MACHO Searches Find Most Candidates in Unexpected Places

For two years now, three groups of astronomers and refugees from particle physics have been measuring millions of stars night after night in a stellar photometry campaign of unprecedented scale. But in fact they're not really looking for stars at all. What they seek are dark “massive compact halo objects” at least as massive as planets but generally less than a tenth the mass of the Sun. These substellar MACHOs, as they are called, would not be massive enough to ignite the fusion that makes stars shine. Heavier bodies, like neutron stars and old white dwarfs, that are dark because their stellar fuel has run out are also included under the MACHO rubric.