- 行业: Printing & publishing
- Number of terms: 178089
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McGraw Hill Financial, Inc. is an American publicly traded corporation headquartered in Rockefeller Center in New York City. Its primary areas of business are financial, publishing, and business services.
A star with a detectable change in brightness that is often accompanied by other physical changes. Every star varies in brightness sometime during its life, usually in the early stages (while it is forming) or in the late stages (close to its death). Therefore, variability provides important clues about the evolution and nature of stars. Depending upon the type of star, variability in brightness can provide information about its size, radius, mass, temperature, luminosity, internal and external structure, composition, and distance from the Earth.
Industry:Science
A starlike body whose mass is too small to sustain nuclear fusion reactions in its core. All stars, including the Sun, shine because they engage in nuclear fusion in their hot and dense cores. In the early 1960s, S. S. Kumar noted that, if they existed, stars with mass less than 8% that of the Sun would not have the high temperatures in their cores necessary to sustain nuclear fusion reactions.
These objects, called brown dwarfs, would not truly be normal stars because their lack of nuclear fusion would inhibit their ability to shine. Indeed, brown dwarfs would grow dimmer as they aged. At even lower masses are the planets, such as Earth and Jupiter (which is approximately 0.1% the mass of the Sun). The distinction between planets and brown dwarfs has been debated; at the end of 2003, no consensus on the scientific definition of the word “planet” had been reached. Some scientists maintained that planets ought to be distinguished from brown dwarfs by the way they form. Planets form in disks of dust and gas swirling about nascent stars, while brown dwarfs are thought to form like stars, out of the gravitational collapse of a huge cloud of gas in space. However, a basic physical distinction is that brown dwarfs are hot enough when they are young to host evanescent nuclear reactions while planets never host any fusion reactions.
Industry:Science
A state in which the metabolic rate of an organism is reduced to an imperceptible level. The several kinds of cryptobiosis (hidden life) include anhydrobiosis (life without water), cryobiosis (life at low temperatures), and anoxybiosis (life without oxygen). Of these, most is known about anhydrobiosis; therefore, the discussion will be restricted to that type.
Industry:Science
A state of fluid flow in which the distribution of body forces along the direction of the net body force is unstable and will thus break down. Fluid flows are subject to a variety of instabilities, which may be broadly viewed as the means by which relatively simple flows become more complex. Instabilities are an important step in the transition between smooth and turbulent flow, and in the atmosphere they are responsible for phenomena ranging from thunderstorms to low- and high-pressure systems. Meteorologists and oceanographers divide instabilities into two broad classes: convective and dynamic.
Industry:Science
A state of fluid flow in which the distribution of mass and momentum is unstable. Fluid flows are subject to a variety of instabilities which generally cause the flow to become more complex and which often lead to turbulent, chaotic flow. Instabilities are responsible for a variety of phenomena in natural flows, including cyclones, hurricanes, and thunderstorms in the atmosphere; mantle convection in the Earth's interior; and granules and supergranules in stellar atmospheres. A great deal of research in the geophysical and astrophysical sciences has focused on flow instabilities; and instability plays an important role in engineering problems ranging from naval engineering and aeronautics to the design of efficient heating and cooling systems.
Industry:Science
A state of matter characterized by large specific surface areas, that is, large surfaces per unit volume or unit mass. The term colloid refers to any matter, regardless of chemical composition, structure (crystalline or amorphous), geometric form, or degree of condensation (solid, liquid, or gas), as long as at least one of the dimensions is less than approximately 1 micrometer but larger than about 1 nanometer. Thus, it is possible to distinguish films (for example, oil slick), fibers (spider web), or colloidal particles (fog) if one, two, or three dimensions, respectively, are within the submicrometer range.
Industry:Science
A state of matter intermediate between that of crystalline solids and gases. Macroscopically, liquids are distinguished from crystalline solids in their capacity to flow under the action of extremely small shear stresses and to conform to the shape of a confining vessel. Liquids differ from gases in possessing a free surface and in lacking the capacity to expand without limit. On the scale of molecular dimensions liquids lack the long-range order that characterizes the crystalline state, but nevertheless they possess a degree of structural regularity that extends over distances of a few molecular diameters. In this respect, liquids are wholly unlike gases, whose molecular organization is completely random.
Industry:Science
A state of matter with an orientational order of building units—individual molecules or their aggregates—and complete or partial absence of the long-range positional order. Liquid crystals, discovered more than 100 years ago, are one of the best-studied classes of soft matter, along with colloids, polymer solutions and melts, gels, and foams.
Industry:Science
A state of resistance to an agent, the pathogen, that normally produces an infection. Pathogens include microorganisms such as bacteria and viruses, as well as larger parasites. The immune response that generates immunity is also responsible in some situations for allergies, delayed hypersensitivity states, autoimmune disease, and transplant rejection.
Industry:Science
A state wherein the immune mechanisms are inadequate in their ability to perform their normal function, that is, the elimination of foreign materials (usually infectious agents such as bacteria, viruses, and fungi). Immune mechanisms are also responsible for the rejection of transplanted organs. The processes described above are accomplished by white blood cells known as lymphocytes, of which there are two major types, T lymphocytes (thymus-derived) and B lymphocytes (bone marrow–derived).
Industry:Science
