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Glossary ACS: Advanced Camera for Surveys Instrument to be installed on Hubble Space Telescope in 1999 servicing mission. ACS consists of three cameras, covering the wavelength range from 120 to 1,000 nm. AGN: Active galactic nuclei-The nuclei of certain galaxies harbor a compact, solar-system-sized object capable of outshining the rest of the galaxy by a factor of 100 or more. The ultimate energy source for active galaxies is now widely thought to be accretion of matter onto a massive black hole (see also blazar and quasar). Air shower Cascades of particles produced when highly energetic cosmic rays collide with atoms and molecules in Earth's upper atmosphere. Annihilation radiation-The radiation produced when a particle and its antiparticle collide and destroy one another. In the case of collisions between electrons and positrons, gamma rays with a characteristic energy of 51 1 keV are produced. ASCA: Advanced Satellite for Cosmology and Astrophysics Low Earth-orbiting x-ray telescope designed to operate in the 0.3- to 12-keV bands. ASCA was launched by Japan in 1993. Astro-E Low Earth-orbiting x-ray telescope in the 0.4- to 10-keV range. Astro-E is to be launched by Japan in 2000. - ~ ---I, ~-~-for- ^-^ BACK %~ ^~ 4~- ~lo. trolls-~ 1O tV US 1~ll~llcu By Jap~l1 Astrometry The branch of astronomy concerned with measuring the position of astronomical objects. AU: Astronomical unit The mean distance from the Sun to Earth, about 150 million kilometers, often used to gauge solar system distances. AXAF: Advanced X-Ray Astrophysics Facility Space-based x-ray telescope with a highly elliptical orbit operating in the range of 0.1 to 10 keV. The facility is to be launched by NASA in 1998. Axion A hypothetical elementary particle whose existence might explain certain particle physics experiments. A candidate for dark matter. Baryons-Heavy subatomic particles that interact strongly in nuclei. The lowest-mass examples are the proton and neutron. BATSE: Burst and Transient Source Experiment-A gamma-ray burst monitoring instrument on NASA's Compton Gamma-Ray Observatory. Big bang-The leading theory about the origin of the universe. It postulates that some 10 billion to 20 billion years ago the universe was in an initial state of very high density and temperature and has been expanding and cooling ever since. 63

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64 A NEW SCIENCE STRATEGY FOR SPACE ASTRONOMY AND ASTROPHYSICS Bipolar flows Outflows, or jets, of matter from opposite poles of astronomical bodies such as protostars. The composition of, and mechanisms responsible for, such flows are not yet well understood. Blackbody radiation The characteristic radiation from a hot body. Its spectral properties are determined by the body's temperature. Black hole A region of space where the density of matter is so great, and the pull of gravity so strong, that not even light can escape. Black holes are thought to be the end point in the evolution of some stars. Massive black holes may be located at the centers of some active galaxies. Blazar A class of active galactic nuclei characterized by high variability in flux and polarization; a large fraction exhibits apparent superluminal motion, indicating relativistic flow. Blue stragglers- Anomalously bright, blue stars found in certain star clusters; they may be binary stars. Bolometric flux A star's total energy output as measured over the entire electromagnetic spectrum. Cataclysmic variables Binary stars consisting of one normal star and a white dwarf, where the normal star loses matter to the white dwarf via an accretion disk. Cataclysmic variables experience repeated eruptive outbursts that increase their apparent brightness by a factor of 10 or more for short periods of time. The periods for these eruptive variables range from days to more than 104 years. The category includes stars such as classical novae (outbursts caused by a thermonuclear explosion on the white dwarf's surface), and dwarf novae (outbursts caused by unstable and irregular mass flow between the stars). CCD: Charge-coupled device-An electronic detector used for low-light-level imaging and astronomical obser- vations. CCDs have now replaced photographic emulsions for sensing visible light in most astronomical applications. Cepheid variables A type of supergiant star whose brightness varies in a predictable manner over a period of between 5 and 10 days. Since a Cepheid's oscillation period is directly related to its intrinsic luminosity, its distance can be calculated. Cepheid variable are one of the key yardsticks used to calculate extragalactic distances. CGRO: Compton Gamma-Ray Observatory Low Earth-orbiting satellite with instruments monitoring gamma- ray emissions in the 0.1- to 30-GeV range. CGRO was launched by NASA in 1991. Chandrasekhar mass The mass above which a white dwarf star would formally shrink to zero radius owing to its inability to counter its own gravity. CMBR: Cosmic microwave background radiation Radiation emitted approximately 100,000 years after the big bang that created the universe. As the universe expanded, the radiation cooled. It is now detectable as microwave blackbody radiation like that emitted by an object with a temperature of 2.73 K. COBE: Cosmic Background Explorer Low Earth-orbiting satellite designed to study the cosmic microwave background radiation using a series of infrared and microwave instruments operating in the 1.25- to 210- micron range. COBE was launched by NASA in 1989. COBRAS/SAMBA See Planck. Cosmological constant, 1Y A hypothetical repulsive force of unknown origin introduced by Einstein in an attempt to construct a static model of the universe. Today, its value is usually assumed to be zero, but this has not been formally demonstrated. Cyclotron emission The characteristic radiation emitted by electrons spiraling around magnetic field lines. This emission is circularly polarized and its wavelength is inversely proportional to the strength of the magnetic field. Dark matter The matter in the universe that has so far escaped our detection. The presence of this unseen matter has been inferred from its gravitational influence on the motions of stars and gas in galaxies and the behavior of galaxy clusters. Deceleration parameter, q~A measure of the rate at which the expansion of the universe is retarded by gravity. Delta Scuti star A type of main sequence variable star that oscillates in brightness with periods of several hours. Like the Cepheid variables, Delta Scuti stars are used to measure cosmic distances.

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GLOSSARY 65 Double degenerate systems A binary star system in which both companions are compact objects (any combinn- tion of white dwarfs, neutron stars, or black holes). r~~~~~~~ ~Or----v ~.~ _~^lV-lA~ EGRET: Energetic Gamma-Ray Experiment Telescope A gamma-ray telescope designed to study faint point-source and diffuse emissions, on NASA's Compton Gamma-Ray Observatory. ESA: European Space Agency. EUVE: Extreme Ultraviolet Explorer Low Earth-orbiting ultraviolet telescope equipped with instrumentation operating in the 7- to 76-nm range. EUVE was launched by NASA in 1992. Event horizon The border around a black hole beyond which nothing, not even light, can escape the pull of gravity. EXOSAT: European X-Ray Observing Satellite-Space-based x-ray telescope with a highly elliptical orbit sensitive to radiation in the 0.05- to 50-keV range. The telescope was launched by the European Space Agency in 1983. False-vacuum energy The difference between the current value of the energy density of empty space and its value at the beginning of the universe. Fermi acceleration A theoretical process by which cosmic rays may achieve their observed high energies. Fly's Eye An air-shower detector array located in Utah and capable of detecting cosmic rays with energies above 10~4 keV. FUSE: Far-Ultraviolet Spectroscopic Explorer Low Earth-orbiting ultraviolet telescope working in the 90- to 120-nm range. FUSE, nominally the first of NASA's mid-size Explorer (Midex) missions, is to be launched in 1998. Gamma-ray burst A sudden, intense multipeaked burst of gamma rays with energies greater than 100 keV. These bursts typically last for a few seconds to a minute and do not appear to repeat. Their source is currently unknown, but their uniform distribution across the sky suggests that they arise in either a spherical halo about the galaxy or at cosmological distances. GBT: Greenbank Telescope-A very large, ground-based radio telescope currently under construction at the National Radio Astronomy Observatory in Greenbank, West Virginia. Ginga Low Earth-orbiting x-ray telescope and gamma-ray burst detector operating in the 1- to 37-keV range in x rays and in the 1- to 400-keV range for gamma-ray bursts. This Japanese spacecraft was placed into orbit in 1987. GMC: Giant molecular cloud Region in interstellar space in which nearly all the matter is in the form of cold molecular gas, mainly hydrogen, but with many organic compounds as well. These clouds are the largest, most massive, and coldest known objects in our galaxy and are the current sites of star formation. Gravitational lens A consequence of the ability of gravity to bend the path of light rays. Astronomers have observed that the light from a distant galaxy or star can be "tensed" by the gravitational fields of intervening objects to form multiple, typically distorted images of the background object. Gravothermal instability- A consequence of the action of the second law of thermodynamics on the intrinsic gravitational dynamics of globular clusters, resulting in a continuing flow of energy from the cluster to its environment. Heavier stars in the cluster move toward the center, releasing gravitational binding energy that imparts kinetic energy to the core stars, which then share this energy with the lighter stars through subsequent interactions. The lighter stars nearer the edge of the cluster pick up velocity, increasing their likelihood of ejection from the cluster while the heavier stars continue to contract toward the center. Great Attractor A supercluster-sized enhancement of dark matter and galaxies postulated to explain observed, local perturbations of the Hubble expansion. The Great Attractor has a mass of some 10~6 solar masses spread over 100 to 200 Mpc and is located at a distance of approximately 50 Mpc from the group of galaxies in which the Milky Way is located. Great Wall A linear concentration of galaxies seen in large-scale redshift surveys.

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66 A NEW SCIENCE STRATEGY FOR SPACE ASTRONOMY AND ASTROPHYSICS Hipparcos Space-based optical telescope dedicated to precise measurements of the positions, parallaxes, and proper motions of stars. Hipparcos was launched by the European Space Agency in 1989. H-R diagram: Hertzsprung-Russell diagram A graph relating stars' colorist, or equivalently temperatures, to their luminosity. Stars at different stages in their evolution occupy specific regions of this diagram. HST: Hubble Space Telescope-Low Earth-orbiting visible, ultraviolet, and near-infrared telescope. HST was placed into orbit by NASA in 1990. HUT: Hopkins Ultraviolet Telescope Shuttle-based ultraviolet telescope sensitive to radiation in the spectral range of 42 to 185 nm. HUT was flown on NASA's STS 35 (Astro-1) and STS 67 (Astro-2), in 1990 and 1995, respectively. Hubble constant, Ho The rate at which the universe is expanding. Actually, the Hubble constant is not really a constant at all, as it is subject to a gradual slowdown (see deceleration parameter). IRAS: Infrared Astronomical Satellite Low Earth-orbiting infrared telescope designed to survey the entire sky in the spectral range between 12 and 100 microns. IRAS, a cooperative program between NASA, the United Kingdom, and the Netherlands, was launched in 1983 and ceased operation in 1984. ISO: Infrared Space Observatory Space-based infrared telescope, designed as a follow-on to IRAS and sensitive to radiation in the 8- to 1 20-micron band. The observatory was launched into a highly elliptical orbit by the European Space Agency in 1995. Isochrones Lines on the Hertzsprung-Russell diagram connecting stars of constant age. ISM: Interstellar medium The gas and dust found between the stars. IUE: International Ultraviolet Explorer- High Earth-orbiting ultraviolet telescope designed to study astro- nomical objects in the 115- to 335-nm wavelength range. IUE, a cooperative mission of NASA, the United Kingdom, and the European Space Agency, was launched in 1978 and ceased operation in 1996. KAO: Kuiper Airborne Observatory-A 0.9-meter telescope located aboard a Lockheed C-141A aircraft and designed to study radiation in the 0.3- to 1,500-micron range from altitudes in excess of 12,000 meters. KAO was first flown in 1974 and ceased operation in 1995. Keck (William M.) telescopes A pair of 10-meter-aperture, optical and infrared telescopes located on the summit of Mauna Kea, Hawaii. Keplerian frequency- Orbital frequency of an object moving under the gravitational attraction of a central point mass. Kerr geometry The geometry of space-time near a rotating black hole. Kuiper Belt A disk-shaped distribution of small icy bodies, extending hundreds of astronomical units beyond Neptune, believed to be the source of the short-period comets seen in the inner solar system. Light curve A plot of the variation in the brightness of an astronomical body as a function of time. LIGO: Laser Interferometer Gravitational Wave Observatory Interferometric gravitational-wave detector under construction at sites in Washington and Louisiana. Lyman-Alpha line The spectral line with a wavelength of 121.6 nm that results from a transition between the lowest two major energy levels of the hydrogen atom. MACHOS: Massive compact halo objects A population of objects in the halo of the Milky Way discovered via their gravitational tensing effect on the starlight from more distant stars. Magellanic Cloud, Large and Small The two closest galaxies to our own Milky Way. They are located about 180,000 light-years away and are visible in the southern sky. A bright supernova, SN1987A, was observed in the Large Magellanic Cloud in 1987. Magnetic monopoly A massive elementary particle carrying a free north or south magnetic pole, postulated by some theories of elementary-particle physics but as yet unobserved. Main sequence The region on the Hertzsprung-Russell diagram occupied by stars burning hydrogen into helium in their cores.

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GLOSSARY 67 MAP: Microwave Anisotropy Prob~A Midex mission designed to detect small variations in the temperature of the cosmic microwave background radiation. It is scheduled for launch in 2000. Mass function The relative number as a function of mass of a given class of astronomical objects. Midex: Mid-size Explorer A continuing line in NASA's budget dedicated to missions designed to address important questions in astrophysics and space physics. They have limited development schedules and their budgets are capped at $70 million. While FUSE is nominally the first Midex mission, it was initiated as a traditional Explorer mission and its budget exceeds the cost cap. MMA: Millimeter Array A National Radio Astronomy Observatory proposal to construct an interferometer consisting of 40 or more ground-based telescopes to study millimeter-wavelength radiation. Sites in Hawaii and Chile are under consideration. Neutron star The final evolutionary stage for larger stars, in which they have exhausted their thermonuclear fuel and radiate relic heat. Neutron stars are extremely dense and are supported by neutron degeneracy pressure. NICMOS: Near-Infrared Camera and Multi-Object Spectrometer Near-infrared camera installed on HST in February 1997 and designed to operate in the 0.8- to 2.5-micron band. Nova An extreme example of the cataclysmic variable phenomenon in which a star's brightness suddenly increases by a factor of a million and then fades over a period of weeks. Novae occur in binary systems of one normal and one white dwarf star, where the normal star transfers matter to the dwarf via an accretion disk. The accreted matter accumulates until such time that it spontaneously ignites in a thermonuclear outburst on the white dwarf's surface. O-B association A group of O and B stars close together in space that are young, massive, and ultraviolet- luminous. The members of an O and B association were formed roughly at the same time. OSS: Office of Space Science (NASA). Pair plasmas A plasma consisting of electrons and positrons. Parallax The apparent shift in position of a nearby object, relative to more distant ones, as the observer changes position. Using basic trigonometry, it is possible to derive the distance of a star as observed from opposite points on Earth's orbit. pc: Parsec A distance equal to 3.3 light-years. Pierre Auger Array- Cosmic-ray air-shower detector approved to be built, consisting of two sites, one in the United States and the other in Argentina. Planck: Cosmic background radiation measuring mission (formerly COBRAS-SAMBA) currently under study by the European Space Agency. Its objective is to extend COBE's discovery of anisotropy in the cosmic microwave background radiation with higher resolution and greater sensitivity. Positronium A short-lived analog of a hydrogen atom in which the central proton is replaced by a positron. Principle of equivalence basic postulate of general relativity stating that it is impossible to distinguish locally between a gravitational field and an accelerating reference frame. Pulsars Highly magnetic neutron stars spinning at a rapid rate and emitting regular bursts of radiation from their poles. Pulsars have been observed to emit at radio, visible, x-ray, and gamma-ray wavelengths. QSO: quasi-stellar object see quasar. Quark stars-A hypothetical end point of stellar evolution related to a neutron star. Quasar- An extremely distant and luminous active galactic nucleus that may outshine its parent galaxy by a factor of 1,000 or more. Sometimes called a quasi-stellar object (QSO). q0 see deceleration parameter. Redshift, z The shift in frequency (making objects appear redder than they actually are) of radiation emitted from a body that is receding from the observer. The expansion of the universe makes objects recede from us, causing the light we see from distant galaxies to be redshifted; the higher the redshift, the farther away the

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68 A NEW SCIENCE STRATEGY FOR SPACE ASTRONOMY AND ASTROPHYSICS object. Redshift is often denoted by z, where z = v/c and c is the velocity of light and v the velocity of the object. The wavelength shift is then given by the factor (1 + z). Relic elementary particles Elementary particles left over from the first moments after the big bang. ROSAT: Roentgensatellit Low Earth-orbiting x-ray telescope functioning in the 0.1- to 2.0-keV range. ROSAT, a cooperative mission between Germany, NASA, and the United Kingdom, was launched in 1990. reprocess The process by which elements are formed when an atomic nucleus captures neutrons on a time scale that is very short compared to the time required for the nuclei to decay via the emission of a beta particle. This mechanism, believed to operate in supernovae, is responsible for the creation of many of the elements in the periodic table more massive than iron. See also, s-process. RR Lyrae star A type of star that oscillates in brightness with a period of approximately 12 hours. Such stars are sometimes called short-period cluster variables. All RR Lyrae stars have approximately equal intrinsic luminosities and so are used to determine the distances to globular clusters. RXTE: Rossi X-Ray Timing Explorer-Low Earth-orbiting x-ray telescope working in the 2- to 250-keV range. The telescope was launched by NASA in 1995. SAX: Satellite per Astronomia in Raggi X An Italian-Dutch x-ray satellite designed to operate in the 0.1- to 200-keV waveband. This spacecraft, also known as BeppoSAX, was launched in April 1996. Schwarzschild geometry The geometry of space-time near a non-rotating black role. Soft gamma-ray repeaters Extremely short-duration (typically less than 1 second) bursts of relatively low- energy gamma rays. Unlike the classical gamma-ray bursts, soft gamma-ray repeaters undergo repeated bursts. Recent observations suggest that they are associated with supernova remnants in our galaxy. SIRTF: Space Infrared Telescope Facility Low Earth-orbiting infrared telescope, the last of NASA's Great Observatories, functioning in the range of 4 to 180 microns. The facility is scheduled for launch in 2001. Sloan Digital Sky Survey Five-year program to measure the positions and redshifts of some 106 galaxies using a dedicated telescope located at Apache Point Observatory in New Mexico. SOFIA: Stratospheric Observatory for Infrared Astronomy A Boeing 747 aircraft equipped with a 2.5- meter telescope, designed to study infrared and submillimeter emissions in the 0.3- to 1,600-micron band. The first flight of this NASA-German cooperative project is scheduled for 2000. Spallation The shattering of a nucleus by a highly energetic cosmic-ray particle. Spiral density waves The mechanism by which the arms of spiral galaxies are formed and maintained by a periodic perturbation in a galaxy' s gravitational field. s-process The mechanism by which elements are formed when atomic nuclei capture neutrons on time scales that are very long compared to the time required for the nuclei to decay via the emission of a beta particle. In contrast to the reprocess operating in supernovae, the s-process occurs in normal stars. This process is responsible for the creation of many of the elements heavier than iron that are not created by the reprocess. Starburst galaxies Bright sources of infrared radiation created when a galaxy forms stars at a rate hundreds of times greater than that observed in the Milky Way. These bursts of star formation may be triggered when a galaxy is perturbed by a close encounter or collision with another galaxy. Stark broadening The broadening of spectral lines due to the electric fields of nearby charged particles. The degree of broadening increases with pressure and is therefore a measure of high gravity. Stellar dynamo-The as yet unknown internal process or processes by which a star's magnetic field is generated. STIS: Space Telescope Imaging Spectrograph A two-dimensional spectrograph operating at ultraviolet, opti- cal, and near-infrared wavelengths. The instrument was installed in HST in February 1997. Sunyaev-Zeldovich effect-A temperature dip in the cosmic microwave background radiation associate with hot, x-ray-emitting gas in clusters of galaxies. Supernova A star that, due to accretion of matter from a companion star (Type Ia) or exhaustion of its own fuel supply (Types Ib, Ic, and II), can no longer support itself against gravity and thus collapses, throwing off its outer layers in a burst of energy that may briefly outshine an entire galaxy.

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GLOSSARY ~ ~a___ A . r ~ 69 burl stair-~ type or Irregularly varying star whose spectrum shows broad and very intense emission lines. T Tauri stars are believed to be very young and have not yet settled onto the main sequence depicted in the Hertzsprung-Russell diagram. Two-Degree Field Survey A project designed to measure the redshifts of a large number of galaxies in the Southern Hemisphere. It will be performed with the 3.9-meter Anglo-Australian Telescope using a flexible fiber-optic system capable of simultaneously measuring the. ~n~.otrn of fin to Ann r~hiPrtc ^`,Pr ~ t`~_~1mrr'mm field of view. ---D ~rim e ~ v__ Viol ~ L~VV-~ 2MASS: 2-Micron All-Sky Survey All-sky infrared survey using telescopes located on Mt. Hopkins, Arizona, and Cerro Tololo, Chile. Type Ia supernova-A supernova occurring in a binary system that contains a white dwarf which is accreting mass from a companion star. Their spectra show a strong silicon line but no evidence of hydrogen or helium. All Type Ia supernova are believed to have the same brightness, allowing them to be used as beacons to measure extragalactic distances. Type Ib supernovae show strong helium lines; Type Ic display no hydrogen, helium, or silicon lines. Both of these types occur in the arms of spiral galaxies and are assumed to result from the core collapse of massive stars that would have ended their lives as Type II supernovae except that they have lost their hydrogen (and helium, for Type Ic) envelopes to some combination of winds and mass transfer. VIRGO A laser gravitational wave interferometer under construction near Pisa in Italy. VLA: Very Large Array Ground-based radio interferometer, located in New Mexico and operated by the National Radio Astronomy Observatory, consisting of 27 telescopes arranged in a Y-shaped array with arms 21 km long. VLBA: Very Long Baseline Array-A network of radio telescopes with elements located in Hawaii, the continen- tal United States, and the U.S. Virgin Islands. The array, operated by the National Radio Astronomy Obser- vatory, is dedicated to VLBI observations. VLBI: Very Long Baseline Interferometry-The simultaneous observation of a single object with an array of radio telescopes that are separated by very large distances, even across or between continents. The signals received at each telescope can be combined to synthesize what would have been seen by a single telescope as large as the array. VSOP: VLBI Space Observatory Program-An 8-meter-diameter radio telescope placed in a highly elliptical orbit about Earth in February 1997 as a part of an Earth-space VLBI network. This Japanese project is also known as the Highly Advanced Laboratory for Communication and Astronomy (HALCA). White dwarf-The final evolutionary stage for an average star (around 0.7 to 1.4 solar masses), when it has exhausted all of its nuclear fuel and is radiating away its internal heat. White dwarfs are highs Len. ~nr1 Irk supported against gravity by the pressure of degenerate electrons. ~C7~11~ ^~_~d _~_ A _ _ _ _ ~1 , . ~. . ~___J ___ _ BAND wol~-xayel star A type or very hot star whose spectrum shows very broad emission lines of helium and hydrogen. XMM: X-Ray Multi-Mirror-Space-based x-ray telescope with a highly elliptical orbit' dedicated to spectro- scopic observations in the 0.1- to 5-nary range. This European Space Agency satellite is to be launched in 1999. z- See redshift.