nucleosynthesis \ l nu-kle-o-'sin-th3-s3s\ Production on a cosmic scale of all the chemical elements from one or perhaps two simple types of atomic nuclei (see nucleus), those of hydrogen and helium. Elements differ in the number of protons and isotopes of each element by the number of neutrons in their nuclei. One type of nucleus can be transformed into another by adding or removing protons, neutrons, or both, processes that go on in stars. Many of the first 26 elements (up to iron) and their present cosmic abundances can be accounted for by successive nuclear fusion reactions, beginning with hydrogen, in stellar cores. Heavier elements are believed to be created in the death of stars during supernova explosions, by capture of successive neutrons by lighter nuclei and decay of some of these neutrons into protons (with ejection of an electron and a neutrino each time).
nucleotide Vnii-kle-o-.tldV Any of a class of organic compounds, includ¬ ing the structural units of nucleic acids. Each consists of a nucleoside and one or more phosphate groups. In nucleic acids, the phosphate of one nucleotide joins to the sugar of the next to form the backbone. Important nucleotides that are not part of nucleic acids include ATP, cyclic AMP (needed in glycogen breakdown), and certain coenzymes.
nucleus Specialized structure occurring in most cells (except bacteria) and separated from the rest of the cell by the nuclear membrane. This membrane seems to be continuous with the cell’s endoplasmic reticulum and has pores that permits the passage of large molecules. The nucleus con¬ trols and regulates the cell’s activities (e.g., growth and metabolism) and carries the genes. Nucleoli are small bodies often seen within the nucleus that play an important part in the synthesis of RNA and protein. A cell normally contains only one nucleus.
nucleus Central, positively charged core of an atom. It consists of posi¬ tively charged protons and neutral neutrons, known collectively as nucle¬ ons, held together by the strong force. The number of nucleons can range from 1 to about 270, depending on the element. Isotopes are atoms of the same element that have the same number of protons but different num¬ bers of neutrons. Some nuclei, especially heavier ones, are unstable, or radioactive (see radioactivity), emitting energy in the form of alpha rays (see alpha decay), beta rays (see beta decay), or gamma rays. The nucleus makes up nearly all the mass but only a minute fraction of the volume of the atom.
nuclide or nuclear species Species of atom as characterized by the number of protons, neutrons, and the energy state of the nucleus. A nuclide is characterized by its mass number and its atomic number. To be regarded as distinct, a nuclide must have an energy content sufficient for a mea¬ surable lifetime, usually more than 10 -10 second. Nuclear isomers, which have the same number of protons and neutrons but differ in energy con¬ tent and radioactivity, are also distinct nuclides. Nuclides are associated
with radioactive decay and may be stable or unstable. There are about 1,700 known nuclides, of which about 300 are stable and the rest radio¬ active.
nudibranch \ , n(y)u-d3-,brai]k\ or sea slug Any marine gastropod in the order Nudibranchia. Most nudibranchs lack a shell, mantle cavity (see mollusk), and gills, and breathe through the body surface. The delicately colored body, up to 16 in. (43 cm) long, has bizarre defensive outgrowths, called cerata, that discharge nematocysts ingested from cnidarian prey. Antennalike organs arise from the head. Nudibranchs occur in shallow waters of all oceans, where they feed chiefly on other invertebrates, par¬ ticularly sea anemones. Some species can swim; others are bottom creep¬ ers. The term sea slug sometimes refers to all members of the subclass Opisthobranchia.
nuee ardente \nu-'a-ar- , dant\ French "fiery or glowing cloud"
Highly destructive, incandescent mass of gas-enveloped particles that is associated with volcanic eruptions. These glowing avalanches, as they are sometimes called, can move down even slight inclines at speeds as high as 100 mph (160 kph). The temperature of the gases can reach 1,100— 1,300 °F (600-700 °C). Nuees ardentes are exceedingly destructive, kill¬ ing all living things in their path. Most occur in the circum-Pacific region known as the Ring of Fire. See also tuff.
Nuer Vnti-ar\ People of the marsh and savanna living on both banks of the Nile River in southern Sudan. They speak an Eastern Sudanic lan¬ guage of the Nilo-Saharan family. The Nuer are cattle-raising people who also cultivate millet and spear fish. They spend the rainy season in per¬ manent villages on the higher ground and the dry season in riverside camps. Feuding between clans is common, as is warfare with the Dinka. They number 1.5 million. See also Nilot.
Nuevo Leon \'nwa-v6-la-'6n\ State (pop., 2000: 3,834,141), northeast¬ ern Mexico. It covers an area of 25,067 sq mi (64,924 sq km), and its capital is Monterrey. The Sierra Madre Oriental crosses the state from northwest to southeast. The region became a state in 1824 and was occu¬ pied by U.S. troops during the Mexican War. Its iron and steel industries were the first heavy industry in Latin America; the state also supports agricultural and textile enterprises.
nuisance In law, an act, object, or practice that interferes with anoth¬ er’s rights or interests by being offensive, annoying, dangerous, obstruc¬ tive, or unhealthful. Such activities as obstructing a public road, polluting air and water, operating a house of prostitution, or keeping explosives are public nuisances and constitute criminal violations. A private nuisance is an activity or condition (e.g., excessive noise, disagreeable odor) that interferes with the use and enjoyment of one’s property and that may be a cause of action in civil litigation. An attractive nuisance is something on one’s property that poses a risk to children or others who may be attracted to it.
Nujoma Vnu-'jo-moV Sam(uel Shafiihuma) (b. May 12, 1929, Owambo, South West Africa) First president (1990-2005) of independent Namibia. In the late 1950s Nujoma helped found the Ovamboland Peo¬ ple’s Organization, forerunner of the South West Africa People's Organi¬ zation (SWAPO). He became SWAPO’s first president in 1960, and, after years of petitioning the UN to compel South Africa to release control of South West Africa, he authorized armed resistance (1966). In 1989, after some 30 years in exile, he led SWAPO to victory in the UN-supervised elections.
Nukualofa V.nu-kii-a-'lo-foX Town (pop., 1999 est.: 36,360), capital, and chief port of Tonga. Located on the northern shore of Tongatapu Island in the southern Pacific Ocean, it has a deep-draft harbour that is protected by reefs. Commercial activities centre on the export of copra and bananas. Landmarks include the 19th-century royal palace, a chapel, and royal tombs.
Nullarbor Vno-lo-.boiA Plain Vast limestone plateau, southern Austra¬ lia. It lies along the south coast of western South Australia and eastern Western Australia, extending north from the Great Australian Bight to the Great Victoria Desert. It occupies 100,000 sq mi (260,000 sq km) of gen¬ erally flat surface in bedrock. The name Nullarbor is derived from the Latin nullus arbor (“no tree”). The Nullarbor National Park preserves rare vegetation and fauna. The plain has many limestone caves, including Koonalda Cave, an archaeological site. The park is crossed by the world’s longest stretch of straight railroad track (330 mi [530 km]).
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nullification Doctrine upholding the right of a U.S. state to declare null and void an act of the federal government. First enunciated in the Virginia and Kentucky Resolutions (1798), it was expanded by John C. Calhoun in response to the Tariff of 1828. Calhoun maintained that a state “interpo¬ sition” could block enforcement of a federal law. The South Carolina leg¬ islature agreed by passing the Ordinance of Nullification (1832), threatening to secede if the federal government forced collection of the 1828 tariff duties. Pres. Andrew Jackson asserted the supremacy of the federal government. The U.S. Congress passed a compromise tariff bill reducing the duties but also passed the Force Bill, which authorized fed¬ eral enforcement of the law. The South Carolina legislature rescinded its ordinance, but the conflict highlighted the danger of nullification.