Pismis 24, the star cluster seen here in an image released on Dec. 11, 2006, lies within the much larger emission nebula called NGC 6357, located about 8,000 light-years from Earth. The brightest object in the picture was once thought to be a single star with an incredibly large mass of 200 to 300 solar masses. That would have made it by far the most massive known star in the galaxy and would have put it considerably above the currently believed upper mass limit of about 150 solar masses for individual stars. Measurements from NASA's Hubble Space Telescope, however, discovered that Pismis 24-1 is actually two separate stars, and, in doing so, "halved" their mass to around 100-150 solar masses each.
Image credit: NASA, ESA and Jesús Maíz Apellániz (Instituto de Astrofísica de Andalucía, Spain); Acknowledgment: Davide De Martin (ESA/Hubble)
