How heavy can a star be? This conundrum has haunted astronomers for decades. Theory indicates that there should be an upper stellar mass limit somewhere between 120 and 300 solar masses. Even though heavy stars are very bright, measurements of their masses can be complicated. The majority of the heaviest stars -- some researchers say as many as 90% or maybe even more -- tend to be born in multiple systems that can easily disguise themselves as single very heavy stars. Moreover, these stellar heavyweights are so rare that only a few are close enough to the Sun to be examined in detail.
One of the top candidates for the title of "Milky Way stellar heavyweight champion" was, until now, Pismis 24-1, a bright young star that lies in the core of the small open star cluster Pismis 24 (the bright stars in the Hubble image) about 8,000 light-years away from Earth. Pismis 24-1 was thought to have an incredibly large mass of 200 to 300 solar masses. This would have made it by far the most massive known star in the Milky Way. New NASA/ESA Hubble measurements of the star, have, however, resolved Pismis 24-1 into two separate stars, and, in doing so, have "halved" its mass to around 100 solar masses.
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