Those behind the find say the star, called R136a1, may once have weighed as much as 320 solar masses. Astrophysicist Paul Crowther said the obese star — twice as heavy as any previously discovered — has already slimmed down considerably over its lifetime.
In fact, it's burning itself off with such intensity that it shines at nearly 10 million times the luminosity of the sun.
"Unlike humans, these stars are born heavy and lose weight as they age," said Crowther, an astrophysicist at the University of Sheffield in northern England. "R136a1 is already middle-aged and has undergone an intense weight loss program."
Crowther said the giant was identified at the center of a star cluster in the Tarantula Nebula, a sprawling cloud of gas and dust in the Large Magellanic Cloud, a galaxy about 165,000 light-years away from our own Milky Way.
The star was the most massive of several giants identified by Crowther and his team in an article in the Monthly Notices of the Royal Astronomical Society.
While other stars can be larger, notably the swollen crimson-colored ones known as red giants, they weigh far less.
Still, the mass of R136a1 and its ilk means they're tens of times bigger than the Earth's sun and they're brighter and hotter, too.
Surface temperatures can surpass 40,000 degrees Celsius (72,000 degrees Fahrenheit), seven times hotter than the sun. They're also several million times brighter, because the greedy giants tear through their energy reserves far faster than their smaller counterparts.
That also means that massive stars live fast and die young, quickly shedding huge amounts of material and burning themselves out in what are thought to be spectacular explosions.
"The biggest live only 3 million years," Crowther said. "In astronomy that's a very short time."
Small lifespans are one of several reasons why these obese stars are so hard to find. Another is that they're extremely rare, forming only in the densest star clusters.
Astronomers also have a limited range in which to look for them. In clusters that are too far away, it isn't always possible to tell if a telescope has picked up on one heavyweight star or two smaller ones in close proximity.
In this case, Crowther's team re-examined previously known stars to see if they could find an accurate measurement of their weight. The team reviewed archival data from the Hubble Space Telescope and gathered new readings from the European Southern Observatory's Very Large Telescope at Paranal in Chile.
Scientists who weren't involved in the find said the results were impressive, although they cautioned it was still possible, although unlikely, that scientists had confused two very close stars for a bigger, single one.
"What they're characterizing as a single massive star could in fact be a binary system too close to be resolved," said Mark Krumholz, an astronomer at the University of California, Santa Cruz.
Both he and Phillip Massey, an astronomer with the Lowell Observatory in Arizona, also cautioned that the star's weight had been inferred using scientific models and that those were subject to change.
But both scientists said the authors had made a strong case, arguing that the solar material being thrown off from feuding stars in a binary system would produce much more powerful X-rays than have been detected.
Crowther acknowledged that R136a1 could have a partner, but he said it was likely to be a much smaller star, meaning that the star's its birth weight was still considerable — perhaps 300 solar masses instead of 320.