An Etymological Dictionary of Astronomy and Astrophysics
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An obsolete term denoting a negatively charged electron, as opposed to a positron.

From → negative + → electron.

A colorless, odorless, and tasteless gaseous chemical element; symbol Ne. Atomic number 10, atomic weight 20.179; melting point -248.67°C; boiling point -246.048°C. It was discovered from its bright red spectral lines by the Scottish chemist William Ramsay and the English chemist Morris William Travers in 1898 from a liquefied air sample. Neon is produced by carbon burning in massive stars and released into the Galaxy when they explode.

From Gk. neon neuter of neos, → new, so called because it was a newly discovered element.

A → nuclear fusion process that takes place in → massive stars and leads to the → production of → oxygen and → magnesium. It requires high temperatures and densities (around 1.2 × 10⁹ K and 4 × 10⁹ kg/m³).

→ neon; → burning.

In computer programing, a function that is defined inside the definition of another function.

→ nested; → function.

A method in the evaluation of polynomials which involves fewer basic operations and allows simpler computation, especially for polynomials of high degree. More specifically, the polynomial P(x) = a₀ + a₁x + a₂x² + a₃x³ + ... + a_nxⁿ can be written in the nested form as: P(x) = a₀ + x(a₁ + x(a₂ + ... + x(a_{n - 1} + a_nx) ...)). For example, the polynomial P(x) = x³ - 5x² + 10x - 3 has the following nested form: P(x) = ((x - 5)x + 10)x - 3. Same as the → Ruffini-Horner method.

→ nested; → multiplication.

In optics, the process of combining two lenses having equal and opposite powers to produce a result having no power.

Verbal noun of → neutralize.

The transition between neutrino types (→ neutrino flavor) which is a probabilistic consequence of → quantum mechanics. A neutrino, when produced, is in a quantum state which has three different masses. Therefore, an electron neutrino emitted during a reaction can be detected as a muon or tau neutrino. In other words, the flavor eigenstates are different from the propagation eigenstates. This phenomenon was discovered in → solar neutrinos as well as in → atmospheric neutrinos. Neutrino oscillation violates the conservation of the → lepton number; it is possible only if neutrinos have a mass. First predicted by Bruno Pontecorvo in 1957, neutrino oscillation has since been observed by several experiments. It resolved the long-standing → solar neutrino problem. The smaller the mass difference between the flavors, the longer the oscillation period, so that oscillations would not occur if all of the flavors were equal in mass or were massless. Moreover, the oscillation period increases with neutrino energy.

→ neutrino; → oscillation.

An uncharged → subatomic particle found in the nucleus of every → atom heavier than → hydrogen. It has a → rest mass of 1.67492 x 10^-24 g, 939.566 → MeV, slightly greater than that of the → proton. The neutron is composed of three → quarks (two down and one up). Although the neutron is electrically neutral, it owns a → spin of 1/2 and a → magnetic moment; it can therefore interact magnetically with matter. A free neutron is unstable and disintegrates by → beta decay to a proton, an → electron, and → antineutrino of the electron type: n→ p + e^- + ν^__e + 0.7823 MeV. Its → mean life is about 15 minutes. The decay of the neutron is associated with a → quark transformation in which a down quark is converted to an up by the → weak interaction.

From neutro-, a combining form representing → neutral, + → -on a suffix used in the names of → subatomic particles.

The → nuclear reaction that occurs when an → atomic nucleus captures a → neutron. Neutron capture is the primary mechanism (principally, the → s-process and → r-process) by which very massive nuclei are formed in stars and during → supernova explosions. Instead of → fusion of similar nuclei, heavy, → neutron-capture elements are created by the addition of more and more neutrons to existing nuclei.

→ neutron; → capture.

The state of degeneracy created when the density of matter is so high that neutrons cannot be packed any more closely together. This condition occurs in the core of stars above 1.44 solar masses (→ Chandrasekhar limit) where under the gravitational collapse electrons and protons are forced to combine into neutrons. Therefore, in a → neutron star all the lowest neutron energy levels are filled and the neutrons are forced into higher and higher energy levels, since according to Pauli Exclusion Principle no two neutrons (fermions) can occupy identical states. This creates an effective pressure which prevents further gravitational collapse. However, for masses greater than 3 solar masses, even neutron degeneracy cannot prevent further collapse and it continues toward the black hole state.

→ neutron; → degeneracy.

A type of radioactive decay of atoms containing excess neutrons, in which a neutron is ejected from the nucleus.

→ neutron; → emission.

The excess of → neutrons over → protons in an → atomic nucleus: η = (N_n - N_p) / (N_n + N_p).

→ neutron; → excess.

An extremely compact ball of matter created from the central core of a star that has collapsed under gravity to such an extent that it consists almost entirely of → neutrons. Neutron stars result from two possible evolutionary scenarios: 1) The → collapse of a → massive star during a → supernova explosion; and 2) The accumulation of mass by a → white dwarf in a → binary system. The mass of a neutron star is the same as or larger than the → Chandrasekhar limit (1.4 → solar masses). Neutron stars are only about 10 km across and have a density of 10¹⁴ g cm^-3, representing the densest objects having a visible surface. The structure of neutron stars consists of a thin outer crust of about 1 km thickness composed of → degenerate electrons and nuclei, which becomes progressively neutron rich with increasing depth and pressure due to → inverse beta decays. In the main body the matter consists of → superfluid neutrons in equilibrium with their decay products, a few percent protons and electrons. Neutron stars have extremely strong magnetic fields, from 3 x 10¹⁰ to 10¹⁵ gauss. As of 2010 more than 2000 neutron stars have been catalogued, which show a large variety of manifestations, mainly → pulsars.

→ neutron; → star.

A → binary system composed of two → neutron stars.

→ neutron; → star; → binary; → system.

A → nucleosynthesis process responsible for the generation of the → chemical elements heavier than the → iron peak elements. There are two possibilities for → neutron capture: the slow neutron-capture process (the → s-process) and the rapid neutron-capture process (the → r-process). The s-process is further divided into two categories: the weak s-component and the main s-component. Massive stars are sites of the weak component of s-process nucleosynthesis, which is mainly responsible for the production of lighter neutron-capture elements (e.g. Sr, Y, and Zr). The s-process contribution to heavier neutron-capture elements (heavier than Ba) is due only to the main s-component. The low- to intermediate-mass stars (about 1.3-8 Msun) in the → asymptotic giant branch (AGB) are usually considered to be sites in which the main s-process occur. There is abundant evidence suggesting that → Type II supernova (SNe II) are sites for the synthesis of the r-process nuclei, although this has not yet been fully confirmed. The observations and analysis on → very metal-poor stars imply that the stars with [Fe/H] ≤ -2.5 might form from gas clouds polluted by a few supernovae (SNe). Therefore, the abundances of → heavy elements in → metal-poor stars have been used to learn about the nature of the nucleosynthetic processes in the early Galaxy (See, e.g., H. Li et al., 2013, arXiv:1301.6097).

→ neutron;→ capture; → element.

The reaction that transforms a → proton into a → neutron when a proton and an → electron are forced together to make a neutron: p + e^-→ n + ν^__e. In astronomy, this process occurs during the → core collapse of → massive stars which leads to the formation of → neutron stars.

→ neutron; → -ize; → -tion.

A space mission by → NASA whose main goal is to study the → dwarf planet Pluto and it satellites. New Horizons was launched on January 19, 2006; it swung past → Jupiter for a → gravity assist and scientific studies in February 2007, and conducted a six-month-long reconnaissance → flyby study of → Pluto and its moons in summer 2015, culminating with Pluto closest approach on July 14, 2015. It flew 12,500 km above the surface of Pluto, making it the first spacecraft to explore the dwarf planet. Its science payload includes seven instruments: Ralph (visible and infrared imager/spectrometer), Alice (ultraviolet imaging spectrometer), REX (Radio Science EXperiment), LORRI (Long Range Reconnaissance Imager), SWAP (Solar Wind Around Pluto), PEPSSI: (Pluto Energetic Particle Spectrometer Science Investigation), and SDC: (Student Dust Counter). As part of an extended mission, New Horizons has maneuvered for a flyby of → Kuiper belt object 2014 MU69, expected to take place on January 1, 2019, when it is 43.4 → astronomical units (AU) from the Sun.

→ new; → horizon.

The Moon's phase when it is at the same celestial longitude as the Sun and thus totally un-illuminated as seen from Earth.

→ new; → moon.

The unit of force in the SI system of units. 1 newton (N) is defined as the force required to give a mass of 1 kilogram an acceleration of 1 m s^-2. 1 N = 10⁵  → dynes.

Named after Sir Isaac Newton (1642-1727), the English highly prominent physicist and mathematician.

Same as the → gravitational constant.

→ Newton; → constant.