Star explodes once again about 50 years after its last supernova

Cosmic revenant: An ordinary supernova astounds astronomers, because the glow of the star explosion lasted six times as long as usual and wavered, instead of fading. Even stranger, however: the same star exploded in 1954 in a supernova. Why it survived and has now exploded again, can not be explained with the current models, as the researchers report in the journal "Nature".

Normally, with a supernova the star core collapses and only one neutron star or black hole remains -
but not with iPTF14hls. © ESO/M. Kornmesser

A so-called type IIP supernova ordinarily follows clear rules: The core of a star rich in mass collapses and causes a violent explosion. With this the star cover rich in hydrogen is blown off and in the center only a neutron star remains – an extremely thick object consisting of exotic matter. The shock wave of the explosion lights up the thrown out gases and generates mostly about 100 days of constant lighting up of this supernova.

"Impossibly" long glow

Now, however, astronomers around Iair Arcavi at the Las Cumbres Observatory in California have discovered a supernova that does not fit this scheme or any other type of supernova. When they registered the flare of a star 800 million light-years away in September 2014, everything seemed quite normal: the spectrum of iPTF14hls' light was exactly the same as that of an IIP supernova, and the temperature of around 5,000 to 6,000 Kelvin was just right.

But when the astronomers re-targeted the same position in January 2015, they discovered the surprising : The supernova was still shining and did not seem to want to stop at all.

Instead of a 100-day plateau the light curve from iPTF14hls lasted more than 600 days, report Arcavi and his colleagues. It seems almost, as if in this explosion everything happens 10 times slower than normally.

The first explosion already in 1954

Also strange: The brightness of the explosion did not simply decrease after the initial summit, as is usual with such a supernova. Instead, iPTF14hls has brightened and darkened at least five times over the last three years, as the astronomers report. In addition, according to the spectral lines, the ejected or exploded elements move differently than usual.

However, it becomes even more puzzling: When the researchers looked in archive data for hints to the prehistory of the star, they bumped into astonishing: In 1954 a supernova had already occurred at the exactly same location.

All these data speak for the fact that there must have been a high-energy pre-supernova outbreak, the astronomers report.

Survived the star death

In concrete terms: The precursor's star of PTF14hls exploded more than 50 years ago for the first time, but was somehow able to keep enough hull material to survive this ordinarily sure star death. In 2014 this "zombie" exploded for the second time in a supernova. This again fits to none of the current models.

This supernova contradicts all what we believed to know about these events, says Arcavi. It is the biggest riddle which I have met in nearly one decade of research in stellar explosions.

Astronomers suspect that the parent star must have been at least 50 times more massive than the sun, perhaps even up to 130 solar masses. Because only such giants under the stars could have survived the first supernova.

A "dinosaur of the universe"?

Perhaps, according to the researchers, iPTF14hls is the first known representative of a supernova type that has so far only been postulated theoretically. In these "pulsational pair instability" supernovae, the core of massive stars becomes so hot that energy inside is transformed into antimatter and matter. This triggers an explosion that blows away the outer hull of the star but leaves the core intact. Only after multiple repetitions the core then collapses and a black hole is created.

However, these explosions were typical for the early universe – today they might seem not to be really valid any more, says Arcavis Mr. Andy Howell. It's like finding a live dinosaur today.

Furthermore, some of the signs of this supernova do not fit to the theory of this supernova type, under it the persistent presence of hydrogen or the enormously high energy of the explosion.

Only one thing seems to be clear: The supernova iPTF14hls fits in none of the current models and could show an absolutely new type of star explosion. Which mechanisms are behind this puzzling event, remains a secret which the astronomers will have to solve.

Source: Nature, in 2017; doi: 10. 1038