An Etymological Dictionary of Astronomy and Astrophysics
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Describing a quantity of substance that is proportional to its molecular weight. Relating to a → mole.

→ mole + → -ar.

Of a gas included in the composition of a → gas mixture, the ratio of the number of moles of this gas to the total number of moles of all the gases in the mixture. Same as mole fraction and mole-fraction concentration.

→ molar; → concentration.

The → heat capacity of one → mole of substance: C_μ = μ C, where μ is the → molecular weight and C the → specific heat capacity. The molar heat capacity of water is practically 18 cal/mole.C°.

Molar, adj. of → mole; → heat; → capacity.

The amount of heat energy required to vaporize 1 mole of a liquid at its → boiling point, usually expressed in kJ/mol.

→ molar; → heat; → vaporization.

The → SI unit of amount of → substance; symbol mol. One mole contains exactly 6.022 140 76 × 10²³ elementary entities. This number is the fixed numerical value of the → Avogadro constant, N_A, when expressed in the unit mol^-1.

From Ger. Mole, short for Molekül, from Fr. → molecule.

Of or pertaining to or caused by molecules.

From → molecule + -ar variant of the adjective-forming suffix → -al, joined to words in which an l precedes the suffix.

Molekuli, from molekul→ molecule + -i adj. suffix.

A band of molecular origin present in a spectrum. See for example → cyanogen band, → S star.

→ molecular; → band.

A relatively dense, cold region of interstellar matter where the atoms are primarily bound together as molecules rather than free atoms or ionized particles. Molecular clouds represent the coldest and densest phase of the → interstellar medium. They consist primarily of → molecular hydrogen (H₂), with temperatures in the range 10-100 K. Molecular hydrogen is not directly observable under most conditions in molecular clouds. Therefore, almost all current knowledge about the properties of molecular clouds has been deduced from observations of molecules such as → carbon monoxide (CO), which have strong emission lines mainly in the → millimeter portion of the → electromagnetic spectrum. So far 129 molecular species have been detected in molecular clouds, among which complex organic molecules. → Dust grains in molecular clouds play a crucial role in the formation of molecules. Molecular clouds are the principal sites where stars form. → giant molecular cloud; → Orion molecular cloud.

→ molecular; → cloud.

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 hypothetical, large molecular sphere, as a component of the → stellar atmosphere, suggested to encompass → red giant and → supergiant stars. This suggestion has offered satisfactory explanations to the spectrum of → Mu Cephei (Tsuji 2003), but it is not clear whether MOLspheres are common features of the atmospheres of all such stars.

MOL, from → molecular; → sphere.