(Astronomie) Ugs. ein Himmelskörper, der dem bloßen Auge als punktförmige Lichtquelle erscheint, in der Astronomie eine selbstleuchtende Gaskugel wie unsere Sonne. Ein S. bildet sich als Verdichtung in der interstellaren Materie. Er wird durch ein Gleichgewicht zw. der Gravitation im Innern u. dem nach außen gerichteten Gasdruck sowie dem Strahlungsdruck stabil gehalten. S.e entwickeln sich zu Roten Riesen oder Überriesen, die ausbrennen. Dadurch entstehen Weiße Zwerge. Weitere Entwicklungsstadien sind Supernovae, Neutronen-S.e u. Schwarze Löcher.
ETYM Latin astronomicus, Greek astronomikos: cf. French astronomique.
Of or pertaining to astronomy; in accordance with the methods or principles of astronomy.
Konstruktionsvariante bei Netzwerken. Die Arbeitsstationen sind alle direkt an den Server angeschlossen, Server und Arbeitsstationen bilden so eine sternförmige Anordnung.
ETYM Latin asteriscus, Greek, dim. of aster star. Related to Aster.
A star-shaped character * used in printing; SYN. star.
'*' Starlike punctuation mark (*) used to link the asterisked word with a note at the bottom of a page, and in place of certain letters in a word (usually a taboo word).
ETYM Old Eng. diamaund, diamaunt, French diamant, corrupted, from Latin adamas, the hardest iron, steel, diamond. Related to Adamant, Tame.
1. Very hard native crystalline carbon valued as a gem; SYN. adamant.
2. A transparent piece of diamond that has been cut and polished and is valued as a precious gem.
Generally colorless, transparent mineral, an allotrope of carbon. It is regarded as a precious gemstone, and is the hardest substance known (10 on the Mohs' scale). Industrial diamonds, which may be natural or synthetic, are used for cutting, grinding, and polishing.
Diamond crystallizes in the cubic system as octahedral crystals, some with curved faces and striations. The high refractive index of 2.42 and the high dispersion of light, or “fire”, account for the spectral displays seen in polished diamonds.
Diamonds were known before 3000 BC and until their discovery in Brazil 1725, India was the principal source of supply. Present sources are Australia, Zaire, Botswana, Russia (Yakut), South Africa, Namibia, and Angola; the first two produce large volumes of industrial diamonds. Today, about 80% of the world's rough gem diamonds are sold through the De Beers Central Selling Organization in London.
Diamonds may be found as alluvial diamonds on or close to the Earth’s surface in riverbeds or dried watercourses; on the sea bottom (off SW Africa); or, more commonly, in diamond-bearing volcanic pipes composed of “blue ground”, kimberlite or lamproite, where the original matrix has penetrated the Earth’s crust from great depths. They are sorted from the residue of crushed ground by X-ray and other recovery methods.
There are four chief varieties of diamond: well-crystallized transparent stones, colorless or only slightly tinted, valued as gems; boart, poorly crystallized or inferior diamonds; balas, an industrial variety, extremely hard and tough; and carbonado, or industrial diamond, also called black diamond or carbon, which is opaque, black or gray, and very tough. Industrial diamonds are also produced synthetically from graphite. Some synthetic diamonds conduct heat 50% more efficiently than natural diamonds and are five times greater in strength. This is a great advantage in their use to disperse heat in electronic and telecommunication devices and in the production of laser components.
Because diamonds act as perfectly transparent windows and do not absorb infrared radiation, they were used aboard NASA space probes to Venus 1978. The tungsten-carbide tools used in steel mills are cut with industrial diamond tools.
Rough diamonds are often dull or greasy before being polished; around 50% are considered “cuttable” (all or part of the diamond may be set into jewelry). Gem diamonds are valued by weight (carat), cut (highlighting the stone’s optical properties), color, and clarity (on a scale from internally flawless to having a large inclusion clearly visible to the naked eye). They are sawn and polished using a mixture of oil and diamond powder. The two most popular cuts are the brilliant, for thicker stones, and the marquise, for shallower ones. India is the world’s chief cutting center.
Noted rough diamonds include the Cullinan, or Star of Africa (3,106 carats, over 500 g/17.5 oz before cutting, South Africa, 1905); Excelsior (995.2 carats, South Africa, 1893); and Star of Sierra Leone (968.9 carats, Yengema, 1972).
ETYM Old Eng. sterre, as. steorra.
Luminous globe of gas, mainly hydrogen and helium, which produces its own heat and light by nuclear reactions. Although stars shine for a very long time—many billions of years —they are not eternal, and have been found to change in appearance at different stages in their lives.
Stars are born when nebulae (giant clouds of dust and gas) contract under the influence of gravity. As each new star contracts, the temperature and pressure in its core rises. At about 10 millionşC the temperature is hot enough for a nuclear reaction to begin (the fusion of hydrogen nuclei to form helium nuclei); vast amounts of energy are released, contraction stops, and the star begins to shine. Stars at this stage are called main-sequence stars; the Sun is such a star and is expected to remain at this stage for the next 5 billion years. Their surface temperatures range from 2,000şC/3,600şF to above 30,000şC/54,000şF and the corresponding colors range from red to blue-white.
The smallest mass possible for a star is about 8% that of the Sun (80 times the mass of the planet Jupiter), otherwise nuclear reactions do not occur. Objects with less than this critical mass shine only dimly, and are termed brown dwarfs.
When all the hydrogen at the core of a main-sequence star has been converted into helium, the star swells to become a red giant, about 100 times its previous size and with a cooler, redder surface. When, after this brief stage, the star can produce no more nuclear energy, its outer layers drift off into space to form a planetary nebula, and its core collapses in on itself to form a small and very dense body called a white dwarf.
Eventually the white dwarf fades away, leaving a non-luminous dark body.
Some very large main-sequence stars do not end their lives as white dwarfs—they pass through their life cycle quickly, becoming red supergiants that eventually explode into brilliant supernovae. Part of the core remaining after such an explosion may collapse to form a small superdense star, which consists almost entirely of neutrons and is therefore called a neutron star. Neutron stars, called pulsars, spin very quickly, giving off pulses of radio waves (rather as a lighthouse gives off flashes of light). If the collapsing core of the supernova is very massive it does not form a neutron star; instead it forms a black hole, a region so dense that its gravity not only draws in all nearby matter but also all radiation, including its own light.
See also binary star, Hertzsprung–Russell diagram, and variable star.
1. (Astronomy) A celestial body of hot gases that radiates energy derived from thermonuclear reactions in the interior.
2. A plane figure with 5 or more points; often used as an emblem.
3. Any celestial body visible (as a point of light) from the Earth at night.
Isaac, 21.7.1920, US-amerik. Geiger russ. Herkunft; spielte u. a. mit P. Casals.
(1920-) Russian-born US violinist. He is both a fine concert soloist and chamber music player; his tone is warm and his style impeccable. He has premiered works by the US composers William Schuman and Leonard Bernstein.
Otto, 1888, 1969, dt.-amerik. Physiker; wies zus. mit W. Gerlach die auf dem Elektronenspin beruhende Aufspaltung eines Strahls von Silberatomen durch ein inhomogenes Magnetfeld in Strahlen versch. Richtung nach (Richtungsquantelung); Nobelpreis 1943.
(1888-1969) German physicist who demonstrated by means of the Stern–Gerlach apparatus that elementary particles have wavelike properties as well as the properties of matter that had been demonstrated. Nobel Prize for Physics 1943.
Stern was born in Sohrau, Upper Silesia (now Zory in Poland), and studied at a number of German universities. He then worked with Albert Einstein in Prague and Zürich. In 1923 Stern became professor at Hamburg and director of the Institute of Physical Chemistry. With the rise of the Nazis, he emigrated to the US in 1933 and set up a department for the study of molecular beams at the Carnegie Technical Institute in Pittsburgh.
In 1920 Stern and Walther Gerlach (1899–1979) carried out their experiment, which consisted of passing a narrow beam of silver atoms through a strong magnetic field. Classical theory predicted that this field would cause the beam to broaden, but quantum theory predicted that the beam would split into two separate beams. The result, showing a split beam, was the first clear evidence for space quantization—the phenomenon that, in a magnetic field, certain atoms behave like tiny magnets which can only take up particular orientations with respect to the direction of the field. Stern went on to improve this molecular-beam technique and in 1931 was able to detect the wave nature of particles in the beams.
In 1933, Stern measured the magnetic moment of the proton and the deuteron, and demonstrated that the proton's magnetic moment was 2.5 times greater than expected.