An Etymological Dictionary of Astronomy and Astrophysics
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A metallic chemical element; symbol Ca. → Atomic number 20; → atomic weight 40.08; → melting point about 839°C; → boiling point 1,484°C; → specific gravity 1.55 at 20°C; → valence +2. It is fifth in abundance in the Earth's crust, of which it forms more than 3%. It is an essential constituent of leaves, bones, teeth, and shells. Never found in nature uncombined, it occurs abundantly as limestone, gypsum, and fluorite. Calcium has several radioactive isotopes. It was first isolated by the British chemist Humphry Davy in 1808.

Coined by Sir Humphry Davy from L. calx (genitive calcis) "lime (CaO) or limestone (CaCO₃)" in which it was found, from Gk. khalix "small pebble," see also → calculate, + → -ium.

A discontinuity in the spectrum of galaxies near the Ca II → H and K lines at about 4000 Å. The Ca break is the most prominent feature in the spectra of elliptical galaxies. Its strength is given by the → calcium break index.

→ calcium; → break.

The strength of the → calcium break, as measured from the fluxes in the intervals 3750-3950 Å and 4050-4250 Å. It is given by the expression Ca-break[%] = 100 · (f_upper - f_lower)/f_upper, where f_upper and f_lower are the mean fluxes measured in the 3750-3950 Å and 4050-4250 Å bands, respectively, in the rest frame (Dressler & Shectman 1987, AJ 94, 899).

→ calcium; → break; → index.