An Etymological Dictionary of Astronomy and Astrophysics
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A giant cloud, or complex of clouds, of interstellar gas and dust associated with the Orion nebula (M42). It is about 1,500 light-years away and measures about 240 light-years across. Besides M42 and M43 it contains a number of famous objects, including Barnard's Loop, the Horsehead Nebula, and the reflection nebulae around M78. Within this cloud, stars have formed recently, and are still in the process of formation.

→ Orion; → molecular cloud.

The best known → ionized nebula and one of the nearest regions to the Sun in which stars are presently being formed. It is visible to the naked eye in the constellation → Orion south of → Orion's Belt as a fuzzy patch. It lies about 1,500 → light-years away and measures about 30 light-years across. The Orion Nebula is ionized and made visible by a small group of → O-type and → B-type stars known as the → Trapezium cluster. Other designations: M 42, NGC 1976. See also: → Orion molecular cloud; → Huygens Region ; → Orion association; → Orion Bar; → Orion Bright Bar.

→ Orion; → nebula.

An → OB association consisting of several dozen → hot stars of → spectral types O and B (→ O star, → B star). The Orion OB1 association consists of several subgroups, first divided by Blaauw (1964) into four subgroups. The subgroups differ in age and gas and dust content:
Orin OB1a, which contains the stars to the northwest of → Orion's Belt stars. Within this group lies another subgroup, known as the → 25 Orionis group;
Orion OB1b, containing the group of stars located in and around the Belt;
Orion OB1c, including the stars around → Orion's Sword; and
Orion OB1d, which contains the stars in and close to the → Orion Nebula (including the → Trapezium cluster).

→ Orion; → OB association.

Same as → Orion Arm.

→ Orion; → → spur.

Three prominent stars in the central regions of the constellation → Orion that align to form the "belt" of the mythological Hunter. They are → Alnitak (ζ Ori), → Alnilam (ε Ori), and → Mintaka (δ Ori). The easternmost star Alnitak is separated from the middle one, Alnilam, by 1°.36, and the westernmost Mintaka has an angular distance of 1°.23 from Alnilam. Their distance is between 800 and 1,300 → light-years from Earth. They probably formed inside the same → molecular cloud less than 10 million years ago.

→ Orion; → belt.

An astronomical → asterism in the constellation → Orion forming an almost vertical line beneath → Orion's Belt. From north to south, the most prominent objects in the Sword are the cluster NGC 1981, the star → 42 Orionis, the famous → Orion Nebula, and the Sword's brightest star → Iota Orionis (→ Hatsya). None of these objects is particularly bright in itself, but their proximity to one another and the nebulosity across much of this region makes the Sword stand out clearly in the night sky.

→ Orion; → sword.

Same as → Orion Arm.

→ Orion; → Cygnus; → arm.

A meteor shower that appears to emanate from the constellation → Orion. It peaks between October 14 and 20 with about 20 meteors per hour.

→ Orion + → -ides.

A set of functions, any two of which, by analogy to orthogonal vectors, vanish if their product is summed by integration over a specified interval.

→ orthogonal; → function.

The state of any quantity when the value of that quantity is continually changing so that it passes through maximum and minimum values.

Verbal noun of → oscillate.

Same as → pulsation mode.

→ oscillation; → mode.

The contact between two osculating curves or the like.

Verbal noun of → osculate

An approximate empirical criterion for the stability of a → galactic disk against its collapse to form a bar. The disk is stable if the following relation holds: T/|W| < 0.14, where T is the rotational → kinetic energy and |W| is the absolute value of the gravitational → potential energy. While the → Toomre criterion applies only to small linear perturbations, the Ostriker-Peebles criterion describes global modes.

Ostriker & Peebles, 1973, ApJ 186, 467; → criterion.

The combination of → oxygen with a substance, or the removal of → hydrogen from it.

Verbal noun of → oxidize.

The total number of electrons that an atom either gains or loses in order to form a chemical bond with another atom. In other words, the charge that atom would have if the compound was composed of ions. The oxidation number of an atom is zero in a neutral substance that contains atoms of only one element. Same as → oxidation state.

→ oxidation; → number.

Same as → oxidation number.

→ oxidation; → state.

The difference between the isotopic mass of a nuclide and its mass number divided by its mass number. The packing fraction is a measure of the stability of the nucleus.

Packing, from the verb pack "to put together in a pack," from the noun pack, from M.E. pak, packe, from M.D. pac or perhaps M.L.G. pak; → fraction.

Barxé, → fraction; anbâštegi quality noun of anbâštan, anbârdan "to fill, to replete;" Mid.Pers. hambāridan "to fill;" from Proto-Iranian *ham-par-, from prefix ham- + par- "to fill;" cf. Av. par- "to fill," parav-, pauru-, pouru- "full, much, many;" O.Pers. paru- "much, many;" Mid.Pers. purr "full;" Mod.Pers. por "full, much, very;" PIE base *pelu- "full," from *pel- "to be full;" cf. Skt. puru- "much, abundant;" Gk. polus "many," plethos "great number, multitude;" O.E. full.

Mutual destruction of a particle and its antiparticle, such as an electron-positron pair, when they collide. The total energy of the two particles is converted into energy as gamma rays. It is the inverse process to → pair production.

→ pair; → annihilation

The creation of an → elementary particle and its → antiparticle from a → boson. For example, the formation of an → electron and a → positron in the interaction of high-energy → gamma ray photons, having at least 1.02 Mev, with an → atomic nucleus (γ → e^- + e⁺). The → rest masses of the electron and positron being 0.51 MeV each, the excess energy will be carried away by these two particles. Pair production is the inverse process to → pair annihilation.

→ pair; → production

The proper motion of a star due to the effect of the Sun's motion relative to the → local standard of rest.

→ parallactic; → motion.