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This is the brightest supernova astronomers have ever seen

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

When massive stars die, they do not fade gently into the good night. They collapse in on themselves and explode. It's called a supernova, and "it is the largest explosion that takes place in space," as NASA explains.

Today in Science, astronomers report they've found the brightest such explosion ever observed. (To be more specific, it’s the most luminous ever observed. Luminosity is the measure of how much light and energy an object emits. The supernova still looks dim to us so so far away.)

This supernova — unceremoniously called ASASSN-15lh — is about twice as powerful as the last record breaker and some 200 times more powerful than a typical supernova. At its peak, it was about 50 times brighter than the entire Milky Way galaxy.

The supernova occurred not in our own galaxy, but in one extremely far away (its exact distance is unknown). The image at the top of this post is an artist's conception of what the supernova would look like from a (relatively close) planet about 10,000 light-years away from the supernova's galaxy. From that vantage point, it would be the brightest thing in the night sky.

By contrast, the image below shows what the supernova looks like to us. On the left is a reference image of the galaxy taken in 2014, before the supernova. The image on the right was taken after the explosion.

Pseudo-color images showing the host galaxy before the explosion of ASASSN-15lh taken by the Dark Energy Camera (DECam) [left], and the supernova by the Las Cumbres Observatory Global Telescope Network (LCOGT) 1-meter telescope network [right].
The Dark Energy Survey, B. Shappee and the ASAS-SN team

The difference looks subtle in the images, but scientists can examine changes in the wavelengths of light and radiation from the galaxy to determine that a supernova occurred, and guess what elements exploded from the star's core.

This supernova was discovered by the All Sky Automated Survey for Supernova (called ASAS-SN or "assassin") a network of six telescopes around the world. By comparing real-time images of the sky to reference images, the telescope system can automatically spot changes in luminosity that suggest a supernova occurred.

This chart compares the relative luminosity of the supernova (called ASASSN-15lh) with the light of the Milky Way and typical supernova. ASASSN-151h tops the chart, and is about twice as powerful as the last record breaker, called ipTf13ajg. (Note to astronomers: Give supernovae better nicknames!)
ASAS-SN

So far, "the power source for ASASSN-15lh is unknown," the discovery team reports in Science. (Their analysis also doubts that the explosion was a type 1a supernova, which is when a white dwarf star is ignited by a neighboring star.)

That astronomers can see evidence of the supernova at all is kind of amazing. The light from the supernova has been traveling for millions of years to reach ASAS-SN's telescope lenses. It's as if a tsunami took a million years to travel across the ocean. We'd see the wave but could only guess at the cataclysm that caused it. Similarly, when we see a supernova go off in the sky, we're seeing an event that happened before humanity even existed.