An exploding star that seems to contravene the laws of physics is puzzling astronomers.
'The wierdest type of supernova yet'
The supernova is twice the brightness expected, suggesting it arose from a star much too massive to exist.
Past observations of supernovae led to the discovery that the Universe's expansion is speeding up.
The findings could affect their use as probes of dark energy, the mysterious entity behind the expansion, scientists report in the journal Nature.
The object, known as SNLS-03D3bb, was discovered in April 2003, in a small, young, star-forming galaxy. It is classified as a type 1a supernova, based on chemicals detected in the atmosphere around it.
Type 1a supernovae have always been regarded as uniform in brightness and distances are traditionally calculated on how bright they appear to be through telescopes.
They are thought to form when a white dwarf - the remains of a low or medium mass star - pulls enough matter from a nearby companion star to explode in a violent thermonuclear reaction.
According to the Indian physicist Subrahmanyan Chandrasekhar, no white dwarf can be more massive than about 1.4 solar masses before it self destructs. SNLS-03D3bb, however, appears to contravene this rule.
"Type 1a supernovae are thought to be reliable distance indicators because they have a standard amount of fuel - the carbon and oxygen in a white dwarf star - and they have a uniform trigger," said Peter Nugent, an astrophysicist at Lawrence Berkeley National Laboratory in Berkeley, California, who worked on the study.
"They are predicted to explode when the mass of the white dwarf nears the Chandrasekhar mass, which is about 1.4 times the mass of our Sun.
"The fact that SNLS-03D3bb is well over that mass kind of opens up a Pandora's box."
Astronomers now have to explain how a white dwarf could grow so massive. One possibility is that two white dwarfs could spiral together and eventually merge.
Another, more likely explanation, is that matter accumulated by a white dwarf from a companion star could add extra angular momentum causing it to rotate more rapidly.
This would provide extra support against gravity and allow the white dwarf to become extra-massive before exploding.
Astronomers are now considering whether objects such as SNLS-03D3bb should be screened out in cosmological studies, to avoid them "contaminating" results.
"As this supernova does not obey the relations that allow type 1a supernovae to be calibrated as standard candles, and as no counterparts have been found at low redshift, future cosmology studies will have to consider possible contamination from such events," the authors write in Nature.