Is there an upper limit to the mass of a star? The answer to this long-standing question seems to be yes — and it has important consequences for our understanding of the evolution of galaxies. Stars form from interstellar gas and synthesize elements heavier than helium in their cores. Carbon, oxygen, silicon and iron, which are crucial for the existence of planets — and for life — are produced mostly by stars that are more massive than the Sun. To understand how galaxies evolve and are enriched in these elements, we need to know exactly how massive stars can be. Using the Hubble Space Telescope, Donald Figer (page 192 of this issue) has analysed the stellar content of the Arches cluster, a highly populous, nearby star cluster. He finds that there are no stars heavier than about 150 solar masses, providing direct evidence for the idea that the spectrum of stellar masses has an upper limit.
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