Artist impression of supernova as seen from nearby exoplanet
About 3.8 billion light years away sits a record-busting exploding star. Dubbed ASASSN-15lh, it shined more than twice as bright as the previous record holder at peak intensity. The massive blast is believed to be a perfect example of a “superluminous supernova.”
‘Superluminous supernova’ are a rare class of explosions created by certain stars. What causes them? Scientists are stumped. Subo Dong, who led an international team of astronomers described the explosion as “the most powerful supernova discovered in human history.” He added, “the explosion mechanism and power source remain shrouded in mystery because all known theories meet serious challenges in explaining the immense amount of energy ASASSN-15lh has radiated.”
Let me try to put in perspective how insanely bright this explosion was. At its brightest, it shined 570 billion times brighter than the Sun. That’s nuts, but it gets even crazier. It shined 20 times brighter than the entire Milky Way galaxy. That’s more than 100 billion stars!
In June 2015, twin telescopes in Cerro Tololo, Chile were busy sweeping the sky as part of the All Sky Automated Survey for SuperNovae (ASAS-SN). The pair of telescopes are armed with 14-centimeter diameter lenses and scan the skies for objects that suddenly appear. They soon spotted the object that would become ASASSN-15lh.
The pair of 14-centimeter telescopes that made the first observation.
Dong and his fellow astronomers sprang into action. They alerted observatories across the world in a bid to gather as much information about the supernova as possible. Even NASA’s Swift satellite joined in. Observatories on the ground and in space continue to observe ASASSN-15lh to this day.
These follow-up observations are telling astronomers a couple of things about this massive star explosion. Some features point to ASASSN-15lh being a ‘hydrogen-poor’ (Type I) superluminous supernova. The rate of temperature decrease and radius expansion also lines up with known Type I superluminous supernova.
That’s where the similarities appear to stop. ASASSN-15lh shocks astronomers by being much brighter and hotter than previous supernova before it. It’s home galaxy also has astronomers scratching their heads. Every Type I superluminous supernova discovered so far comes from galaxies that are smaller than the Milky Way. But ASASSN-15lh appears to originate in a galaxy that is bigger and brighter than the Milky Way.
Point the Hubble
The Hubble Space Telescope may be able to shed some light on the many questions surrounding ASASSN-15lh. Later this year, Hubble will train its sophisticated instruments on the supernova and give Dong and his fellow astronomers the most detailed look yet at what’s left of ASASSN-15lh’s explosion.
One question the astronomers hope to clear up is exactly where the supernova explosion is located. It appears to be in a bigger and brighter galaxy than the Milky Way. But it could also sit in a small, dimmer neighboring galaxy.
There’s no good theory
Astronomers are at a loss. One theory points to neutron stars called magnetars. They are leftover cores of massive, exploded stars and could boost the brightness of superluminous supernova. But even this scenario doesn’t explain the sheer amount of energy seen in ASASSN-15lh.
“The honest answer is at this point that we do not know what could be the power source for ASASSN-15lh,” says Dong. “ASASSN-15lh may lead to new thinking and new observations of the whole class of superluminous supernova, and we look forward to plenty more of both in the years ahead.”
Dong and his colleagues will just have to wait and see what the Hubble makes of the gigantic explosion.