For a variety of reasons, many think the first stars to form in the Universe were monsters, hundreds of times the mass of the Sun. With that much mass, the stars were destined to have short lives that ended violently in a supernova, seeding the first galaxies with the elements that would form future stars and planets. There have been a few reports of gamma-ray bursts that are distant enough that they probably signal the end of the first generation of stars, but we have not observed any of these supernovae directly.

Now, researchers have used a survey that ran for several years to identify distant superluminous supernovae—explosions that are so bright they are thought to occur when the most massive stars convert light into matter/antimatter particle pairs at their core. Although these don't appear to have come from the explosion of the Universe's first stars, the more distant of the events dates from only 1.5 billion years after the Big Bang. Based on this success, they suggest their approach can be used to spot the demise of the Universe's first stars.

Why were these stars so big? A star is a balancing act between gravity and energy. Too much energy, and the gas cloud that could form a star would dissipate. Too much gravity, and it will quickly collapse into a black hole. As the gas cloud collapses into a star, this collapse liberates energy, which threatens to push the cloud apart if it can't be radiated away. And, as it turns out, pure hydrogen (which is most of the early Universe) is lousy at dissipating this energy. So it's really hard to form a star out of nothing but hydrogen (the complex mix of elements present after the first stars exploded makes the process easier).

To get around this inefficiency, the first stars formed from very dense gas clouds, where there was enough gravity to overcome their internal heat. Models indicate the results were enormous, 200 to 300 times the mass of the Sun, and they burned through their hydrogen quickly.

Theoretical considerations suggested stars with this mass would suffer a rather unusual fate. At their cores, the density of gamma-ray light would reach levels that were so high, some of it would spontaneously convert into pairs of particles and antiparticles (remember, e = mc2, so the energy of the gamma-rays can be converted to matter). Having antimatter at the core of the star was bad enough, but the sudden loss of radiation would essentially pull the legs out from under the core of the star. Without the steady outward pressure from the energy, the entire core of the star would suddenly collapse.

With stars this size, that's a rather extreme event. About 60 solar masses worth of carbon and oxygen fuse into heavier elements almost instantaneously. The resulting thermonuclear explosion tears the star to pieces, leaving nothing behind but a rapidly expanding gas cloud.

For years, theorists were waiting for astronomers to catch up with them and observe one of these supernovae, termed "pair instability." In recent years, they seem to have done so, spotting a couple examples of supernovae that reach extreme brightness, 10 times more luminous than the strongest type Ia explosions. They also have a distinctive pattern of brightening, with a peak several weeks after the initial explosion, powered by the decay of 56Ni formed when the core collapsed.

Normally, we can't detect supernovae out to high enough red-shifts to spot them in the early Universe, but the authors of the new paper note that these pair-instability events are extremely bright, saying, "The extreme luminosities of superluminous supernovae extend the redshift limit for supernova detection using present technology." They went back and examined a survey done from the years 2003-2008 called the Canada-France-Hawaii Telescope Legacy Survey Deep Fields. That ran for six months every year from 2003 to 2008. The authors simply looked for objects that weren't present in the first observations but appeared in later ones, and were bright in the ultraviolet at the source (though redshifted to longer wavelengths by the time they reach Earth).

As expected from pair-instability supernovae, these slowly climb to peak brightness, and that peak brightness is enormous: 1044 ergs/second, or 1037 watts. The authors estimate the progenitor of one of these explosions weighed in at 250 solar masses. The other one reached levels of brightness in the UV that aren't possible from the supernova alone, which suggests the explosion ran into gas that had been ejected from the star earlier.

The redshift values of these objects are 2.1 and 3.9, which places them at a time when the Universe was 3 billion and 1.5 billion years old. Follow-up observations with the Keck telescope suggest there were heavier elements around, which indicate these stars were not the first to form in the Universe.

It's hard to extrapolate too much with only two examples, but if the rate of discovery holds up, the authors say that pair instability supernovae may have occurred at a rate 10 times higher than they do in the current Universe. They say that a continued search for these events, along with follow-up observations that check their composition in greater detail, may ultimately reveal the details of the explosion of one of the Universe's first stars.

Nature, 2012. DOI: 10.1038/nature11521 (About DOIs).