Black holes may be harder to make than once thought. When stars go supernova, theoretical calculations suggested that those about 25 times the sun's mass or more turn into black holes; meanwhile less massive stars become dense, whirling balls of neutrons. One such neutron star, in the Westerlund 1 star cluster and examined by NASA's Chandra X-ray Observatory, defies this calculation. Based on the size of the largest nearby stars, astronomers deduced that the neutron star's parent was among the biggest of them all—at least 40 solar masses. They suggest that extremely weighty stars might shed mass so effectively before they die that enough is left only for a neutron star. The findings, appearing in an upcoming Astrophysical Journal Letters, may severely limit the formation of black holes to stars between 25 and 40 solar masses.