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diffraction

(Encyclopedia)diffraction, bending of waves around the edge of an obstacle. When light strikes an opaque body, for instance, a shadow forms on the side of the body that is shielded from the light source. Ordinarily...

promethium

(Encyclopedia)promethium prōmēˈthēəm [key], artificially produced radioactive chemical element; symbol Pm; at. no. 61; mass no. of most stable isotope 145; m.p. 1,042℃; b.p. 3,000℃ (estimated); sp. gr. unk...

hydrogen bomb

(Encyclopedia)hydrogen bomb or H-bomb, weapon deriving a large portion of its energy from the nuclear fusion of hydrogen isotopes. In an atomic bomb, uranium or plutonium is split into lighter elements that togethe...

stellar populations

(Encyclopedia)stellar populations, two broadly contrasting distributions of star types that are characteristic of different parts of a galaxy. Population I stars are young, recently formed stars, whereas population...

Jeter, Derek Sanderson

(Encyclopedia)Jeter, Derek Sanderson, 1974–, American baseball player, b. Pequannock, N.J. Drafted by the New York Yankees' in 1992, he played in the minors until he was first called up in 1995. His first full se...

Mattis, James

(Encyclopedia)Mattis, James, 1950–, American general and secretary of defense (2017–18), b. Pullman, Wash., grad. Central Washington Univ. (1971). Commissioned as a second lieutenant (1972) in the Marines after...

Heinlein, Robert Anson MacDonald

(Encyclopedia)Heinlein, Robert Anson MacDonald hīˈlīn [key], 1907–88, American science-fiction writer, b. Butler, Mo. His best-known novel, Stranger in a Strange Land (1961), concerns a young man who is raised...

Frederick, city, United States

(Encyclopedia)Frederick, city (2020 pop. 78,171), seat of Frederick co., NW Md.; settled 1745, inc. 1817. The processing center of a fertile farm and dairying area, i...

Gill, Sir David

(Encyclopedia)Gill, Sir David gĭl [key], 1843–1914, Scottish astronomer, educated at the Univ. of Aberdeen. He made observations of the transits of Venus and Mars and investigated the solar parallax. As astronom...

Fort McHenry

(Encyclopedia)Fort McHenry, former U.S. military post in Baltimore harbor; built 1794–1805. In the War of 1812 it was bombarded (Sept. 13–14, 1814) by a British fleet under Sir Alexander Cochrane, but the fort,...