Massive Compact Halo Object - Types of MACHOs

Types of MACHOs

MACHOs may sometimes be considered to include black holes. Black holes are truly black in that they emit no light and any light shone upon them is absorbed and not reflected. It is thought possible that there is a halo of black holes surrounding the galaxy. A black hole can sometimes be detected by the halo of bright gas and dust that forms around it as an accretion disc being pulled in by the black hole's gravity. Such a disk can generate jets of gas that are shot out away from the black hole because it cannot be absorbed quickly enough. An isolated black hole, however, would not have an accretion disk and would only be detectable by gravitational lensing. Cosmologists doubt they make up a majority of dark matter because the black holes are at isolated points of the galaxy. The largest contributor to the missing mass must be spread throughout the galaxy to balance the gravity. A minority of physicists, including Chapline and Laughlin, believe that the widely accepted model of the black hole is wrong and needs to be replaced by a new model, the dark energy star; in the general case for the suggested new model, the cosmological distribution of dark energy would be slightly lumpy and dark energy stars of primordial type might be a possible candidate for MACHOs.

Neutron stars are somewhat like black holes, but are not heavy enough to collapse completely, instead forming into a material rather like that of an atomic nucleus (sometimes informally called neutronium). After sufficient time these stars could radiate away enough energy to become cold enough that they would be too faint to see. Likewise, old white dwarfs may also become cold and dead, eventually becoming black dwarfs, although the universe is not thought to be old enough for any stars to have reached this stage.

The next candidate for MACHOs are the brown dwarfs mentioned above. Brown dwarfs are sometimes called "failed stars" as they do not have enough mass for nuclear fusion to begin and simply glow a dull brown. Hence, their only source of energy is released through their own gravitational contraction, and may therefore be faintly visible in some circumstances. Most brown dwarfs are about thirteen to seventy-five times the mass of Jupiter.