An Etymological Dictionary of Astronomy and Astrophysics
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One of many compact and dense sub-structures in a → molecular cloud. Their typical sizes are about 0.5-10 → parsecs and their masses range from about 50 to 10³ → solar masses. The gas temperatures are about 10-20 K, and the number densities from about 10³ to 10⁴ cm^-3.

→ molecular; → clump.

A mixing process in a → fluid caused by the → random relative motions due to → Brownian motion of the individual particles. See also → eddy diffusion.

→ molecular; → diffusion.

An → electromagnetic radiation emitted by → interstellar molecules through → transitions between → energy states of → molecules.

→ molecular; → emission.

The formula of a chemical compound, showing the kind and arrangement of atoms.

→ molecular; → formula.

A molecule consisting of two hydrogen atoms (H₂) which is the most abundant molecule in the Universe. Molecular hydrogen plays a fundamental role in many astrophysical contexts. It is found in all regions where → self-shielding against the ultraviolet photons, responsible for its → photodissociation, is sufficiently large. Containing two identical hydrogen atoms, H₂ is highly symmetric. Due to this property, the molecule has no → dipole moment and all → rotation-vibrational transitions within the electronic → ground state are → quadrupolar with low → spontaneous emission  → Einstein coefficient values. The molecule exists in two almost independent states, namely → orthohydrogen and → parahydrogen. H₂ may be excited through several mechanisms, including: 1) → far ultraviolet (FUV) induced → optical pumping and → collisional excitation in → photodissociation regions (PDRs) associated with → star formation; 2) → hard X-rays penetrating and heating regions within → molecular clouds, which in turn excite H₂ via collisions with electrons or hydrogen atoms; and 3) collisional excitation of H₂ due to acceleration produced by interstellar → shock waves. H₂ is thought to be chiefly produced via surface reactions on → interstellar grains, but the exact formation mechanism is not fully understood.

→ molecular; → hydrogen.

An outflow of molecular material, often → bipolar, observed in the regions of → star formation. Molecular outflows are thought to be driven by → bipolar jets from → protostars. They are probably → bow shocks which have had time to cool and be observable in molecular lines. Molecular outflows are poorly → collimated compared to the jets and tend to be slow moving (velocities 10-20 km s^-1). Some bipolar outflows may be driven by → stellar winds.

→ molecular; → outflow.

The ability of a molecular entity to be distorted from its normal shape by an external → electric field. When a molecule is subjected to an electric field there is a small displacement of electrical centers which induces a dipole in the molecule. More specifically, the molecular polarizability α is defined as the ratio of the induced → dipole moment (p) to the local electric field (E) that produces this dipole moment: α = p/E (in cgse units).

→ molecular; → polaizable; → -ity.

In → propositional logic, a → sentence containing at least one → connectives. See also → atomic proposition.

→ atomic; → proposition.

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.

The smallest unit of a chemical compound. A molecule consist of two or more atoms held together by covalent bonds.

From Fr. moléclue, from Mod.L.molecula, diminutive of L. moles "mass, massive structure, barrier;" → -ula

A molecule that is normally found in or produced by living systems. Organic molecules typically consist of carbon atoms in rings or long chains, where other atoms (e.g. hydrogen, oxygen, and nitrogen) are attached, except for → carbon monoxide (CO) and → carbon dioxide (CO₂), etc.

→ organic; → molecule.

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 molecule initially produced when a comet nucleus sublimates, soon changed to different daughter molecules because of solar radiation.

→ parent; → molecule.

A molecule in which the centers of positive and negative charge distribution do not converge and therefore has a mostly positive charge on one side and a mostly negative charge on the other. Different atoms around a central atom will always be polar molecules. Some polar molecules are H₂O, HF, COS, and CH₃Cl. Polar molecules are characterized by a → dipole moment.

→ polar; → molecule.

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.

The chemical combination of one → oxygen (O) atom bonded to two → hydrogen (H) atoms. The bonding between the oxygen atom and each hydrogen atom is known as → covalent bonds. The two hydrogen atoms are bonded to the oxygen atom at a 105° angle. This geometry of the water molecule causes it to have positively and negatively changed ends, known as → polarity. Water is referred to a polar or dipolar molecule. The large nucleus of the oxygen atom attracts the shared electrons causing this side of the water molecule to be negatively charged while the hydrogen side is positively charged. This polarity allows water to bond easily with adjacent water molecules.

→ water; → molecule.