Number 223 (Story #1), April 24, 1995 by Phillip F. Schewe and Ben Stein MACHOs MAKE UP LESS THAN 20% OF THE PRESUMED DARK MATTER HALO shrouding our galaxy. The presence of massive compact halo objects (MACHOs), such as non- radiating neutron stars or white dwarfs and substellar objects such as planets, is invoked to partially explain the rapid rotation of the outer parts of the Milky Way. Several groups search for MACHOs by scanning stars in the central galactic bulge and in the overhead Large Magellanic Cloud (LMC). They look for instances of microlensing, a phenomenon in which the star's light is brightened by the gravitational focusing effect of a foreground MACHO. (On an extragalactic scale, large lensing effects have been observed in which a distant quasar's image is split in two by the gravity of foreground galaxies.) One of the groups, the "MACHO collaboration," has now finished its full analysis of lensing events. A representative of this group, Livermore physicist Kem Cook, reported at last week's American Physical Society meeting in Washington, DC that he and his colleagues were now convinced that the LMC events are indeed related to the influence of truly nonluminous objects. They therefore assert that these measurements constitute the first definitive observation of dark matter in our galaxy. Furthermore, they calculate that the mass of the MACHOs in the halo added up to about 7.6 x 10**10 solar masses and that the MACHO fraction of the dark halo was about 19%. Unlike the LMC events, the lensing events seen in the direction of the galactic bulge are probably caused by ordinary stars and not dark matter objects. Still, the bulge events are of interest partly because they may offer a way of looking for extra-solar planets. One observed lensing event entailed a double-cusped brightening, suggesting to the MACHO scientists that some lensing objects are binary systems; some of these might be star-planet systems.