Newly discovered supernova most powerful stellar explosion ever seen

PHOTO: An artist's impression of the super-luminous supernova ASASSN-15lh as it would appear from an exoplanet located about 10,000 light-years from the blast. (Beijing Planetarium/Jin Ma)

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Astronomers have detected the most powerful stellar explosion ever seen — a massive supernova, twice as powerful as anything previously recorded.

Key points

• Massive supernova is 570 billion times brighter than our Sun, and 20 times brighter than all the stars in our Milky Way Galaxy combined.
• Scientists unsure about what could be powering the supernova named ASASSN-151h
• Unlike other super-luminous supernovae, ASASSN-151h exploded in a large, calm galaxy

The supernova, named ASASSN-15lh, occurred in a distant galaxy 3.8 billion light-years away, and is 200 times more powerful than the average supernova.

The huge blast, reported in the journal Science, is at the very upper limit of researchers' understanding of stellar physics, raising new questions about how such a powerful event could be generated.

"It's an extreme example — it's so extreme that it's really unclear what's actually causing the supernova — if that's what it is," one of the study's authors, Dr Benjamin Shappee of the Carnegie Observatories in Pasadena, said.

"This was so exciting and so far outside what's expected, I was a little sceptical until we got the follow-up data.

"Then when we saw it on two different telescopes and confirmed the data I started to get extremely excited."

The record-breaking explosion, thought to be a super-luminous supernova, was discovered in June using the All-Sky Automated Survey for SuperNovae system (ASASSN).

Most super-luminous supernovae occur in small, busy star-forming galaxies. However, ASASSN-15lh exploded in a large and rather calm galaxy.

Mystery surrounds super-luminous supernovae

PHOTO: Pseudo-colour images showing the host galaxy before the explosion of ASASSN-15lh (Left), and the supernova (Right).(The Dark Energy Survey/B. Shappee/ASAS-SN team)

"There are lots of different types of supernovae ... but it's not clear how you could get such a luminous supernova," Dr Shappee said.

The supernova could have been spawned by an extremely rare type of star called a millisecond magnetar, a very rapidly spinning and extremely dense neutron star with a very strong magnetic field.

This shows that in supernovae studies we're still uncertain on a number of the details -- even the common ones. We're still finding many things which are completely unanticipated in nature.

Dr Benjamin Shappee

To shine so bright, the magnetar would also have to spin at least 1,000 times a second, and convert all that rotational energy to light with nearly 100 per cent efficiency.

"There's an upper limit as to how fast they spin before they rip themselves apart," Dr Shappee said.

"So if you take a neutron star and you dump all of its energy into its (gaseous) envelope, then you can reproduce what we see. It could be this object — but it's right on the edge of what models can reproduce."

Dr Shappee said another possibility is a very large star — hundreds of times the mass of our Sun.

"This could be telling us that such massive stars really do exist and then explode," he said.

"We have some Hubble Space Telescope time to further investigate this object."

If the team finds that ASASSN-15lh lies in the very centre of its galaxy, then perhaps it is not a supernova at all, but instead some unusual nuclear activity around a supermassive black hole.

"We think it's a supernova because spectroscopically it looks very similar to super-luminous type 1 supernovae, and it also fades with time and gets cooler like supernovae," Dr Shappee said.

"This shows that in supernovae studies we're still uncertain on a number of the details — even the common ones. We're still finding many things which are completely unanticipated in nature."