An Etymological Dictionary of Astronomy and Astrophysics
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A neutron with an energy between 1 eV and 0.01 MeV.

→ resonance; → region; → neutron.

An extent of wavelengths into which the electromagnetic spectrum is divided; e.g. infrared or ultraviolet region.

→ spectral; → region.

A region in the → interstellar medium where processes of → star formation are going on or have occurred in the past.

→ star; → formation; → region.

A region in which → star formation is going on.

→ star; → formation; → region.

A very young → H II region fully embedded in its natal molecular cloud. Ultracompact H II regions are distinguished from classical H II regions by their small sizes (diameter ≥ 0.1 pc), high densities (Ne ≥ 10⁵ cm^-3), and high emission measures (EM ≥ 10⁷ pc cm^-6). Their typical ionized gas content is about 10^-2  → solar masses, in contrast to classical H II regions with a mass of about 10⁵ solar masses. Due to very important extinction, ultracompact H II regions are not accessible to visible wavelengths.

→ ultra-; → compact; → H II region.

A region of → molecular clouds whose chemistry is affected by → X-rays from neighboring sources.

→ X-ray; → dissociation; → region.