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Astrophysics, the branch of astronomy that seeks to understand the birth, evolution, and end states of celestial objects and systems in terms of the physical laws that govern them. For each object or system under study, astrophysicists observe radiations emitted over the entire electromagnetic spectrum and variations of these emissions over time (see Electromagnetic Radiation ; Spectroscopy ; Spectrum ). This information is then interpreted with the aid of theoretical models. It is the task of such a model to explain the mechanisms by which radiation is generated within or near the object, and how the radiation then escapes. Radiation measurements can be used to estimate the distribution and energy states of the atoms, as well as the kinds of atoms, making up the object. The temperatures and pressures in the object may then be estimated using the laws of thermodynamics .

Steady-State Theory, theory of cosmology, or the study of the universe and its origins, that was once a rival to the big bang theory , which proposes that the universe was created in a giant explosion. The steady-state theory holds that the universe looks, on the whole, the same at all times and places. The Austrian-British astronomer Hermann Bondi and the Austrian-American astronomer Thomas Gold formulated the theory in 1948. The British astronomer Fred Hoyle soon published a different version of the theory based on his mathematical understanding of the problem. Most astronomers believe that astronomical observations contradict the predictions of the steady-state theory and uphold the big bang theory.

Gold, Thomas (1920- ), Austrian-American astronomer, born in Vienna and educated at the University of Cambridge. He is best known as the developer—with Austrian-British mathematician Hermann Bondi and British astronomer Fred Hoyle—of the steady-state theory of the universe. The theory, proposed in 1948, holds that the universe is homogeneous and that matter is continuously being created as the universe expands. Most scientists currently endorse the big-bang theory instead. Gold also developed the accepted explanation of pulsars as being spinning neutron stars .

Spectroscopy, in physics and physical chemistry, the study of spectra (see Spectrum ). The basis of spectroscopy is that each chemical element has its own characteristic spectrum. This fact was recognized in 1859 by German scientists Gustav Robert Kirchhoff and Robert Wilhelm Bunsen . They developed the prism spectroscope in its modern form and applied it to chemical analysis. One of two principal spectroscope types, this instrument consists of a slit for admitting light from an external source, a group of lenses, a prism, and an eyepiece. Light that is to be analyzed passes through a collimating lens, which makes the light rays parallel, and the prism; then the image of the slit is focused at the eyepiece. One actually sees a series of images of the slit, each a different color, because the light has been separated into its component colors by the prism. The German scientists were the first to recognize that characteristic colors of light, or the spectra, are emitted and absorbed by

Refrigeration, process of lowering the temperature and maintaining it in a given space for the purpose of chilling foods, preserving certain substances, or providing an atmosphere conducive to bodily comfort. Storing perishable foods, furs, pharmaceuticals, or other items under refrigeration is commonly known as cold storage. Such refrigeration checks both bacterial growth and adverse chemical reactions that occur in the normal atmosphere. The use of natural or manufactured ice for refrigeration was widespread until shortly before World War I, when mechanical or electric refrigerators became available. Ice owes its effectiveness as a cooling agent to the fact that it has a constant fusion temperature of 0° C (32° F). In order to melt, ice must absorb heat amounting to 333.1 kJ/kg (143.3 Btu/lb). Melting ice in the presence of a dissolving salt lowers its melting point by several degrees. Foodstuffs maintained at this temperature or slightly above have an increased storage life. Solid c

Compression systems employ four elements in the refrigeration cycle: compressor, condenser, expansion valve, and evaporator. In the evaporator the refrigerant is vaporized and heat is absorbed from the material contents or the space being cooled. The vapor next is drawn into a motor-driven compressor and elevated to high pressure , which raises its temperature. The resulting superheated, high-pressure gas is then condensed to liquid in an air- or water-cooled condenser. From the condenser the liquid flows through an expansion valve, in which its pressure and temperature are reduced to the conditions that are maintained in the evaporator.

For every refrigerant there is a specific boiling, or vaporization, temperature associated with each pressure, so that it is only necessary to control the pressure in the evaporator to obtain a desired temperature. A similar pressure-temperature relationship holds in the condenser. One of the most widely used refrigerants for many years has been dichlorodifluoromethane, known popularly as Refrigerant-12. This synthetic chlorofluorocarbon (CFC) when used as a refrigerant would, for example, vaporize at -6.7° C (20° F) in its evaporator under a pressure of 246.2 kPa (35.7 psi), and after compression to 909.2 kPa (131.9 psi) would condense at 37.8° C (100° F) in the condenser. The resulting condensed liquid would then enter the expansion valve to drop to evaporator pressure and repeat the cycle of absorbing heat at low temperature and low pressure and dissipating heat at the much higher condenser pressure and temperature. In small domestic refrigerators used for food storage, the condense

A few household units, called gas refrigerators, operate on the absorption principle. In such gas refrigerators a strong solution of ammonia in water is heated by a gas flame in a container called a generator, and the ammonia is driven off as a vapor, which passes into a condenser. Changed to a liquid state in the condenser, the ammonia flows to the evaporator as in the compression system. Instead of the gas being inducted into a compressor on exit from the evaporator, however, the ammonia gas is reabsorbed in the partially cooled, weak solution returning from the generator, to form the strong ammonia solution. This process of reabsorption occurs in a container called the absorber, from which the enriched liquid flows back to the generator to complete the cycle. Increasing use of absorption refrigeration now occurs in refrigeration units for comfort space cooling, for which purpose refrigerant temperatures of 45° to 50° F (7.2° to 10° C) are suitable. In this temperature range, water c

Refrigerant-12 and related CFCs, Refrigerant-11 and Refrigerant-22, are currently the major compounds used in the cooling and insulation systems of home refrigeration units. It has been found, however, that CFCs are posing a major threat to the global environment through their role in the destruction of the ozone layer . A search has therefore begun for replacements, and some manufacturers of CFCs have already pledged to phase out these products by the end of the century.

Mendel’s Laws, principles of hereditary transmission of physical characteristics. They were formulated in 1865 by the Augustinian monk Gregor Johann Mendel . Experimenting with seven contrasting characteristics of pure-breeding garden peas, Mendel discovered that by crossing tall and dwarf parents, for example, he got hybrid offspring that resembled the tall parent rather than being a medium-height blend. To explain this he conceived of hereditary units, now called genes, which often expressed dominant or recessive characteristics. Formulating his first principle (the law of segregation), Mendel stated that genes normally occur in pairs in the ordinary body cells, but segregate in the formation of sex cells (eggs or sperm), each member of the pair becoming part of the separate sex cell. When egg and sperm unite, forming a gene pair, the dominant gene (tallness) masks the recessive gene (shortness). To corroborate the existence of such hereditary units, Mendel went on to interbreed the

Fallopio, Gabriello (1523?-1562), also known as Gabriello Fallopio and Gabriel Fallopius, Italian anatomist, physician, botanist, and surgeon. Born in Modena, Fallopio studied medicine at the University of Ferrara, and after receiving his degree he worked and studied at various European medical schools. Fallopio became professor of anatomy at Ferrara in 1548 and professor of surgery and anatomy at the University of Pisa about a year later. In 1551 Cosimo I dè Medici, grand duke of Tuscany, called him to a similar post at Pisa to succeed Andreas Vesalius , the Belgian anatomist. There he also held the chair of botany and materia medica and was superintendent of the botanical gardens. Fallopio's work dealt primarily with cranial anatomy , and he added considerably to the knowledge of the ear. He was the first to use the ear speculum instrument to diagnose diseases of the ear and the first to show the connection between the mastoid, a part of the skull that houses the ear, and the mid

Vesalius, Andreas (1514-1564), Belgian anatomist and physician, whose dissections of the human body and description of his findings helped to correct misconceptions prevailing since ancient times and to lay the foundations of the modern science of anatomy . Vesalius was born in Brussels. The son of a celebrated apothecary, he attended the University of Leuven and later the University of Paris, where he studied from 1533 to 1536. At the University of Paris he studied medicine and showed a special interest in anatomy. Through further study at the University of Padua in 1537, Vesalius obtained his medical degree and an appointment as a lecturer on surgery. During his continuing research, Vesalius showed that the anatomical teachings from antiquity of the Greco-Roman physician Galen , then revered in medical schools, were based on dissections of animals, even though they were intended to provide a guide to the structure of the human body. Vesalius went on to write an elaborate anatomi

Galen (129-199?), the most outstanding physician of antiquity after Hippocrates. His anatomical studies on animals and observations of how the human body functions dominated medical theory and practice for 1400 years. Galen was born of Greek parents in Pergamum, Asia Minor, which was then part of the Roman Empire. A shrine to the healing god Asclepius was located in Pergamum, and there young Galen observed how the medical techniques of the time were used to treat the ill or wounded. He received his formal medical training in nearby Smyrna and then traveled widely, gaining more medical knowledge. About 161 he settled in Rome, where he became renowned for his skill as a physician, his animal dissections, and his public lectures. Galen dissected many animals, particularly goats, pigs, and monkeys, to demonstrate how different muscles are controlled at different levels of the spinal cord. He noted the functions of the kidney and bladder and identified seven pairs of cranial nerves. He also

Ultrasonics, branch of physics dealing with high-frequency sound waves, usually in the range above 20,000 hertz (Hz), that is, above the audible range. It is to be distinguished from supersonics (see Aerodynamics), which deals with phenomena arising when the velocity of a solid body exceeds the speed of sound. Modern ultrasonic generators can produce frequencies up to more than several gigahertz (1 GHz = 1 billion Hz) by transforming alternating electric currents into mechanical oscillations. Detecting and measuring ultrasonic waves are accomplished mainly through the use of a piezoelectric receiver or by optical means (see Crystal ), because ultrasonic waves are rendered visible by the diffraction of light. The science of ultrasonics has many applications in various fields of physics, chemistry, technology, and medicine. Ultrasonic waves have long been used for detection and communication devices called sonar, of great importance in present-day navigation, and especially in submarine

Aerodynamics, branch of fluid mechanics that deals with the motion of air and other gaseous fluids, and with the forces acting on bodies in motion relative to such fluids. The motion of an airplane through the air, the wind forces exerted on a structure, and the operation of a windmill are all examples of aerodynamic action such as airplanes. One of the fundamental forces studied in aerodynamics is lift, or the force that keeps an airplane in the air. Airplanes fly because they push air down. The leading edge of an airplane wing is slightly higher than the trailing edge when the plane is maintaining altitude. As the wing moves through the air, it deflects the air that flows underneath it downward. Air flowing over the top of the wing follows the surface of the wing and is also deflected downward. The third law of motion formulated by English physicist Sir Isaac Newton states that every action causes an equal and opposite reaction (see Newton’s Third Law of Motion ). As the wing pushes

Fluid Mechanics, physical science dealing with the action of fluids at rest or in motion, and with applications and devices in engineering using fluids. Fluid mechanics is basic to such diverse fields as aeronautics, chemical, civil, and mechanical engineering meteorology, naval architecture, and oceanography.. Fluid mechanics can be subdivided into two major areas, fluid statics, which deals with fluids at rest, and fluid dynamics, concerned with fluids in motion. The term hydrodynamics is applied to the flow of liquids or to low-velocity gas flows where the gas can be considered as being essentially incompressible. Aerodynamics is concerned with the theory of flight, and compressible fluid flow or gas dynamics with the behavior of gases under flow conditions, where velocity and pressure changes are sufficiently large to require inclusion of the compressibility effects. Applications of fluid mechanics involve all kinds of flow machinery, including jet propulsion, hydraulics, turbine,

Newton’s third law of motion states that an object experiences a f orce because it is interacting with some other object. The force that object 1 exerts on object 2 must be of the same magnitude but in the opposite direction as the force that object 2 exerts on object 1. If, for example, a large adult gently shoves away a child on a skating rink, in addition to the force the adult imparts on the child, the child imparts an equal but oppositely directed force on the adult. Because the mass of the adult is larger, however, the acceleration of the adult will be smaller. Newton’s third law also requires the conservation of momentum, or the product of mass and velocity. For an isolated system, with no external forces acting on it, the momentum must remain constant. In the example of the adult and child on the skating rink, their initial velocities are zero, and thus the initial momentum of the system is zero. During the interaction, internal forces are at work between adult and child, bu

Bernoulli’s Principle, in physics, the concept that as the speed of a moving fluid ( liquid or gas ) increases, the pressure within that fluid decreases. Originally formulated in 1738 by Swiss mathematician and physicist Daniel Bernoulli , it states that the total energy in a steadily flowing fluid system is a constant along the flow path. An increase in the fluid’s speed must therefore be matched by a decrease in its pressure. Bernoulli’s principle applies in nozzles, where flow accelerates and pressure drops as the tube diameter is reduced. It is also the principle behind orifice or Venturi flow meters. These meters measure the pressure difference between a low-speed fluid in an approach pipe and the high-speed fluid at the smaller orifice diameter to determine flow velocities and thus to meter the flow rate. Bernoulli’s principle is sometimes used to explain the net force in a system that includes a moving fluid, such as lift on an airplane wing, thrust of a ship’s propeller, or d

The major premises of Copernicus's theory are that the earth rotates daily on its axis and revolves yearly around the sun. He argued, furthermore, that the planets also circle the sun, and that the earth precesses on its axis (wobbles like a top) as it rotates. The Copernican theory retained many features of the cosmology it replaced, including the solid, planet-bearing spheres, and the finite outermost sphere bearing the fixed stars. On the other hand, Copernicus's heliocentric theories of planetary motion had the advantage of accounting for the apparent daily and yearly motion of the sun and stars, and it neatly explained the apparent retrograde motion of Mars, Jupiter, and Saturn and the fact that Mercury and Venus never move more than a certain distance from the sun. Copernicus's theory also stated that the sphere of the fixed stars was stationary. Another important feature of Copernican theory is that it allowed a new ordering of the planets according to their periods

Einstein’s third major paper in 1905, “On the Electrodynamics of Moving Bodies,” contained what became known as the special theory of relativity . Since the time of the English mathematician and physicist Sir Isaac Newton, natural philosophers (as physicists and chemists were known) had been trying to understand the nature of matter and radiation, and how they interacted in some unified world picture. The position that mechanical laws are fundamental has become known as the mechanical world view, and the position that electrical laws are fundamental has become known as the electromagnetic world view. Neither approach, however, is capable of providing a consistent explanation for the way radiation (light, for example) and matter interact when viewed from different inertial frames of reference, that is, an interaction viewed simultaneously by an observer at rest and an observer moving at uniform speed. In the spring of 1905, after considering these problems for ten years, Einstein realiz

Mammogram, X-ray image of the breast that helps physicians detect and evaluate breast abnormalities. Mammography is typically performed on women who do not have symptoms of breast cancer . The procedure can detect cancer in its early stages, when treatment is most effective. A mammogram can detect a breast abnormality as small as 0.5 cm (0.2 in), a size too small for a woman or her doctor to feel it as a lump.

Magnetic Resonance Imaging (MRI), medical diagnostic technique that creates images of the body using the principles of nuclear magnetic resonance. A versatile, powerful, and sensitive tool, MRI can generate thin-section images of any part of the body—including the heart, arteries, and veins—from any angle and direction, without surgical invasion and in a relatively short period of time. MRI also creates “maps” of biochemical compounds within any cross section of the human body. These maps give basic biomedical and anatomical information that provides new knowledge and may allow early diagnosis of many diseases. MRI is possible in the human body because the body is filled with small biological “magnets,” the most abundant and responsive of which is the proton , the nucleus of the hydrogen atom. The principles of MRI take advantage of the random distribution of protons, which possess fundamental magnetic properties. Once the patient is placed in the cylindrical magnet, the diagnostic p

Plastic Surgery, branch of surgery, dealing with the remodeling of any portion of the human body that has been damaged or deformed. The malformation may have occurred congenitally, that is, at birth, as a child born with a cleft palate or a cleft lip (see Birth Defects ). Disfigurement may also be the result of injury or of deforming surgery required in treating such diseases as cancer. The primary objectives of plastic surgery are the correction of defects, the restoration of lost function, and the improvement of appearance.

Gynecology, study of women’s diseases, with special emphasis on the female reproductive organs. Areas of special concentration for gynecologists include disorders of the uterus , or womb, the organ where an unborn fetus develops; ovaries , the organs that produce ova, or eggs, which are the female sex cells; fallopian tubes , the channels connecting the uterus and ovaries; cervix, the organ that connects the vagina and uterus; vagina, the canal between the cervix and vulva, or external female organs; and breasts. See Reproductive System ; Human Sexuality .

Transit Instrument, one of the most important of astronomical instruments, consisting of a telescope fixed to a horizontal axis, so as to revolve in the plane of the meridian. It is employed in the observation of the meridian transits of the heavenly bodies. See Transit . The meridian is marked by a reticle in the telescope composed of from 5 to 15 vertical wires and 2 horizontal wires. A lamp is used to illuminate the wires so that they can be seen at night through the telescope. The horizontal mounting axis is composed of two metal shafts fixed to the telescope tube and exactly perpendicular to it. The axis shafts rest on two support arms and bearings, which allow the telescope to be pointed from the horizontal through to the vertical along the meridian. These axis shafts must be so precisely machined and positioned in line with each other as to be essentially the same, as if they were portions of a solid shaft resting across the support arms. Four adjustments are necessary before a

Olbers, Heinrich Wilhelm Matthäus (1758-1840), German physician and astronomer, born in Abergen (now part of Bremen). He was educated at the University of Göttingen and practiced medicine at Bremen. In 1779 he devised a method, still employed by astronomers, for calculating the orbits of comets. (Olbers discovered several comets, the first, named after him, in 1815.) In 1781 he identified Uranus as a planet rather than as a comet, as had previously been assumed. He discovered the asteroids Pallas in 1802 and Vesta in 1807 and first proposed that all asteroids are fragments of a shattered planet that formerly revolved around the sun. He observed, in 1826, that the night sky should be uniformly illuminated if the universe were infinite and homogeneous, with stars in every direction. This observation, called Olbers’s paradox, was only resolved with the discovery of the Redshift , along with the realization that stars have finite lifetimes.

Astrology, the study of how events on earth correspond to the positions and movements of astronomical bodies, particularly the sun , moon , planets , and stars . Astrologers believe that the position of astronomical bodies at the exact moment of a person’s birth and the subsequent movements of the bodies reflect that person’s character and, therefore, destiny. For many years, scientists have rejected the principles of astrology. However, millions of people continue to believe in or practice it. Astrologers create charts called horoscopes, which map the position of astronomical bodies at certain times, such as when a person is born. A horoscope is illustrated by a circle, called the ecliptic . The ecliptic is the plane on which the earth orbits around the sun in a year. It is divided into twelve sections, called the signs of the Zodiac , which include Aries , Taurus , Gemini , Cancer , Leo , Virgo , Libra , Scorpio , Sagittarius , Capricorn , Aquarius , and Pisces . Astrologers assign e

Hoyle, Sir Fred (1915-2001), English astronomer and mathematician, who was one of the first to apply relativity equations and modern physics to cosmology, born in Bingley, Yorkshire. He graduated from the University of Cambridge in 1939 and was elected a fellow of its St. John's College. In the field of astrophysics , Hoyle is noted for his computations of the ages and temperatures of stars, the prediction of the existence of quasi-stellar objects that were later found, and his major contributions to the theory that the heavier elements evolved in succession from hydrogen . He was knighted in 1972. A prolific writer, Hoyle has published many popular books on astronomy and works of science fiction.

Gamow, George (1904-68), Russian American theoretical physicist, born in Odesa (Odessa), Ukraine, and educated at the University of Leningrad (now Saint Petersburg). His early work in nuclear physics was done at the universities of Leningrad, Göttingen, Copenhagen, and Cambridge. Gamow became professor of physics at Leningrad in 1931 but left the Soviet Union in 1933. The following year he moved to the United States, and he became a naturalized citizen in 1940. He was professor of theoretical physics at George Washington University (1934-56) and professor of physics at the University of Colorado (1956-68). Gamow made important contributions in a wide variety of fields, including radioactivity and cosmogony, as well as astrophysics and nuclear physics. He was one of the leading exponents of the theory of the evolutionary universe. He wrote many books for the general public, including The Birth and Death of the Sun (1940) and One, Two, Three ... Infinity (1947).

Leavitt, Henrietta Swan (1868-1921), American astronomer, whose work made possible the first accurate determination of extragalactic distances. While working at the Harvard College Observatory on a survey of Cepheid variable stars (stars the luminosity, or brightness, of which varies in an extremely regular manner) she discovered (1912) that the Cepheids having the greatest average brightness also had the longest periods of variation. When, in 1913, the Danish astronomer Ejnar Hertzsprung accurately estimated the distances of a few Cepheids, the distances of all Cepheids could be calculated from Leavitt's period-luminosity correlation. This method of distance determination greatly increased the scientific knowledge of the physical universe.

Hale, George Ellery (1868-1938), American astronomer, born in Chicago, Illinois, and educated at the Massachusetts Institute of Technology. While Hale was still in college, his father built the Kenwood Observatory, a small observatory near Chicago. Hale used the observatory for original research and in 1889 invented the spectroheliograph, a device used to study the surface of the sun . In 1892 Hale was appointed associate professor of astrophysics at the University of Chicago and in 1895 he organized the Yerkes Observatory, in Williams Bay, Wisconsin, of which he served as director until 1904. In 1904 he organized the Mount Wilson Observatory, near Los Angeles, California, which he directed until 1923. In 1908, Hale discovered that sunspots have magnetic fields. Hale conceived and helped design the first giant reflecting telescope. The instrument, a reflector with a 200-in (5.08-m) mirror, was installed at Mount Palomar Observatory near San Diego, California, in 1948. It was named the

Hubble Space Telescope (HST), first general-purpose orbiting observatory. Named after American astronomer Edwin P. Hubble , the Hubble Space Telescope was launched on April 24, 1990. The HST makes observations in the visible, infrared, and ultraviolet regions of the electromagnetic spectrum (see Electromagnetic Radiation ). The primary mirror of the HST has a diameter of 94.5 in (240 cm), and the optics of the telescope are designed so that, theoretically, when making a visible-light observation, the telescope can resolve astronomical objects that are at an angular distance of 0.05 arcsecond apart. In comparison, traditional large ground-based telescopes, under very good sky conditions, have an image resolution of about 0.5 arcsecond. Originally, the HST was equipped with five detectors: the Wide-Field Planetary Camera, the Faint Object Camera, the Faint Object Spectrograph, the High-Resolution Spectrograph, and the High Speed Photometer (see Spectroscopy ). It also has three fine guid

Messier, Charles (1730-1817), French astronomer, born in Badonviller, noted for the valuable catalog of nebulous-appearing celestial objects that he compiled from 1758 to 1784. Messier called these objects nebulae, and the catalog's purpose was to help other astronomers to distinguish such objects from comets. Messier was also noted for his discoveries of comets. Today his catalog is known to consist of galaxies and star clusters as well as true nebulas. The catalog numbers are still used in designating the objects that he listed.

Hubble, Edwin Powell (1889–1953), American astronomer, who made important contributions to the study of galaxies , the expansion of the universe, and the size of the universe. Hubble was the first to discover that fuzzy patches of light in the sky called spiral nebula were actually galaxies like Earth’s galaxy, the Milky Way . Hubble also found the first evidence for the expansion of the universe, and his work led to a much better understanding of the universe’s size. Hubble was born in Marshfield, Missouri. He attended high school in Chicago, Illinois, and received his bachelor’s degree in mathematics and astronomy in 1910. He was awarded a Rhodes Scholarship to study at the University of Oxford in England, where he earned a law degree in 1912. In 1919 Hubble finally accepted the offer from Mount Wilson Observatory, where the 100-in (2.5-m) Hooker telescope was located. The Hooker telescope was the largest telescope in the world until 1948. While Hubble was working at the Yerkes Obse

Big Bang Theory, currently accepted explanation of the beginning of the universe. The big bang theory proposes that the universe was once extremely compact, dense, and hot. Some original event, a cosmic explosion called the big bang, occurred about 13.7 billion years ago, and the universe has since been expanding and cooling. The theory is based on the mathematical equations, known as the field equations, of the general theory of relativity set forth in 1915 by Albert Einstein . In 1922 Russian physicist Alexander Friedmann provided a set of solutions to the field equations. These solutions have served as the framework for much of the current theoretical work on the big bang theory. American astronomer Edwin Hubble provided some of the greatest supporting evidence for the theory with his 1929 discovery that the light of distant galaxies was universally shifted toward the red end of the spectrum (see Redshift ). Once “tired light” theories—that light slowly loses energy naturally, bec

Telescope, device that permits distant and faint objects to be viewed as if they were much brighter and closer to the observer. Telescopes are typically used to observe the skies. For hundreds of years, telescopes were the only instruments available for studying the planets and stars. Even today, space probes can reach only our closest neighbors in the heavens, and scientists continue to rely on telescopes to learn about distant stars , nebulas , and galaxies. Telescopes are the fundamental research instruments that enable astronomers to tackle scientific questions about the birth of the universe (see Big Bang Theory ; Cosmology ); the emergence of structure in the early universe; the formation and evolution of stars, galaxies, and planetary systems; and the conditions for the emergence of life itself. Most telescopes work by collecting and magnifying visible light that is given off by stars or reflected from the surface of planets. Such instruments are called optical telescopes. Conve

Brahe, Tycho (1546-1601), Danish astronomer, who made precise, comprehensive astronomical measurements of the solar system and more than 700 stars . The data Brahe accumulated was superior to all other astronomical measurements made until the invention of the telescope in the early 17th century. Brahe was born in Knudstrup in southern Sweden (then part of Denmark). He studied law and philosophy at the universities of Copenhagen and Leipzig; at night, however, Brahe busied himself with observing the stars. With no instruments other than a globe and a pair of compasses, he succeeded in detecting grave errors in the standard astronomical tables, and set about correcting them. In 1572 he discovered a supernova in the constellation Cassiopeia . After Brahe had spent some time traveling and lecturing, Frederick II, king of Denmark and Norway, offered to provide Brahe with funds to construct and equip an astronomical observatory on the island of Hven (now Ven). Brahe accepted the proposal,

Copernican System, systematic explanation of the movement of the planets around the sun; advanced in 1543 by the Polish astronomer Nicolaus Copernicus . The Copernican system advanced the theories that the earth and the planets are all revolving in orbits around the sun, and that the earth is spinning on its north-south axis from west to east at the rate of one rotation per day. These two hypotheses superseded the Ptolemaic system , which had been the basis of astronomical theory until that time. The Copernican system first described the precession of the equinoxes (see Ecliptic ) but did not explain it. Publication of the Copernican system stimulated the study of astronomy and mathematics and laid the basis for the discoveries of the German astronomer Johannes Kepler and the British astronomer Sir Isaac Newton.

Ptolemaic System, in astronomy, theory of the order and action of the heavenly bodies. It was advanced in the 2nd century ad by the Alexandrian astronomer Ptolemy. The Ptolemaic theory held that Earth is stationary and at the center of the universe; closest to Earth is the Moon, and beyond it, extending outward, are Mercury, Venus, and the Sun in a straight line, followed successively by Mars, Jupiter, Saturn, and the so-called fixed stars. Later, astronomers supplemented this system with a ninth sphere, the motion of which supposedly produced the precession of equinoxes (see Ecliptic ). A tenth sphere or primum mobile, which was thought to motivate the other heavenly bodies, was also added. To explain the various observed motions of the planets, the Ptolemaic system described them as having small circular orbits called epicycles; the centers of the epicycles, on circular orbits around Earth, were called deferents. The motion of all spheres is from west to east. After the decline of cl

Kepler’s Laws, three laws concerning the motions of planets formulated by the German astronomer Johannes Kepler early in the 17th century. See Orbit ; Planet ; Solar System . Kepler based his laws on planetary data collected by the Danish astronomer Tycho Brahe, to whom he was an assistant. The proposals broke with a centuries-old belief based on the Ptolemaic system advanced by the Alexandrian astronomer Ptolemy , in the 2nd century ad, and the Copernican system , put forward by the Polish astronomer Nicolaus Copernicus , in the 16th century, that the planets moved in circular orbits. According to Kepler's first law, the planets orbit the sun in elliptical paths, with the sun at one focus of the ellipse. The second law states that the areas described in a planetary orbit by the straight line joining the center of the planet and the center of the sun are equal for equal time intervals; that is, the closer a planet comes to the sun, the more rapidly it moves. Kepler's third la

Kepler, Johannes (1571-1630), German astronomer and natural philosopher, noted for formulating and verifying the three laws of planetary motion. These laws are now known as Kepler's laws . Kepler was born on December 27, 1571, in Weil der Stadt in Württemberg and studied theology and classics at the University of Tübingen. There he was influenced by a mathematics professor, Michael Maestlin, an adherent of the heliocentric theory of planetary motion first developed by the Polish astronomer Nicolaus Copernicus . Kepler accepted Copernican theory (see Copernican System ) immediately, believing that the simplicity of Copernican planetary ordering must have been God's plan. In 1594, when Kepler left Tübingen for Graz, Austria, he worked out a complex geometric hypothesis to account for distances between the planetary orbits—orbits that he mistakenly assumed were circular. (Kepler later deduced that planetary orbits are elliptic; nevertheless, these preliminary calculations agreed w

Cryogenics, study and use of materials at very low temperatures. The upper limit of cryogenic temperatures has not been agreed on, but the National Institute of Standards and Technology has suggested that the term cryogenics be applied to all temperatures below -150° C (-238° F or 123° above absolute zero on the Kelvin scale). Some scientists regard the normal boiling point of oxygen (-183° C or -297° F), as the upper limit (see Absolute Zero ). Cryogenic temperatures are achieved either by the rapid evaporation of volatile liquids or by the expansion of gases confined initially at pressures of 150 to 200 atmospheres. The expansion may be simple, that is, through a valve to a region of lower pressure, or it may occur in the cylinder of a reciprocating engine, with the gas driving the piston of the engine. The second method is more efficient but is also more difficult to apply. See Heat .

Dirac, Paul Adrien Maurice (1902-84), British theoretical physicist and Nobel laureate, renowned for his prediction of the existence of the positron, or antielectron, and for his research in quantum theory . Dirac was born in Bristol, England, and educated at the universities of Bristol and Cambridge. His quantum theory of electron motion led him in 1928 to postulate the existence of a particle identical to the electron in every aspect but charge, the electron having a negative charge and this hypothetical particle a positive one. Dirac's theory was confirmed in 1932 when the American physicist Carl Anderson discovered the positron. In 1933 Dirac shared the Nobel Prize in physics with the Austrian physicist Erwin Schrödinger, and in 1939 he was made a fellow of the Royal Society. He was a professor of mathematics at Cambridge from 1932 to 1968, a professor of physics at Florida State University from 1971 until his death, and a member of the Institute for Advanced Study periodicall

Copernicus, Nicolaus (1473-1543), Polish astronomer, best known for his astronomical theory that the sun is at rest near the center of the universe, and that the earth, spinning on its axis once daily, revolves annually around the sun. This is called the heliocentric, or sun-centered, system. See Astronomy ; Solar System . Copernicus was born on February 19, 1473, in Thorn (now Toruń), Poland, to a family of merchants and municipal officials. Copernicus's maternal uncle, Bishop Łukasz Watzenrode, saw to it that his nephew obtained a solid education at the best universities. Copernicus entered Kraków Academy (now Jagiełłonian University) in 1491, studied the liberal arts for four years without receiving a degree, and then, like many Poles of his social class, went to Italy to study medicine and law. Before he left, his uncle had him appointed a church administrator in Frauenberg (now Frombork); this was a post with financial responsibilities but no priestly duties. In January 1497 C

Match, short, thin piece of wood, cardboard, or waxed string, tipped with a mixture of fire-producing substances, and used to produce a flame. One of the first matches produced was the brimstone match, made by dipping thin strips of wood into melted sulfur; the sulfur points ignited when applied to a spark produced by a flint and steel. In 1812 a chemical match was invented. Coated with sulfur and tipped with a mixture of potassium chlorate and sugar, it ignited when touched to sulfuric acid. Matches made with phosphorus and ignited by friction were invented in 1827 by the British chemist John Walker , and have been used in improved form ever since. In the modern friction match, one end of the bare stick is dipped in a fireproofing agent, so that it will not burn readily, and the other end is coated with paraffin. The head of the match contains an oxidizing agent, such as potassium chlorate; a substance that oxidizes readily, such as sulfur or rosin; a filler of clay; a binding materia

Nobel, Alfred Bernhard (1833-96), Swedish chemist, inventor, and philanthropist, born in Stockholm. After receiving an education in Saint Petersburg, Russia, and in the United States, where he studied mechanical engineering, he returned to Saint Petersburg to work under his father, developing mines, torpedoes, and other explosives. In a family-owned factory in Heleneborg, Sweden, he sought to develop a safe way to handle nitroglycerin, after a factory explosion in 1864 killed his younger brother and four other people. In 1867 Nobel achieved his goal; by using an organic packing material to reduce the volatility of the nitroglycerin , he produced what he called dynamite. He later produced ballistite, one of the first smokeless powders. At the time of his death he controlled factories for the manufacture of explosives in many parts of the world. His will provided that the major portion of his $9 million estate be set up as a fund to establish yearly prizes for merit in physics, chemistry

Cosmology, study of the universe as a whole, including its distant past and its future. Cosmologists study the universe observationally—by looking at the universe—and theoretically—by using physical laws and theories to predict how the universe should behave. Cosmology is a branch of astronomy , but the observational and theoretical techniques used by cosmologists involve a wide range of other sciences, such as physics and chemistry. Cosmology is distinguished from cosmogony, which used to mean the study of the origin of the universe but now usually refers only to the study of the origin of the solar system.

Lavoisier, Antoine Laurent (1743-1794), French chemist, who is considered the founder of modern chemistry . Lavoisier was born on August 26, 1743, in Paris and was educated at the Collège Mazarin. He was elected a member of the Academy of Sciences in 1768. He held many public offices, including those of director of the state gunpowder works in 1776, member of a commission to establish a uniform system of weights and measures in 1790, and commissary of the treasury in 1791. He attempted to introduce reforms in the French monetary and taxation system and in farming methods. As one of the farmers-general, he was arrested and tried by the revolutionary tribunal, and guillotined on May 8, 1794. Lavoisier's experiments were among the first truly quantitative chemical experiments ever performed. He showed that, although matter changes its state in a chemical reaction, the quantity of matter is the same at the end as at the beginning of every chemical reaction. These experiments provided e

Berthollet, Claude Louis, Comte (1749-1822), French chemist, who made contributions to several fields of chemistry . Berthollet was born in Talloires, and educated at the University of Turin. In 1785 Berthollet proposed the use of chlorine as a bleaching agent. After years of skepticism, Berthollet was one of the first to support the correct antiphlogistic combustion theories of the French chemist Antoine Lavoisier , although he opposed Lavoisier's erroneous theory that oxygen is the fundamental acidifying principle. With Lavoisier and others, Berthollet helped devise a new system of chemical nomenclature in 1787 that is the basis of the system currently used. He made important contributions to the knowledge of the chemistry of explosives and the metallurgy of iron. Berthollet's significant work, Essai de statique chimique (Essay on the State of Chemistry, 2 volumes, 1803), presented his theories on chemical affinity and the reversibility of reactions.

Aristotle (384-322 bc), Greek philosopher and scientist, who shares with Plato and Socrates the distinction of being the most famous of ancient philosophers. Aristotle was born at Stagira, in Macedonia, the son of a physician to the royal court. At the age of 17, he went to Athens to study at Plato's Academy. He remained there for about 20 years, as a student and then as a teacher. When Plato died in 347 bc, Aristotle moved to Assos, a city in Asia Minor, where a friend of his, Hermias, was ruler. There he counseled Hermias and married his niece and adopted daughter, Pythias. After Hermias was captured and executed by the Persians in 345 bc, Aristotle went to Pella, the Macedonian capital, where he became the tutor of the king's young son Alexander, later known as Alexander the Great. In 335, when Alexander became king, Aristotle returned to Athens and established his own school, the Lyceum. Because much of the discussion in his school took place while teachers and students wer

Cannizzaro, Stanislao (1826-1910), Italian chemist, born in Palermo, Sicily. After participating in the 1848 Sicilian revolution, Cannizzaro worked (1849-51) in a laboratory in Paris. He was appointed professor of chemistry at the institute in Alessandria (1851) and at the universities of Genoa (1855), Pisa (1861), and Rome (1871). At Alessandria he discovered the reaction that bears his name, Cannizzaro's reaction, which proves that aldehydes in the presence of concentrated alkali are reduced to a mixture of their corresponding alcohol and acid; for example, benzaldehyde yields benzyl alcohol and benzoic acid. Cannizzaro made a great contribution to atomic theory by clarifying (1858) the distinction between atomic weight and molecular weight. He showed how unknown atomic weights of elements in volatile compounds can be arrived at from known molecular weights of the compounds. Cannizzaro also determined that atomic weights of elements in compounds can be determined if specific heat

Barometer, instrument for measuring atmospheric pressure, that is, the force exerted on a surface of unit area by the weight of the atmosphere. Because this force is transmitted equally in all directions through any fluid, it is most easily measured by observing the height of a column of liquid that, by its weight, exactly balances the weight of the atmosphere. A water barometer is far too large to be used conveniently. Liquid mercury, however, is 13.6 times as heavy as water, and the column of mercury sustained by normal atmospheric pressure is only about 760 mm (about 30 in) high. Normal, or standard, atmospheric pressure is usually defined at 1013.25 millibars, which is equivalent to 760 mm (29.9213 in) of mercury or 1.03323 kg/sq cm (14.6960 lb/sq in). An ordinary mercury barometer consists of a glass tube about 840 mm (about 33 in) high, closed at the upper end and open at the lower. When the tube is filled with mercury and the open end placed in a cup full of the same liquid, the

Berzelius, Jöns Jakob, Baron (1779-1848), Swedish chemist, considered one of the founders of modern chemistry . Berzelius was born near Linköping. While studying medicine at the University of Uppsala, he became interested in chemistry. After practicing medicine and lecturing, he became a professor of botany and pharmacy at Stockholm in 1807. From 1815 to 1832 he was professor of chemistry at the Caroline Medico-Chirurgical Institute in Stockholm. He became a member of the Stockholm Academy of Sciences in 1808 and in 1818 became its permanent secretary. For his contributions to science, Berzelius was made a baron in 1835 by Charles XIV John, king of Sweden and Norway. Berzelius's research extended into every branch of chemistry and was extraordinary for its scope and accuracy. He discovered three chemical elements— cerium , selenium , and thorium —and was the first to isolate silicon , zirconium , and titanium . He introduced the term catalyst into chemistry and was the first to ela

Bioenergetics, study of the processes by which living cells use, store, and release energy. A central component of bioenergetics is energy transformation, the conversion of energy from one form to another. All cells transform energy. Plant cells, for example, use sunlight to make carbohydrates (sugars and starches) from simple inorganic chemicals. In this process, called photosynthesis , radiant energy from the sun is converted into stored chemical energy. If these plant carbohydrates are eaten by an animal (see Food Web ), they will be broken down and their chemical energy turned into movement (kinetic energy), body heat (radiant energy), or new chemical bonds (see Adenosine Triphosphate ; Citric Acid Cycle ; Metabolism ). In all such transformations, some energy is lost to the environment. This lost energy, which is no longer available for useful work, is called entropy. The second law of thermodynamics states that any system tends to run down—that is, increase its entropy—over time

Schwarzschild, Karl (1873-1916), German astronomer, mathematician, and physicist, born in Frankfurt, who predicted the existence of black holes . His first two papers on astronomy were published while still a schoolboy. After studying at the universities of Strasbourg and Munich, he was appointed Director of the Göttingen Observatory in 1901, and of the Astrophysical Observatory in Potsdam in 1909. He volunteered for military service at the start of World War I, but was invalided home in 1916 after contracting a rare skin disease, from which he died. His contributions were in the main theoretical, and related to solar physics, relativity, stellar kinematics, photographic magnitudes, the study of rotating fluid masses, and geometrical optics. In 1916 he postulated the Schwarzschild radius, on the basis of the general theory of relativity newly propounded by Albert Einstein . When a massive star explodes as a supernova, it may leave a remnant so compact that it lies wholly within this

Bubble Chamber, device that detects and tracks the paths of high-energy subatomic particles released by radioactive substances. Subatomic particles are invisible to the unaided eye. Bubble chambers provide an indirect way for physicists to “see” a particle and learn about its charge, mass, and energy and how it interacts with other subatomic particles. See also Particle Detectors ; Elementary Particles . A bubble chamber contains liquid, often hydrogen or deuterium, heated beyond its boiling point. This liquid does not boil, however, because it is under pressure and all the impurities have been removed. As a charged particle moves through the liquid, it interacts with atoms and molecules in the liquid, making them ions, which are atoms or molecules with a positive or negative charge. The new ions that form along the path of the charged particles act as impurities, causing the liquid next to them to boil. The tiny bubbles formed by the boiling liquid behind the particle form a line that