An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

A structure that protects against excessive heat, especially that which covers the vulnerable surfaces of a → spacecraft and protects it when re-entering the Earth's atmosphere.

→ heat; → shield.

The ozone layer within the stratosphere that filters out potentially lethal intensities of ultraviolet radiation from the Sun.

→ ozone; shield, from O.E. scield, scild, related to sciell "seashell, eggshell," from P.Gmc. *skeldus (cf. Du. schild, Ger. Schild, Goth. skildus); PIE base *(s)kel- "to cut."

Separ "shield," from Mid.Pers. spar "shield;" cf. Skt. phalaka- "board, lath, leaf, shield," phálati "(he) splits;" Gk. aspalon "skin, hide," spolas "flayed skin," sphalassein "to cleave, to disrupt;" O.H.G. spaltan "to split;" Goth. spilda "board;" PIE base *(s)p(h)el- "to split, to break off;" → ozone.

The phenomenon whereby the → photodissociation transitions of a molecule in interstellar clouds become → optically thick, so that the molecule in question is "shielded" by other molecules against dissociating stellar → far-ultraviolet (FUV) photons. In the case of → molecular hydrogen (H₂), when the → column density exceeds 10¹⁴ cm^-2, the UV absorption bands become optically thick, and H₂ undergoes self-shielding. More specifically, all of the photons that could lead to UV photodissociation are absorbed by H₂ in the outer layers of the cloud, hence protecting the H₂ within the cloud. Self-shielding occurs in → diffuse interstellar clouds exposed to the interstellar → radiation field or in → molecular clouds in proximity to sources of UV photons. Dust can also absorb UV photons, further limiting the photodissociation, but it dominates only when the local UV radiation field is unusually intense relative to the density of the cloud.

→ self-; → shield; → -ing.

1) A broad piece of armor carried on the arm as a defense against swords, arrows, etc.
2) A person or thing that protects or defends. → heat shield; → ozone shield; → self-shielding; → shielding effect.

M.E. shelde, from O.E. scield, scild, related to sciell "seashell, eggshell;" cf. Du. schild, Ger. Schild, Goth. skildus; PIE base *(s)kel- "to cut."

Separ "shield," from Mid.Pers. spar "shield;" cf. Skt. phalaka- "board, lath, leaf, shield," phálati "(he) splits;" Gk. aspalon "skin, hide," spolas "flayed skin," sphalassein "to cleave, to disrupt;" O.H.G. spaltan "to split;" Goth. spilda "board;" PIE base *(s)p(h)el- "to split, to break off."

The decrease in attraction between an electron and the nucleus in any atom with more than one → electron shell. The repulsion forces from other electrons in shells cause the net force on electrons in outer shells to be significantly smaller in magnitude. Also known as → screening effect.

→ shield; → effect;