The point on the celestial sphere directly below the observer. A cloud of gas and dust in space. A fundamental particle with little or no mass and no electric charge. Neutron A fundamental particle with no electric charge, but a mass practically equal to that of a proton . The remnant of a very massive star which has exploded as a supernova .
Neutron stars send out rapidly-varying radio emissions, and are therefore called pulsars .
Nova A star which suddenly flares up to many times its normal brilliancy, remaining bright for a relatively short time before fading back to obscurity. Nutation A slow, slight nodding of the Earth s axis, due to the gravitational pull of the Moon on the Earth s equatorial bulge.