Astronomers have discovered the most luminous supernova ever observed, which is up to 50 times brighter than the entire Milky Way galaxy.

The super-luminous supernova, called ASAS-SN-15lh, was discovered by the All Sky Automated Survey for SuperNovae team (ASAS-SN), an international collaboration at the Ohio State University in US, which uses a network of 14-centimetre telescopes around the world to scan the visible sky every two or three nights looking for very bright supernovae.

Supernovae are violent stellar explosions and some of the brightest objects in the universe. Human records noting their existence date back nearly 2,000 years.

"We spotted a newly occurring explosion in a galaxy of an unknown distance," said Benjamin Shappee, from Carnegie Institution for Science.

Subsequent observations allowed the team to confirm the existence of the supernova ASAS-SN-15lh, Shappee said.

The supernova's spectra matched that of other hydrogen-poor super-luminous supernovae.
However, the researchers realised after further follow-up that it is two times more luminous than any supernova previously discovered.

ASAS-SN-15lh at peak was almost 50 times more luminous than the entire Milky Way galaxy, the researchers said.

The researchers determined that the galaxy where ASAS-SN-15lh formed is very atypical for a super-luminous supernova, which raises questions about how these types of supernovae form.

Its host galaxy is not the typical low-luminosity, star-forming galaxy where previous super-luminous supernovae have been spotted. ASAS-SN-15lh's galaxy is more luminous than our own Milky Way.

Within the past two decades a rare new category of super-luminous supernovae have been discovered, which shine one hundred to a thousand times brighter than the more common supernovae.

It has been theorised that these super-luminous supernovae are powered by so-called magnetars, neutron stars with extremely powerful magnetic fields, with the magnetism providing the engine for the immense luminosity.

According to this theory, the magnetic field's spin magnifies the energy of the explosion, increasing the luminosity.

As counterintuitive as it may sound, super-luminous supernovae are difficult for astronomers to spot.

This is because they are rare and tend to form in low-luminosity galaxies with vigorous star formation, whereas the sky surveys that have been traditionally used to locate supernovae target bright galaxies with low rates of star formation.

The findings were published in the journal Science.