Washington: NASA scientists have spotted the farthest known galaxy in the universe, a primitive cluster of stars just 500 million years old.

An intensive survey deep into the universe by NASA's Hubble and Spitzer space telescopes has yielded the image of the galaxy named SPT0615-JD, stretched and amplified by a phenomenon called gravitational lensing.

Though a few other primitive galaxies have been seen at this early epoch, they have essentially all looked like red dots given their small size and tremendous distances.However, in this case, the gravitational field of a massive foreground galaxy cluster not only amplified the light from the background galaxy but also smeared the image of it into an arc.

"No other candidate galaxy has been found at such a great distance that also gives you the spatial information that this arc image does," said Brett Salmon of the Space Telescope Science Institute in the US."By analysing the effects of gravitational lensing on the image of this galaxy, we can determine its actual size and shape," said Salmon.

First predicted by Albert Einstein a century ago, the warping of space by the gravity of a massive foreground object can brighten and distort the images of far more distant background objects.

Astronomers use this "zoom lens" effect to go hunting for amplified images of distant galaxies that otherwise would not be visible with today's telescopes. SPT0615-JD was identified in Hubble's Reionization Lensing Cluster Survey (RELICS) and companion S-RELICS Spitzer programme.

"RELICS was designed to discover distant galaxies like these that are magnified brightly enough for detailed study," said Dan Coe, Principal Investigator of RELICS.

By combining the Hubble and Spitzer data, Salmon calculated the lookback time to the galaxy of 13.3 billion years. Preliminary analysis suggests the diminutive galaxy weighs in at no more than 3 billion solar masses. It is less than 2,500 light-years across

The object is considered prototypical of young galaxies that emerged during the epoch shortly after the Big Bang. The galaxy is right at the limits of Hubble's detection capabilities, but just the beginning for the upcoming NASA James Webb Space Telescope's powerful capabilities, said Salmon.

"This galaxy is an exciting target for science with the Webb telescope as it offers the unique opportunity for resolving stellar populations in the very early universe," he said.