An Etymological Dictionary of Astronomy and Astrophysics
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The dynamical motion of chemically bound atoms which constantly change their position with each other. The vibration of molecules is treated within → quantum theory. Therefore, the energy of molecular vibration can only take → discrete values. To a first approximation, molecular vibrations can be approximated as → simple harmonic oscillator assigned to each mode.

→ molecular; → vibration.

The sum of the atomic weights of all the atoms in a molecule.

→ molecular; → weight.

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 hydrogen molecule composed of three atoms in which one of the atoms is ionized. The molecular ion H₃⁺ plays a key role in the chemistry of gaseous → interstellar medium. It reacts efficiently with almost any neutral atom or molecule to initiate a network of ion-neutral reactions. It is responsible for producing molecules such as OH, CO, and H₂O. The first detection of H₃⁺ in the interstellar medium, via two closely spaced absorption lines (doublet) near 3.668 μm, were reported in two dense → molecular cloud cores along the lines of sight to the embedded → young stellar objects W33A and GL2136 (Geballe & Oka 1996). Since then H₃⁺ has been detected in several molecular clouds. It is believed that H₃⁺ forms via → cosmic ray → ionization of H₂ producing H₂⁺, which quickly reacts to another H₂ molecule to form H₃⁺ ( H₂ + CR → H₂⁺ and H₂⁺ + H₂→ H₃⁺ + H + 1.7 eV).

Tri-, → three; → atomic; → hydrogen; → molecular; → ion.