Spotted in space: An explosion far brighter than the entire Milky Way

Astronomers detail their discovery of a record-setting supernova that defies easy explanation.

An artist's impression of ASASSN-15lh as it would appear from an exoplanet located about 10,000 light-years away in the host galaxy of the supernova.

Beijing Planetarium/Jin Ma

Scientists announced Thursday that they've witnessed a cosmic explosion in the form of a distant super-luminous supernova so bright it strains our grasp of physics to explain it.

The exploding massive star has the ominous-looking name ASASSN-15lh and is only 10 miles (16 kilometers) across at its core, yet is much brighter than the entire output of the Milky Way. If you're wondering why you haven't seen anything so crazy-sounding lighting up the sky, that's because it's 3.8 billion light-years away. (Polaris, the bright "North Star" is between 300 and 450 light-years away, for comparison.) In fact, at the time the super-supernova explosion took place, life was just beginning to emerge here on Earth.

Supernova explosions happen toward the end of the life of a massive star that goes out with one literal last blast, appearing to create a "new" star in the heavens that then slowly fades away over weeks or months. Supernovas that can be seen with the naked eye are so rare there hasn't really been one since 1604, but some have theorized that the Biblical star of Bethlehem may have been a supernova.

ASASSN-15lh wasn't discovered by a team of snipers, but rather by the All Sky Automated Survey for SuperNovae team (ASAS-SN) -- hence the name. A study on the discovery appears in the latest issue of Science. The ASAS-SN collaboration based at Ohio State University uses a network of 14-centimeter telescopes around the world to look for shining supernovas. Normally it can only find typical supernovas out to a distance of about 350 million light-years away.

On June 14, 2015, the group spotted a new explosion that turned out to be much farther away -- and much, much brighter -- than what they typically find. At its peak intensity, ASASSN-15lh was 570 billion times brighter than our own sun or 20 times brighter than the output of all 100 billion stars in the Milky Way galaxy.

Before and after views of the host galaxy.

The Dark Energy Survey, B. Shappee and the ASAS-SN team

"ASASSN-15lh is the most powerful supernova discovered in human history," Subo Dong, the study's lead author, said in a statement. Dong is an astronomer at the Kavli Institute for Astronomy and Astrophysics at Peking University. "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."

Right now researchers' best guess is that the massive supernova's unthinkable amount of energy might come from a type of rapidly spinning neutron star called a magnetar, but even this explanation doesn't quite adequately account for the insane luminosity of ASASSN-15lh, which might actually be the result of violent death spasms of a star that's more massive than what was previously thought to be possible. Another possibility is that it could be an entirely new kind of event taking place near a supermassive black hole.

"The honest answer is at this point that we do not know what could be the power source for ASASSN-15lh," Dong said. "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."

To get a better picture of what's going on with this massive display of cosmic fireworks, astronomers have secured time to look closer using the Hubble Space Telescope in the coming months.

Meanwhile, no need to worry about a similar explosion taking out our own sun, which is believed to still have a few billion more good years in it.

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