The largest known spiral galaxy in the universe has been discovered by accident.
The galaxy NGC 6872, located about 212 million light years away, is 522,000 light-years across - big enough to contain five Milky Ways.
Found in the constellation of Pavo, it also contains the remnants of another galaxy, IC 4970, which 'recently' - in astronomical terms - crashed through its spiral.
It is thought this titanic collision set off a wave of star formation which made NGC 6872 the giant that it is today.
Nasa said it lends evidence to the theory that large galaxies - including our own - "grew through mergers and acquisitions" by colliding with other smaller galaxies.
Researchers looking at data from the Galaxy Evolution Explorer (Galex) satellite found the remarkably huge galaxy entirely by accident.
They were looking for star-forming regions near to the galaxy. Instead they found a huge amount of ultra-violet light from young stars - which revealed the true scale of the galaxy itself.
It is thought after further observations that the wave of new star formation was kick-started by the collision with IC 4970.
Using other telescopes like the Very Large Telescope, the Two Micron All-Sky Survey and the Spitzer space telescope, they noted that the youngest stars were found in the outer spiral arms, with the older stars in the centre.
The BBC quotes Rafael Eufrasio from Basa's Goddard Space Flight Cente as saying the find was "a gift".
"I was not looking for the largest spiral - it just came as a gift," he told the BBC.
"The northeastern arm of NGC 6872 is the most disturbed and is rippling with star formation, but at its far end, visible only in the ultraviolet, is an object that appears to be a tidal dwarf galaxy similar to those seen in other interacting systems," said team member Duilia de Mello, a professor of astronomy at Catholic University.
The find was reported at the American Astronomical Society meeting in California.
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