Complex sentence: The highest observed neutron star mass is two solar masses, with a conservative upper-limit for the mass of a stable neutron star of three solar masses, so that a pair of neutron stars would not have sufficient mass to account for the merger.
Split into simpler sentences: The highest observed neutron star mass is two solar masses, with a conservative upper-limit for the mass of a stable neutron star of three solar masses. As a result, a pair of neutron stars would not have sufficient mass to account for the merger.