Oort limit

The mass density in the plane of the Galaxy near the Sun's locality, as calculated from the velocities and distribution of stars in relation to the gravitational field of the galactic disc. At one time it was believed that the mass density deduced in this way exceeded the amount that could be seen directly in the form of stars and interstellar material. However, the discovery of fainter stars and more interstellar material has brought the observed and calculated values of local mass density closer together, implying little, if any, "hidden" matter in the solar neighbourhood. The name is that of the Dutch astronomer, Jan Oort, distinguished for his work in the field of galactic dynamics.