernova to be visible to the naked eye in our galaxy.

The "new star" was first noticed by observers in northern Italy on 9 October 1604 and by sky-watchers in the Far East the following night. Bizarrely, people in Europe were monitoring the exact region of the night sky where the supernova flared up. "It sounds incredible, but they were actually expecting something unusual to happen there," says William Blair of Johns Hopkins University in Baltimore.

The reason for the interest was a close approach, or "conjunction", of the planets Jupiter, Saturn and Mars. Such events occur about every 20 years, but in this part of the sky they happen only every 800 years or so. The time before had corresponded with the birth of Charlemagne, king of the Franks, and the time before that with the birth of Christ. This conjunction about 2,000 years ago is believed by some to be the famous star of Bethlehem observed by the Three Wise Men.

The appearance of the new star in 1604, therefore, sent astrologers into paroxysms of excitement. It was an extremely important omen that could only signify the start of a new 800-year cycle and the birth of a powerful king.

In Prague, on 10 October, Emperor Rudolph II's mathematician was shaken awake by a meteorologist colleague, John Brunowski, who claimed he'd glimpsed a "new star" through a gap in the clouds. A sceptical Johannes Kepler saw nothing of Brunowski's star that night or the next. In fact, he had to wait until 17 October. Thereafter, and until it faded into invisibility on 8 October 1605, he recorded its brightness.

Kepler, whose day job was deducing his famous laws of planetary motion from the observations of his mentor Tycho Brahe, observed the star regularly, apart from a month or so between November and January when the Sun encroached on it. Crucially, he determined that the new star did not move against the background stars, as a planet did. It must, therefore, be at a far greater distance and could have no connection with the planetary conjunction. Kepler wrote up all he discovered in a book. De Stella Nova, published in 1606, is the main reason why supernova 1604 came to be known as "Kepler's star".

Kepler monitored the star when it was probably at its peak brightness and rivalled Jupiter until it faded from view a year later. His observations have proved invaluable. In 1943, they were scrutinised by the German-American astronomer Walter Baade, who, along with the Swiss-American Fritz Zwicky, had first recognised the existence of a distinct class of super-powerful exploding stars - supernovae.

Baade reconstructed the supernova's "light curve" - a graph of how it brightened and faded with time. He concluded it was what would be expected for a "Type Ia" supernova, which we now know is due to the explosion of an Earth-sized "white dwarf" star. When a companion star dumps too much material on to it, a white dwarf becomes catastrophically unstable, so that it explodes.

Baade, classed as an enemy alien and unable to serve in the US military, had nothing to do during the Second World War but sit at the "prime focus" of the world's largest telescope in California. With Los Angeles conveniently blacked out below, he was able to use the 100-inch Mount Wilson telescope to photograph the remnant of Kepler's supernova for the first time.

Baade's designation of Kepler's star as a Type Ia supernova, was not, however, the end of the story. From the 1970s, astronomers, including Sidney van den Bergh and Karl Kamper of the Dominion Observatory in British Columbia, had examined the remnant with more powerful instruments such as the five-metre telescope at Mount Palomar in California. The motion of super-hot "filaments" of gas being blown outwards from the site of the explosion revealed that the star that went supernova must have been travelling out of our galaxy at about one million kilometres per hour. "This is abnormally fast," says Blair. "Stars in our galaxy do not normally move so fast."

There is a possible explanation. The explosion of a very massive star in a binary star system can cause the other star to ricochet at high speed. Van den Bergh, Kamper and others reasoned that, millions of years before the explosion of Kepler's star, its companion had also gone supernova, sending Kepler's star racing headlong out of our galaxy. Only later had Kepler's star itself exploded. As it would have been solitary then, it could not have been a Type Ia supernova. The only solitary stars that go supernova are much more massive stars than the Sun at the end of their lives - Type II supernovae.

There was another twist. "Nobody has ever discovered a star; the relic of the object that went supernova in 1604," says Blair. This is not unprecedented; of the six historical supernovae in the past 1,000 years, two others - the supernova of 1006 and Tycho's star of 1572 - have revealed no relic. But they were both Type Ias and, in this type, the white dwarf that goes supernova is destroyed. If supernova 1604 was a Type II supernova, as van den Bergh believed, it should have left a relic - most probably a super-dense "neutron star". Such a star, no bigger than a city such as London, should give out intense X-rays. But no such object has been found by the orbiting super-sensitive Chandra X-ray observatory.

In the mid-1980s, totally confused about what kind of supernova Kepler's star was, astronomers examined other contemporary observations. They found there was a considerable spread in the estimates of the supernova's brightness. The uncertainties meant that the light curve could be stretched to fit either a Type Ia or a Type II.

More recently, Blair and his Johns Hopkins colleague Ravi Sankrit have made use of the full range of modern instruments, including the Chandra and Hubble space telescopes. They believe the case for Kepler's star being a Type Ia is now very strong. "The pendulum has swung back," says Blair.

Why should anyone care what kind of supernova Kepler's star was? The answer is: dark energy. In 1998, astrophysicists in the US dropped a bombshell. They announced that, contrary to all expectations, the expansion of the universe was speeding up. A mysterious invisible stuff - dark energy - permeated all of space and was remorselessly driving the galaxies apart. The evidence for dark energy came from the observation of Type Ia supernovae in ultra-distant galaxies. As white dwarfs are all very similar in mass when they detonate, they all have about the same intrinsic luminosity.

For this reason, they are used by cosmologists as "standard candles", whose apparent brightness can be used to indicate their true distance from Earth. The Type Ias observed in distant galaxies were fainter than they should have been, indicating that the universe's expansion had speeded up since the stars exploded, pushing them farther away than expected and making them appear fainter.

But Kepler's star, if it is a Type Ia supernova, is far from a standard Type Ia. Just look at how much confusion it has caused. "It is disconcerting when possibly the nearest example of a Type Ia supernova is so different from all the others," says Blair.

Alexei Fillipenko of the University of California, Berkeley, the leader of one of the two teams that discovered dark energy, admits that Kepler's supernova is a puzzle. "It is troublesome that we can't tell for sure what kind of supernova Kepler's object was."

Nobody is yet going as far as saying that Supernova 1604 undermines the idea that Type Ia supernovae are standard candles and, therefore, the dark energy claim. The fact remains, however, that cosmologists are putting a great deal of faith in objects that are far from well understood. "It's bothersome," says Blair. "It's a thorn in the side of cosmology."

Marcus Chown is the author of 'The Universe Next Door' (Headline)