Astronomers have spotted the most powerful supernova in human history.
The record-breaking cosmic explosion was 570 billion times brighter than the Sun and about 200 times more powerful than a typical supernova, scientists said.
The blast - known as ASASSN-15lh - is thought to be an example of a "superluminous supernova," a recently discovered type of explosion unleashed by certain stars when they die.
But space experts are at a loss about which stars might be responsible for such an extreme event.
Astronomer Subo Dong said: "ASASSN-15lh is the most powerful supernova discovered in human history.
"The explosion's 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.
"The honest answer is at this point that we do not know what could be the power source.
"ASASSN-15lh may lead to new thinking and new observations of the whole class of superluminous supernova."
The explosion, described in a study published in the Science journal, was first glimpsed in June and took place around 3.8 billion light years away.
It was more than twice the brightness of the previous record-holding supernova and 20 times that of the 100 billion stars in the Milky Way.
The event was spotted by two telescopes in Chile as part of an international space survey based at Ohio State University.
The telescopes sweep the skies to detect suddenly appearing objects that are very bright but are too far away for human observers to notice.
In the four months after the explosion, the amount of energy beaming out of ASASSN-15lh would take the Sun in its current state more than 90 billion years to equal its emissions, scientists said.
The Hubble Space Telescope will be used later this year to obtain more detailed views of the aftermath of the explosion.