Nathan Smith, of the University of California at Berkeley, who led the discovery team, described SN 2006gy as "a special kind of supernova that has never been seen before".
He called it "freakily massive" at 150 times the mass of the sun.
"This one is way above anything else," he said. "It's really astonishing."
He said the star had been shining at levels brighter than other supernovae for several months.
An artist's illustration of what SN 2006gy would look like through a telescope [EPA/Nasa]
And even at 240 million light years away, this star in a distant galaxy does suggest that a similar and relatively nearby star – one 71 quadrillion kilometres away – might explode in similar fashion any day now, or 50,000 years from now, he added.
David Pooley, an astronomer from University of California at Berkeley, said it would not threaten Earth but it would be so bright that people could read by it at night, although it will only be visible to those in the Southern Hemisphere.
Observations from the Chandra X-ray telescope helped show that the star did not become a black hole like other supernovae and skipped a stage of star death.
Unlike other exploding stars, which peak at brightness for a couple of weeks at most, this supernova peaked for 70 days, Nasa said.