AstroGlossary - N
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nebula
A generic term for a fuzzy, diffuse astronomical object. Astronomers have observed four different types of nebulae: H II regions, reflection nebulae, planetary nebulae, and supernova remnants. See also: supernova remnantNeptune
The eighth planet from the Sun in the solar sSystem. Neptune orbits the Sun at an average distance of 30 AU. The planet has a mass 17.1 times the mass of Earth and a radius 3.9 times the radius of Earth. Neptune is a gas planet made of hydrogen, helium, and methane and has no solid surface. It has a small ring system and 13 known moons, the largest of which is Triton. neutrino
A subatomic particle with very small mass and zero charge, that rarely interacts with matter. Many quadrillions of neutrinos emanating from the core of the Sun pass through your body every second, but you'd never know it. Because of their ghostly properties, neutrinos are extremely difficult to capture and measure, but several super-sensitive neutrino detector arrays are now in operation around the world. Unlike optical telescopes, which can see only the surface of the Sun, neutrino telescopes might soon let us directly observe the solar core, so scientists can test their theories of thermonuclear fusion reactions. neutron
A subatomic particle with zero charge and rest mass of 1.6749286 x 10^-27 kilograms. Neutrons, like protons, are found in atomic nuclei. By themselves, neutrons are unstable and decay into a proton, electron, and antineutrino through the beta minus decay process. See also: neutron star
The crushed remnant of a star that exploded as a supernova. Stars that are born with about 8 to 20 times the mass of the Sun blast most of their material into interstellar space in titanic explosions, leaving only their crushed, dense cores. Neutron stars are named after their composition: neutrons. In a star with a core that is 1.4 to 3 times the mass of the Sun, the core collapses so completely that electrons and protons combine to form neutrons. A full bathtub of neutron-star material (instead of water) would weigh as much as two Mount Everests. A neutron star is about 10-15 miles (16-24 km) in diameter, with a liquid neutron core and a crust of solid iron. Some neutron stars, called pulsars, spin rapidly (from once a second to several hundred times per second) and generate powerful magnetic fields. See also: supernova, supernova remnantNewton's Laws
Newton's First Law: Objects in motion stay in motion, and objects at rest stay at rest, unless acted upon by an outside force. Newton's Second Law: For every action, there is an equal and opposite reaction. Newton's Third Law: The force exerted on an object and the acceleration of the object are directly proportional. The constant of proportionality is the mass of the object. Newton's Law of Gravity:The gravitational force exerted on one object by another object is proportional to the product of the masses of the two objects and inversely proportional to the square of the distance between them. The constant of proportionality is the Universal Gravitational Constant, G. See also: Newton, Isaac
A British mathematician and physicist who lived from 1643 - 1727. Newton made major advances in the studies of optics, mathematics, astronomy, and physics, most of which were published in his two major works, Opticks and Principia. Among other innovations, he constructed the first reflecting telescope (now known as a Newtonian style telescope), invented integral calculus, and postulated a Universal Law of Gravity. See also: Newtonian focusNewtonian focus
A style of focus for reflecting telescopes for which the focal point lies to the side of the telescope. Light enters the telescope and is reflected from the primary mirror to a secondary "pickoff" mirror in the middle of the telescope tube. The secondary mirror directs light out the side of the telescope. Newtonian focus telescopes, also known as Newtonian telescopes, are popular for telescopes with eyepieces, because they provide for convenient viewing positions. See also: nova
An explosion from the surface of a white-dwarf star in a binary star system. A nova occurs when the white dwarf, which is the dense core of a once-normal star, "steals" gas from its nearby companion star. When enough gas builts up on the surface of the white dwarf it triggers an explosion. For a brief time, the system can shine up to a million times brighter than normal. As long as it continues to take gas from its companion star, the white dwarf can produce nova outbursts at regular intervals. nuclear fusion
A reaction that involves two atomic nuclei merging to create the nucleus of a new atom. The reaction is accompanied by a tremendous release of energy. The Sun's energy, as with all stars, comes from nuclear fusion reactions that happen in its core. The reactions fuse hydrogen nuclei into helium nuclei. About 0.7 percent of the mass of the hydrogen is converted to energy, which makes the star shine.
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