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WHEN the final thing a giant star does is explode, briefly giving out more light than 100 billion ordinary stars put together, you would be forgiven for thinking that spotting them is pretty easy. Indeed, hundreds of these epic events are seen every year by the armies of astronomers that scan the skies in search of them.

Yet these are just the tip of the iceberg. When astronomers scale up to the number of supernovae they expect to be taking place throughout the entire universe they reach a mind-boggling number: thousands of exploding stars every hour. In the time it has taken to read this introduction, five massive stars have destroyed themselves somewhere in the universe. The energy from these blasts is now racing across space, perhaps to encounter Earth in millions or even billions of years.

But there is a problem. “Up to half the supernovae are going missing,” says , an astronomer at Ohio State University in Columbus. The big question is, what is happening to these stars? Are they simply disappearing without trace: there one minute and gone the next, or are we simply failing to see their explosive finale?

Understanding supernovae isn’t some trivial piece of celestial accounting. Supernovae are the engines of cosmic change. They shape galaxies, sometimes by triggering new stars to be born and at other times by halting star formation in its tracks. For a galaxy small enough, a volley of supernovae can blow it apart.

Perhaps most importantly for us, supernovae help to seed the universe with the atoms that make planets and life possible.

Stars live predictable lives. They are born in clouds of interstellar gas and dust and play out their years governed by the laws of physics. Massive stars, ones that are more than 8 times as massive as the sun, exist for a few million to tens of millions of years – the blink of an eye in cosmic terms – then explode as supernovae. A supernova begins when the core of the massive star becomes so large that it is no longer able to support its own weight. The core collapses from roughly the volume of the Earth to the volume of an asteroid, creating a ball of neutrons just 10 to 15 kilometres across – a neutron star. With its heart suddenly crushed, the rest of the star comes crashing down on top.

Gone without a flash

This inrush of gas strikes the neutron star, creating a shock wave that heads outwards. From here, two things can happen. In the first scenario, the shock wave is powerful enough to move outwards through the downpour of material and blast huge quantities of gas into space, creating a supernova explosion and leaving behind a neutron star and a nebula of shining gas that persists for centuries (see some of the most beautiful examples in our gallery “Ghostly glows mark violent deaths of stars“).

In the second scenario, the shock wave simply stalls. More and more gas falls down onto the neutron star until it eventually collapses into a black hole. Astronomers call this a “failed supernova”. With no supernova, the star will simply disappear: there one moment and gone the next. “It’s like being at a concert and seeing all the lighters in the air, and then one of those lighters just goes out,” says Beacom.

So we know what happens, in theory. The problem comes when we tot up the numbers. Astronomers have developed numerous techniques for measuring the rate at which stars of all sizes are born over the aeons. They have seen that this rate changes so slowly over the course of billions of years that for massive stars living such short lives, the death rate must match the birth rate. It’s when astronomers add together the observations of supernovae and black holes that the death rate comes up well short.

Of course, there could be a simple explanation. Some supernovae may be going off behind great banks of cosmic dust and so be hidden from our view, while others are intrinsically faint. Black holes are fiendishly difficult to spot too. Unless they happen to be close enough to rip another star to pieces, they are essentially invisible (see “Signatures of the invisible”).

To make progress, we need a new way to study supernovae. And astronomers believe the answer lies with ghostly particles called neutrinos. All supernovae, from the failed to the brightest, emit neutrinos. As much as 99 per cent of a supernova’s energy is carried out into space by neutrinos – even by the most luminous supernovae. “Neutrinos are what a supernova is really all about,” says George Fuller, a theoretician at the University of California, San Diego, “The energy of the optical explosion is pretty small in comparison.”

“Neutrinos are what a supernova is all about. The visible explosion is small by comparison”

Supernovae neutrinos are produced during the maelstrom of the star’s initial collapse as the neutron star is built. During this tumultuous event and under eye-watering pressure, electrons are forced into the atomic nuclei, where they combine with the protons to form the neutrons. Each of these reactions gives off a neutrino.

They aren’t alone. Neutrinos are streaming through us at all times. They come from a variety of sources on Earth, notably radioactive decay, lightning strikes and nuclear reactors. The sun drenches us in neutrinos too and then there are the cosmic rays that hit the atmosphere and produce showers of particles, including neutrinos. The reason you don’t feel trillions of neutrinos pouring through your body at this moment is because they interact so weakly with matter.

To catch one, we need giant detectors like the experiment deep under mount Ikenoyama in Japan. Super-K comprises a huge tank filled with 50,000 tonnes of ultra-pure water and works by detecting the fleeting pulse of light given out on the rare occasions when a neutrino collides with one of the water molecules. Such detectors have taught us much about the nature of neutrinos themselves. But when it comes to neutrino astronomy, they are more akin to Galileo’s spyglass than the Hubble Space Telescope.

The sun is such a huge source of neutrinos that distinguishing neutrinos from another celestial object has so far proved next to impossible. Astronomers have only managed to do it once, back in 1987 when a in the nearby Large Magellanic Cloud galaxy, 170,000 light years away. Super-K had not been built back then but its forerunner, Kamiokande II, was in operation. It captured just 12 neutrinos while the detector under Lake Erie in Ohio bagged eight.

Although small in number, they confirmed the overall picture of the star’s core collapse resulting in a burst of neutrinos. Since then, no other supernova has gone off near enough for its neutrinos to be detected, despite significant advances in our ability to capture them over the ensuing decades.

Supernovae in our galaxy have proved rare in the last few centuries, with the last one being seen by Johannes Kepler in 1604. This is frustrating because Super-K would detect an estimated 10,000 neutrinos if a supernova were to explode in the Milky Way. With such a haul of neutrinos, astronomers could begin to check their theories in detail.

All is not lost, though. The next generation of neutrino detectors will drastically extend our reach beyond the Milky Way and its environs. Of the devices on the drawing board and searching for funding, the most advanced is , the proposed big brother of Super-K. It would be 20 to 25 times larger than Super-K and be capable of detecting individual supernovae bursts out to millions of light years away, bringing other large galaxies into view and leading to an expected detection rate of a supernova neutrino burst at least once a decade.

With such capacity, sooner or later, a neutrino burst will be detected that astronomers cannot find with their telescopes; in other words, a failed supernova. “You should hope to get one failed supernova per decade, about the same as the detection rate for a normal supernova,” says at Arizona State University in Tempe, who has been investigating how best to detect some of the elusive particles.

With these observations in hand, a direct comparison with the neutrinos from a successful supernova will be possible. Computer simulations already suggest that neutrinos are the key to whether the supernova explosion triggers or fails. Zoom into the finer details of the simulations and you find that the outward moving shock wave stalls but can sometimes be revitalised if a tiny fraction of the neutrinos streaming out from the nascent neutron star is absorbed in the dense material that accumulates behind the stalled shock wave. With enough power, the shock wave races off through the rest of the star and blows it to pieces.

The simulations also tell us that neutrinos coming from failed supernovae should be more energetic than those from exploding ones. The reason is that the collapses that ultimately form black holes may be hotter, more extreme affairs than those forming neutron stars. “If we see higher-energy neutrinos we will know that failed supernovae are taking place,” says Lunardini.

Neutrinos uncovered

The next generation of neutrino detectors should tell us more. They should be able to reveal the history of supernovae throughout the universe. This is because the weakly interacting characteristic of neutrinos means that they hang around for aeons. They cross the universe mostly passing unhindered through the stars and planets with the result that the majority of the neutrinos given out by each and every supernova throughout history will still be in space. They will exist in a vast sea of neutrinos called the diffuse supernova neutrino background (DSNB), which contains about a tenth of the energy density of the cosmic microwave background radiation left over from the big bang. The neutrinos in the DSNB will carry a range of energies. By comparing the spread of energies to those seen in the individual supernovae neutrino bursts, researchers will be able to work out the proportion of successful to failed supernovae.

“Neutrinos given out by each and every supernova throughout history will still be in space”

Detectors like Hyper-K should routinely detect the DSNB. “It’s a new frontier that we hope to see in the next decade or so,” says Lunardini.

We may not have to wait quite so long. Tantalisingly, calculations suggest that water-based neutrino detectors such as Super-K are already right at the edge of detecting the DSNB. A few neutrinos from it are probably being caught every year but they are swamped by a background signal of neutrinos produced in the atmosphere when cosmic rays strike.

Luckily there is a solution. Neutrinos come in three types, or flavours, depending on how they are produced. For example, neutrinos produced in supernovae are electron-neutrinos, whereas twice as many muon-neutrinos are created in the atmosphere than electron-neutrinos. Only electron-neutrinos are revealed when a neutrino passes through a detector filled with ultra-pure water, as Super-K is. However, other experiments have revealed all three types of neutrino by dissolving salt in heavy water, which is made from a heavier form of hydrogen consisting of a neutron as well as a proton.

To Beacom and his colleagues, these different capabilities suggest a way to find the DSNB. The with the silvery-white metal gadolinium. Dissolving a soupçon of it into regular water can make any detected DSNB neutrinos give out a unique signal; first the burst of light that indicates the capture of the neutrino by a proton in the water, followed 20 nanoseconds later by a gamma ray with a distinctive energy. The gamma ray comes about because the neutrino interaction also produces a neutron that is captured by the gadolinium, which emits gamma radiation as a result.

So promising is this approach that the Kamiokande collaboration is spending more than $4 million dollars on a test facility known as EGADS, short for “evaluating gadolinium’s action on detector systems”. If all goes well there, gadolinium could be a part of the main Super-K detector by 2015. “We could have the first DSNB detections by 2016,” says Beacom.

Lunardini agrees that it could give us the first hints of the DSNB but to achieve the high numbers of neutrinos needed for a detailed analysis she says we’ll need to wait for the next generation of neutrino detectors. Once astronomers have that, they can really begin to work out why stars explode.

“Supernovae confounded the ancient astrologers, now they are confounding modern astronomers,” says Beacom.

But thanks to the growing confidence of the neutrino experimenters, there is now a real chance for progress. Nature is conducting thousands of experiments every hour before our eyes. All we have to do is watch and learn.