Did a tiny star power one of the biggest bangs in the Universe?

A ball of gas only 10 miles wide could have started the most powerful supernova ever recorded, with an explosion brighter than the Milky Way.

14th January 2016
An artist's impression of the record-breakingly powerful, superluminous supernova ASASSN-15lh (© Beijing Planetarium/Jin Ma)

A small mystery object lies at the heart of one of the biggest explosions astronomers have ever seen.

This object is radiating the energy of over 500 billion suns, yet could fit comfortably in a medium-sized crater of the Moon. And scientists haven’t figured what it is.

The explosion event left a hot gas ball that was discovered in June 2015 by the All Sky Survey for Supernovae (ASAS-SN, pronounced ‘Assassin’) collaboration, led by Ohio State University. Despite being so phenomenally bright, at 3.8 billion light years away it’s sadly not visible to the naked eye.

In a paper published in Science this week, the team speculate on the identity of the mystery object at the centre of the bang, dubbed ASASSN-15lh. There are two leading candidates; both pretty extreme even by astronomy’s standards.

Something that might just fit the bill is a rare and exotic star called a millisecond magnetar. An unusual form of neutron star, magnetars are incredibly dense, spin fast and have a strong magnetic field.

However, as team leader Krzysztof Stanek from Ohio State, remarks "If it really is a magnetar, it's as if nature took everything we know about magnetars and turned it up to 11." It would need to spin 1000 times a second and somehow convert its rotational energy into light with near-flawless efficiency. It’s possible, but it would push the laws of physics close to the limit.

Or perhaps a black hole is to blame? Later this year, the Hubble Space Telescope will turn its giant lens to ASASSN-15lh. If it finds the bright gas ball right at the centre of a large galaxy, where supermassive black holes reside – they may have just found their culprit.

Exactly what it might be doing would remain a mystery though, because nobody has seen a black hole behave like this before. We could be witnessing something completely new, perhaps changing our understanding of the Universe.

But for now, as the paper’s lead author Subo Dong admits, ‘the honest answer is at this point we do not know.

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