ESO Astronomical Glossary - N
A nebula is a diffuse cloud of gas and dust in intergalactic space. Nebulae can broadly be classified into emission nebulae and reflection nebulae. In emission nebulae, the gas in the clouds is heated and ionised by nearby stars, causing it to 'glow'. Examples of such nebulae are HII regions and planetary nebulae. Reflection nebulae are only visible because the dust contained within them reflects light from nearby stars. Sometimes giant molecular clouds are referred to as 'dark nebulae'.
A neutron star is the remnant of a massive star that died in a supernova explosion, composed mostly of tightly-packed neutrons. Neutron stars measure just 10-20 km in diameter but have the same mass as the Sun. Thus, a pinhead of neutron star material (1 millimetre across) weighs almost 1 million tons, or about as much as the largest oil carrier ever built, fully loaded. Some neutron stars emit strong pulses of radiation as they rotate, because of their strong magnetic fields; they are called pulsars.
New General Catalogue (NGC)
The New General Catalog (NGC) is a list of over 13,000 deep-sky celestial objects, such as galaxies, globular clusters and nebulae. It was developed in 1888 by the astronomer J.L.E. Dreyer, and often celestial objects are referred to by their list number, e.g. the Great Nebula in Orion is NGC 1976 (and M42).
New Technological Telescope (NTT)
The 3.58-m New Technological Telescope (NTT) is located at La Silla. It was the first telescope equipped with a complete active control system of the primary and secondary systems, always ensuring an optimal image quality. Known as 'active optics', this system was developed by ESO and is used in all large telescopes today.
A nova is an explosion on the surface of a white dwarf that is accreting matter from a companion star (see cataclysmic variable). This causes the system to temporarily brighten by a factor of several hundred to several thousand; this brightening gives the impression a 'new' star is being born, hence the name 'nova'.