An Etymological Dictionary of Astronomy and Astrophysics

1. A Hydroxyl radical formed by abstraction of a hydrogen atom from water.
   
2. An OH group within a molecule.

See also: From → hydro- + ox(y)- a combining form meaning “sharp, acute, pointed, acid,” used in the formation of compound words, from Gk, oxys “sharp, keen, acid” + -yl a suffix used in the names of chemical radicals, from Fr. -yle, from Gk. hyle “matter, substance.”

→ Calabash Nebula.

Emission or absorption lines on an electromagnetic spectrum generated by hydroxyl, → OH molecules. At present, four principal lines are known in the radio domain at frequencies of 1612, 1665, 1667, and 1720 MHz, or wavelengths of approximately 18 centimeters.

Etymology (EN): → OH; → line.

A → maser phenomenon created by → OH molecules with characteristic → OH lines. OH masers are detected toward a variety of astronomical environments, including massive star formation regions and evolved late-type stars.

Etymology (EN): → OH; → maser.

An astronomical source emitting microwave radiation characteristic of the hydroxyl OH molecule, especially one showing a maser effect. OH sources are found in molecular clouds in interstellar medium and in the cool envelopes of evolved stars.

See also: OH, chemical compound hydroxyl; → source.

An evolved Mira-type star which is associated with strong OH maser and strong infrared (IR) emission from the surrounding shell of warm gas and dust.

Etymology (EN): → OH; → infrared; → star.

A unit of electrical resistance equal to that of a conductor in which a current of one ampere is produced by a potential of one volt across its terminals.

See also: Named after Georg Simon Ohm (1789-1854), the German physicist who discovered the law which bears his name.

1. For a → conductor at rest, the → voltage across the conductor is equal to the product of the current flowing through it and its → resistance. In other words, when such a conductor is subjected to an electric field E, the resulting → current density, J, is proportional to the electric field E: J = σE, where σ is the → conductivity, i.e. the reciprocal of → resistivity, ρ = 1/σ.
   

2. Ohm’s law for a moving conductor is expressed by:

J = σ(E + v x B), where v is the velocity and B
the → magnetic induction.

See also: → ohm; → law.

Of or relating to a system which obeys Ohm’s law.

See also: → ohm + → -ic.

An upper bound on the time scale on which the magnetic field of a system would decay in the absence of any other agent. It is expressed as: τ_μ = R² / μ, where R is the scale size of the system, η the magnetic diffusivity (η = 1 / μσ, where μ is the magnetic permeability and σ the electrical conductivity). For a star like the Sun, τ_μ  ≅ 10¹⁰ years, so a fossil magnetic field could survive for the star’s lifetime on the main sequence. For the Earth, τ_μ  ≅ 10⁴ years, so a → dynamo is required to explain the persistence of the geomagnetic field.

See also: → Ohmic; → decay; → time.

1. A loss of electric energy due to conversion into heat when a current flows through a resistance. Same as Ohmic loss.
   

2. In plasma physics, the energy released by charged particles as they make collisions with other particles.

See also: → Ohmic; → dissipation.

Same as → Ohmic dissipation.

See also: → Ohmic; → loss.