FOUR HUNDRED years ago this October, stargazers had their eyes set firmly on a small patch of sky, eagerly following an alignment of the planets, when an unusually bright star flared into life. Burning brightly for many months, the star mystified everyone who saw it. All these years later, it is still puzzling astronomers.
We now know that the celestial spectacle of late 1604 was a supernova, the blazing funeral pyre of a star that blew itself apart. At its peak it was brighter than any other star in the night sky. It was also the last supernova in the disc of our galaxy to have been visible to the naked eye, and for a year its brightness was monitored by astronomer Johannes Kepler.
But today, observations of supernovae offer more than just a snapshot of exploding stars – they can tell us about the nature of the universe. In 1998, observations of very distant supernovae implied the existence of dark energy, a mysterious substance that appears to be speeding up the universe’s expansion. It was a shocking result that catapulted supernovae to the forefront of physics – and meant we had to be sure we understood these objects properly.
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The trouble is that Kepler’s star, as it is popularly known, does not fit astronomers’ expectations about supernovae. “It is very troublesome that we can’t tell for sure what kind of supernova Kepler’s object was,” says Alexei Fillipenko of the University of California at Berkeley. And if we cannot understand one of our nearest supernovae, argue some astronomers, how confident can we be about the others that light up the universe?
At first glance, one supernova looks much like another. For months on end it appears as a brilliant star. Yet careful observations have shown that there are two main types of explosion. The star’s fate depends on how it led its life, which in turn depends on its mass.
Heavyweight stars many times as massive as the sun fuse their hydrogen into helium via the so-called CNO cycle, in which carbon, nitrogen and oxygen act as catalysts. The enormous weight bearing down on the star’s core creates temperatures so extreme that helium nuclei fuse into carbon. Nuclear reactions in the cores of mature massive stars continue to forge successively heavier elements. Eventually, the star resembles an onion, with the heaviest metals sinking to the core and outer shells full of progressively lighter elements.
When a massive star runs out of nuclear fuel, the core collapses rapidly to form a super-dense relic, either a city-sized neutron star or, if the star is massive enough, a black hole. The shock wave produced by the collapse flings huge amounts of gas far out into space. These explosions are type II supernovae.
In contrast, lightweight stars with a similar mass to the sun lead much quieter lives. For billions of years they fuse hydrogen into helium, and possibly helium into carbon or oxygen. After the fuel is used up, the outer layers puff up, float away and leave behind the slowly cooling core, a white dwarf star.
By rights, these stars are too small to explode by themselves. But when they are orbiting around a companion star, they can suffer a very violent end. What makes the difference is gas flowing off the companion. As the white dwarf’s partner puffs up towards the end of its own life, it pours gas onto the surface of the white dwarf. The extra mass squeezes the white dwarf’s core of carbon and oxygen until it becomes catastrophically unstable. The instability sparks a frenzy of nuclear reactions, which release enough energy to blow the star apart, forging even heavier elements in the process. This is a type Ia supernova.
The real value of type Ia supernovae to astronomers is in the light that they emit. Because white dwarfs only detonate when their mass reaches a certain known value, they are all similar in size when they explode. The result is that all type Ia supernovae have the same brightness. Cosmologists exploit this feature to work out the distance to faraway galaxies, by comparing the observed brightness of a distant supernova with its known actual brightness. Such observations have revealed that expansion of the universe is accelerating and is being driven by dark energy.
However, this technique assumes that type Ia supernovae are identical, and Kepler’s star is undermining that assumption. For decades, astronomers have argued over whether it is a type Ia or type II supernova. Now observations from X-ray telescopes are providing answers.
It was German-American astronomer Walter Baade who first classed Kepler’s star as a type Ia supernova in 1943. He came to his conclusion after poring over Kepler’s original observations. From them, he reconstructed the star’s brightness over time. It was exactly what he expected for a type Ia supernova.
But Baade’s classification was thrown into doubt in the 1970s, when Sidney van den Bergh and Karl Kamper of the Dominion Observatory in British Columbia, Canada, and their colleagues examined the supernova remnant with optical telescopes. They studied the motion of super-hot filaments of tortured gas being blown outwards from the site of the explosion. These suggested that the star that blew up must have been travelling out of the plane of the galaxy at 300 kilometres per second. “This is abnormally fast,” says William Blair of Johns Hopkins University in Baltimore, Maryland, who has studied the remnant. “Stars in our galaxy do not normally move so fast.”
“How confident can we be about other supernovae that light up the universe?”
The explosion of an extremely massive star in a binary star system, however, can cause the remaining star to ricochet at high velocity. van den Bergh, Kamper and others therefore reasoned that millions of years before Kepler’s star exploded, it had a companion that also blew up as a supernova. The blast would have sent Kepler’s star racing out of the galaxy. Millions of years later when the lone star reached the end of its life, it too would have undergone a supernova explosion. What their work implied was that both Kepler’s star and its lost companion produced type II supernovae.
The case for Kepler’s star being a type II supernova was strengthened by observations of the remnant with optical telescopes. The light spectrum revealed that the remnant is chock-full of nitrogen. This is never seen in a type Ia supernova because the stars that produce white dwarfs are not heavy enough or hot enough to forge nitrogen. Massive stars, on the other hand, make copious amounts.
But the story of Kepler’s star was not over. Just as there was a problem with it being a type Ia supernova, there was a problem with it being a type II supernova too. “Nobody has ever discovered a star, the collapsed relic of the object that went supernova in 1604,” says Blair.
If the supernova of 1604 was a type II, as van den Bergh believed, it should have left behind a super-dense neutron star. Such neutron stars are invisible to optical telescopes, but any gas that they accrete becomes violently hot and emits X-rays as it falls towards the super-dense object. But despite its exquisite sensitivity, NASA’s orbiting Chandra X-ray observatory, launched in 1999, has failed to find such an object.
Most recently, two groups have tried instead to determine the distance to the remnant, from which they could work out its true size and luminosity. A team led by Blair has made infrared observations of the dust in the supernova remnant with NASA’s Spitzer Space Telescope, which launched last year. Meanwhile, a second group, led by Blair’s colleague Ravi Sankrit at Johns Hopkins, has observed the remnant at visible wavelengths with the Hubble Space Telescope. Together their results have pinned down the distance to 13,000 light years, give or take 1000 light years, five times as accurate as the previous best estimate. Information like this should help astrophysicists understand the mechanisms behind supernovae better.
Blair and Sankrit also reviewed observations from other X-ray telescopes. Chandra can see fine detail in X-ray sources, while the European Space Agency’s XMM-Newton observatory can distinguish closely spaced spectral lines. What these observations reveal is that the cloud around Kepler’s star is emitting bright X-rays from iron and silicon.
This is characteristic of type Ia supernovae. Despite their bland make-up of light elements, white dwarf stars explode in burning fireballs that form heavy metals and hurl them into space. By contrast, massive stars that end in type II supernovae lock most of their heavy elements into the collapsed cores. Iron’s X-ray fingerprints are a sure-fire sign of a type Ia supernova.
“The pendulum has swung back,” says Blair. “It has not been generally acknowledged in the literature yet but the case for Kepler’s star being a type Ia supernova is now very strong.” And if Kepler’s star really is a type Ia supernova, it is not like any other known, which is worrying news for cosmologists. “It is very disconcerting when possibly the nearest example of a type Ia supernova is so different from all the others,” says Blair.
He isn’t going as far as saying that Kepler’s star undermines the dark energy claim. But the fact remains that cosmologists are putting their faith in objects that are far from well understood. “It’s bothersome,” says Blair. “I think it’s a thorn in the side of cosmology.”
Kepler’s star is born
The new star was first noticed by observers in northern Italy on 9 October 1604 and by sky-watchers in east Asia the following night. It was almost guaranteed that someone in Europe would see the star as soon as it appeared because, by chance, many people were watching that precise region of sky. “Remarkably, they were actually expecting something unusual to happen there,” says William Blair of Johns Hopkins University in Baltimore, Maryland.
The reason for their anticipation was an alignment of the planets Jupiter, Saturn and Mars. Although such events occur roughly every 20 years, they happen in this particular region of the sky 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. So when the new star appeared, western astrologers were beside themselves with excitement. For them, it was an important omen signifying the start of a new 800-year cycle and the birth of a powerful king.
When Johannes Kepler heard of the new star on 10 October, he was sceptical about its existence. Bad weather meant he had to wait until 17 October to see it, but for almost a year afterwards he charted its brightness in great detail, until the star faded out of sight. Thanks to his detailed observations, the star became known as Kepler’s star.