Lattimer has made several fundamental contributions to the field of
nuclear astrophysics, with a particular focus on neutron stars. One of his biggest impacts was modeling the birth of neutron stars from supernovae in 1986 with then-research assistant professor
Adam Burrows. This came just six months before the closest supernova in modern history (SN 1987A, in the LMC). Their paper predicted the signature of
neutrinos from supernovae that was subsequently validated by neutrino observations, from SN 1987A on February 23, 1987. In work that led to his PhD thesis, Lattimer and his advisor
David Schramm first argued that the mergers of neutron stars and
black holes would result in the ejection of neutron-rich matter in sufficient quantities to explain the origin of
r-process elements such as gold and platinum. Later, with collaborators, he demonstrated decompressing neutron-star matter from both neutron star-black holes and neutron star-neutron star mergers would form a natural r-process that would match observed patterns. Mass ejection and r-process nucleosynthesis from decompression has been apparently observed in the aftermath of
GW170817, the first merger of two neutron stars detected by LIGO/VIRGO. The inferred r-process mass seems sufficient that neutron star mergers are likely the dominant source of these nuclides. Lattimer and collaborators also proposed that the recently observed rapid cooling of the neutron star in the
Cassiopeia A supernova remnant is the first direct evidence for
superfluidity and
superconductivity in neutron star interiors. He has collaborated extensively with
Madappa Prakash. ==Awards and honors==