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19 December 2016updated 01 Jul 2021 11:38am

Is this the most dramatic death of a star ever recorded?

By Anjuli R. K. Shere

Last year, a spectacular event was recorded – something that scientists were calling “the brightest supernova ever”. Although the suspected supernova (the explosion of a star as it dies) actually occurred 2.8 billion years ago, the event in the southern constellation of Indus was documented in June 2015 by the All Sky Automated Survey for SuperNovae (ASAS-SN) network, a group of four automated 14cm telescopes that search for exploding stars.

The occurrence – named ASASSN-15lh – was over 500 billion times brighter than our Sun, and outshone the entire Milky Way 20 times over. Krzysztof Stanek, co-principal investigator of the ASASSN network, told the New Scientist “this is really on steroids, and then some”, adding that if such a supernova had occurred in our galaxy, there would be no night.

At first the event flummoxed astronomers, but after over a year of research, Queen’s University in Belfast (QUB) has just announced an alternative reason for the bright light – that it marked the death throes of a star being ripped apart by a supermassive black hole.

As star deaths go, this is less exciting than the Starkiller Base’s super-weapon from The Force Awakens needing to drain a star’s energy to function, but much more unusual than most supernovae. However, the argument is far from resolved; there is evidence to support or disclaim both explanations.

There were several characteristics of the event that suggest that it was not a superluminous supernova – which would have been ten-100 times more luminous than an “average” supernova – but initially astronomers did not know how else to classify such a bright star death. Other astronomical occurrences had exhibited similarly atypical parts of their spectra, some of which had also been labelled superluminous supernovae. They have also been attributed to other alarming and unusual space features, for instance magnetars (extraordinarily dense neutron stars, with magnetic fields about 1,000 trillion times more powerful than Earth’s magnetic field).

Some of ASASSN-15lh’s characteristics seemed to confirm the original superluminous supernova narrative, such as magnesium lines in its spectrum, which researchers from the ASASSN network had predicted would be there, based on observations of previous superluminous supernovae. Then, after 100 days, there was another anomaly – ASASSN-15lh’s light, which had been weakening, stubbornly re-brightened. Although there is no consensus on an explanation for this phenomenon yet, other so-called superluminous supernovae had sometimes done this, so it did not directly contradict the dominant narrative.

Still, some astrophysicists, including a group from the Experimental Astrophysics group at the Weizmann Institute of Science, Israel, wrote a paper arguing against the supernova label for ASASSN-15lh. Giorgos Leloudas, lead author of this study, told that this aspect of the star death was inconsistent with fundamental aspects of a supernova, like the temperature evolution – as it is “an expanding ball of gas”, by nature it must cool down as it swells. Rather, he observed, the object “started warming up again, and stayed hot at a constant and very high temperature for a long time, and it continues to do so”.

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The location of the event was also of concern, said Leloudas, as the galaxy in which the star death occurred was both immense and “red” – meaning it is full of old and low-mass, cooling stars that emit red radiation. Without younger, massive stars, it would be impossible to have a superluminous supernova.

Leloudas’ study group proposed an alternative explanation: a tidal disruption event, in which an orbiting star had too closely approached the supermassive black hole at the centre of its galaxy, and been shredded by the tidal forces of the black hole. As last year was the 100th year of Albert Einstein’s gravitational theory, the news that the “brightest supernova ever” was probably actually the echoes of a star being destroyed by excessive gravity seems a fitting tribute. Amusingly, while Einstein’s formulae accounted for the existence of black holes, the man himself never believed that they really occurred.

It’s ironic that a black hole cannot be seen because the strength of its gravitational “sink” is such that any light that enters its event horizon is swiftly pulled to its centre, and yet it could create the conditions for such a terrific burst of high-energy light. If the star had been as massive as our Sun, a Kerr black hole – one that has mass and angular momentum (so spins rapidly), but no electrical charge – could have disrupted it just beyond the event horizon. As it spiralled uncontrollably into the hole, the gases that comprised the star heated up, causing the release of one final luminous flare before being sucked into oblivion.

Black holes “eating” stars is such a rare occurrence that there are only about ten on record, but it is thought that the evidence that ASASSN-15lh was a tidal event does not just extend to the event’s sudden bright flash, or its position in a “passive” galaxy, but is also strengthened by “the presence of highly ionised CNO gas” at different velocities, which is consistent with the idea that there would be material and radiation surrounding the tidal disruption. While there is still the possibility that the flash was caused by something completely different, Professor Smartt from QUB’s Astrophysics Research Centre, who gathered the data that supports the tidal event theory, believes that the new explanation is the result of “an excellent example of international collaboration and scientific team work”.

While it is true that scientific collaboration may be at the heart of the new findings, if this story revolves around anything, it is the supermassive black hole. Lurking at the centre of a galaxy 4 billion lightyears from our tiny blue planet, the black hole is 100 million times more massive than our Sun and has the power to destroy stars – with flair.

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