Astronomers reckon that it is only between one and three million years old, and that when it originally formed it may have had a mass up to 200 times greater than our Sun's - before it threw off much of that in a series of violent eruptions.
Those eruptions formed the pink "clouds" which appear to surround the star - a stellar nebula which the HST had to peer through in order to determine the detail of the picture; the Pistol star is not visible to the eye, but lies beyond the constellation Sagittarius, in the dust clouds of the Milky Way. The HST detected it using the infra-red rays that penetrate the dust and reach Earth.
The star is so energetic that it gives off as much energy in six seconds as our Sun does in a whole year. The nebula is so big - four light-years across - that it would nearly span the distance from the Sun to Alpha Centauri, the nearest star to Earth's solar system.
"This star may have been more massive than any other star, and now it is without question still among the most massive - even at the low end of our estimates," says Don Figer of the University of California at Los Angeles. "Its formation and life stages will provide important tests for new theories about star birth and evolution."
A few million years is a remarkably early age for a star to be so energetic. Its size also indicates that in a few million years it will certainly explode as a supernova, throwing out its contents in a massive explosion.