sourceatmospheric science

biblical source and one of four that, according to the documentary hypothesis, comprise the original literary constituents of the Pentateuch, the first five books of the Bible. It is so called because of its use of the Hebrew term Elohim for God, and hence labelled E, in contrast with another discerned source that uses the term YHWH and is labelled J (after the German transliteration of YHWH). See also Bible.

social movement, begun by computer programmers, that rejects secrecy and centralized control of creative work in favour of decentralization, transparency, and unrestricted (“open”) sharing of information. Source refers to the human-readable source code of computer programs, as opposed to the compiled computer programming language instructions, or object code, that run on computers but cannot be easily understood or modified by people.

In closed-source, or proprietary, software development, only the object code is published; the source code is held secret in order to control customers and markets. Open-source projects reject this practice and publish all their source code on the Internet under licenses that allow free redistribution. An important feature of open-source development is that the resulting extensive peer review seems to do a better job of minimizing computer bugs and computer security risks than the typical in-house process of quality assurance at closed-source vendors.

Beyond computer software, the concept of open source has been used to create free online databases and by commercial Internet vendors to populate reviews of items for sale, such as books, music, and movies.

in astronomy, any of a class of cosmic objects that emit radiation at X-ray wavelength. Because the Earth’s atmosphere absorbs X rays very efficiently, X-ray telescopes and detectors must be carried high above it by spacecraft to observe objects that produce such electromagnetic radiation.

A brief treatment of astronomical X-ray sources follows. For full treatment, see Cosmos.

Advances in instrumentation and improved observational techniques have led to the discovery of an increasing number of X-ray sources. By the late 20th century, thousands of these objects had been detected throughout the universe.

The Sun was the first celestial object determined to give off X rays; rocket-borne radiation counters measured X-ray emissions from its corona (outer atmosphere) in 1949. The Sun, however, is an intrinsically weak X-ray source, and it is prominent only because it is so close to the Earth. The unambiguous detection of X rays from other more distant ordinary stars was achieved 30 years later by the orbiting HEAO 2 satellite known as the Einstein Observatory. It detected more than 150 ordinary stars by the X-radiation from their coronas. The stars observed cover almost the entire range of star-types—main sequence, red giants, and white dwarfs. Most stars emit only an extremely small fraction of their energy in the form of X rays. Young, massive stars are the most powerful X-ray emitters. They usually occur in nebulas, and their hot coronal gases can expand to make a nebula itself a detectable X-ray source.

A more powerful type of X-ray source is a supernova remnant, the gaseous shell ejected during the violent explosion of a dying star. The first to be observed was the Crab Nebula, the remnant of a supernova explosion whose radiation reached the Earth in ad 1054. It is, however, a very atypical remnant because its X...