Accretion, accretion disk—
Astronomical objects as diverse as protostars and active galaxies may derive their energy from the gravitational power released by the infall, or accretion, of material onto a central object. The combined effects of gravity and rotation often force the accreting material into an orbiting accretion disk.
Certain galaxies emit far more energy than can be accounted for by their stars alone. The central regions of these galaxies harbor a compact, solar-system-sized object capable of outshining the rest of the galaxy by a factor of 100. The ultimate energy source for active galaxies may be the accretion of matter onto a black hole. Active galaxies can emit strongly across the entire electromagnetic spectrum, from radio waves to gamma rays. See quasar.
A technique to take account of slowly varying forces, such as gravitational deflections and temperature drifts, that can distort a mirror on time scales of minutes to hours, resulting in imperfect images.
A set of techniques to adjust the mirrors of telescopes on time scales of hundredths of a second to correct for distortions in astronomical images due to turbulence in the earth's atmosphere.
Highly energetic gamma rays collide with atoms and molecules in the earth's upper atmosphere and produce bursts of light and particles, called airshowers, that can be detected from the ground, giving information about the most energetic processes in the universe.
The distribution of galaxies in space is not uniform (isotropic) to the limit of the most sensitive surveys. However, the intensity of the cosmic background radiation from the Big Bang is highly uniform in all directions. Astronomers are searching with more sensitive telescopes for the small anisotropies in the cosmic background radiation that should be present given the non-uniform distribution of galaxies.
A unit of angle corresponding to 1/60th of a degree. The full moon is 30 arcminutes in diameter.
A unit of angle corresponding to 1/3600th of a degree; 1/60th of an arcminute. An arcsecond is approximately the size of a penny viewed from about 2.5 miles.
(1) Groups, or arrays, of telescopes can be combined to simulate a single large telescope, kilometers or even thousands of kilometers across. (2) Astronomical instruments have recently been fabricated using new electronic components called detector arrays or charge-coupled devices (CCDs) that consist of thousands of individual detectors constructed on centimeter-sized wafers of silicon, or other materials.
The branch of astronomy concerned with measuring the positions of celestial objects. Advances in technology may soon permit a 1,000-fold improvement in the measurement of positions, and thus in our ability to determine distances to stars and galaxies. See parallax.
The separation between telescopes in an interferometer. The largest baseline determines the finest detail that can be discerned with an interferometer.
Most astronomers believe that the universe began in a giant explosion called the “Big Bang” some 10 billion to 20 billion years ago. Starting from an initial state of extremely high density, the universe has been expanding and cooling ever since. Some of the most fundamental observed properties of the universe, including the abundance of light elements such as helium and lithium and the recession of galaxies, can be accounted for by modern theories of the Big Bang.
A region in space where the density of matter is so extreme, and the resultant pull of gravity so strong, that not even light can escape. Black holes may be the endpoint in the evolution of some types of stars and may be located at the centers of some active galaxies and quasars.
A glowing object emits radiation in a quantity and at wavelengths that depend on the temperature of the object. For example, a poker placed in a hot fire first glows red-hot, then yellow-hot, then finally white-hot. This radiation is called thermal or blackbody radiation.
A star-like object that contains less than about 0.08 the mass of the sun and is thus too small to ignite nuclear fuels and become a normal star. Brown dwarfs emit small amounts of infrared radiation due to the
slow release of gravitational energy and may be a component of the missing mass.
A unit of information used in reference to computers and quantities of data. A byte consists of 8 bits (0's and 1's) and may correspond to a single character or number.
A molecule (CO) consisting of carbon and oxygen that emits strongly at millimeter and submillimeter wavelengths and that can be used to trace cool gas in our own and other galaxies.
A recently discovered moon of the planet Pluto.
Charge-coupled device, or CCD—
An electronic detector used for low-light-level imaging and astronomical observations. CCDs were developed by NASA for use in the Hubble Space Telescope and the Galileo probe to Jupiter and are now widely used on ground-based telescopes.
Cosmic background radiation—
The radiation left over from the Big Bang explosion at the beginning of the universe. As the universe expanded, the temperature of the fireball cooled to its present level of 3 degrees above absolute zero (−270 degrees celsius). Blackbody radiation from the cosmic background is observed at radio, millimeter, and submillimeter wavelengths.
Protons and nuclei of heavy atoms that are accelerated to high energies in the magnetic field of our galaxy and that can be studied directly from the earth or from satellites.
Approximately 90 percent of the matter in the universe may so far have escaped direct detection. The presence of this unseen matter has been inferred from motions of stars and gas in galaxies, and of galaxies in clusters of galaxies. Candidates for the missing mass include brown dwarf stars and exotic subatomic particles. Dark matter was called “missing mass” for many years. However, because it is the light, not the mass, that is missing, astronomers have given up this terminology. Also called missing mass.
The finest detail that can be discerned with a telescope. The physical principle of diffraction limits this to a value proportional to the wavelength of the light observed divided by the diameter of the telescope.
An x-ray telescope launched in 1978, one of a series of High-Energy Astrophysics Observatories, HEAO-2.
Radiation can be represented as electric and magnetic fields vibrating with a characteristic wavelength or frequency. Long wavelengths (low frequencies) correspond to radio radiation, intermediate wavelengths to millimeter and infrared radiation, short wavelengths (high frequencies) to visible and ultraviolet light, and extremely short wavelengths to x-rays and gamma rays. Most astronomical observations measure some form of electromagnetic radiation.
Expansion of the universe—
The tendency of every part of the universe to move away from every other part due to the initial impetus of the Big Bang; also known as the Hubble expansion, after the American astronomer Edwin Hubble,
whose observations of receding galaxies led to our present understanding of the expanding universe. See redshift.
Objects outside our galaxy, more than about 50,000 light-years away, are referred to as extragalactic.
A National Research Council report (NRC, 1982) on astronomy in the 1980s chaired by G.B. Field.
A telescope used to monitor gamma rays from astronomical sources.
An isolated grouping of tens to hundreds of billions of stars ranging in size from 5,000 to 20,000 light-years across. Spiral galaxies like our own Milky Way are flattened disks of stars and often contain large amounts of gas out of which new stars can form. Elliptical galaxies are shaped more like footballs and are usually devoid of significant quantities of gas.
The study of astronomical objects using the most energetic form of electromagnetic radiation.
One billion (109) bytes. A unit of information used to describe quantities of data or the storage capacity of computers.
A consequence of Einstein's general relativity theory is that the path of light rays can be bent by the presence of matter. Astronomers have observed that the light from a distant galaxy or quasar can be “lensed” by the matter in an intervening galaxy to form multiple and often distorted images of the background object.
A NASA program to launch four major observatories to cover the optical (HST), gamma-ray (GRO), X-ray (AXAF), and infrared (SIRTF) portions of the electromagnetic spectrum.
Green Bank Telescope—
A 100-m steerable radio telescope under construction in Green Bank, W. Va.
A National Research Council report (NRC, 1972) on astronomy in the 1970s chaired by J.L. Greenstein.
The study of the internal vibrations of the sun. In a manner analogous to terrestrial seismology, helioseismology can reveal important information about the sun's internal condition.
Hubble Space Telescope (HST)—
A 2.4-m-diameter telescope orbiting in space, designed to study visible, ultraviolet, and infrared radiation; the first of NASA's Great Observatories.
The most abundant element in the universe. It can be observed at a variety of wavelengths, including 21-cm radio, infrared, visible, and ultraviolet wavelengths, and in a variety of forms, including atoms (HI), molecular form (H2), and ionized form (HII).
The study of astronomical objects using intermediate-wavelength radiation to which the atmosphere is mostly opaque and the human eye insensitive. Humans sense infrared energy as heat. The infrared part of the electromagnetic spectrum generally corresponds to radiation
with wavelengths from 1 µm to 1000 µm (1 µm is one-millionth of a meter). Objects with temperatures around room temperature or lower emit most of their radiation in the infrared.
A spatial interferometer combines beams of light from small, widely separated telescopes to synthesize the aperture of a single large telescope; see array. A different form of interferometer can be used on a single telescope to break up the light into its constituent colors; see spectroscopy.
A moon of the planet Jupiter on which volcanoes have been observed from the Voyager spacecraft and from terrestrial telescopes.
The largest field of view over which a distortion-free image can be formed looking through the earth's atmosphere; a few arcseconds at visible wavelengths.
A Japanese underground observatory used to detect neutrinos.
Kuiper Airborne Observatory—
A 1-m-diameter telescope for infrared and submillimeter observations that is carried above most of the earth's water vapor in a C-141 aircraft.
A unit of astronomical distance equal to the distance light travels in a year: about 9 trillion miles. The nearest star is 4 light-years away. The center of our galaxy is about 24,000 light-years away. The closest galaxy is about 180,000 light-years away.
Magellanic Clouds, Large and Small—
The two closest galaxies to our own Milky Way, located about 180,000 light-years away and visible only from the Southern Hemisphere. A bright supernova, SN1987A, was observed in the Large Magellanic Cloud in 1987.
The study of the motion of gases in the presence of magnetic fields.
A unit of brightness for stars. Fainter stars have numerically larger magnitudes. The brightest stars, excluding the sun, are about magnitude 0; the faintest star visible to the unaided eye is about magnitude 6. A star with V = 15 is one-millionth as bright as the half-dozen brightest stars with V = 0. Stars as faint as magnitude 28 can be seen with powerful terrestrial or spaceborne telescopes.
One million bytes. A unit of information used to describe quantities of data or the storage capacity of computers. A single image from the Hubble Space Telescope comprises about 5 megabytes.
Our sun is located in the Milky Way Galaxy, which consists of some 100 billion stars spread in a disk over 50,000 light-years across and hundreds of light-years thick. (See the cover of this report.)
Approximately 90 percent of the matter in the universe may so far have escaped direct detection. The presence of this unseen matter has been inferred from motions of stars and gas in galaxies, and of galaxies
in clusters of galaxies. Candidates for the missing mass include brown dwarf stars and exotic subatomic particles. Dark matter was called “missing mass” for many years. However, because it is the light, not the mass, that is missing, astronomers have given up this terminology. Also called dark matter.
One of a family of subatomic particles with little or no mass. These particles are generated in nuclear reactions on the earth, in the centers of stars, and during supernova explosions and can give unique information about these energetic processes. Because neutrinos interact only weakly with matter, they are difficult to detect.
The process by which heavy elements such as helium, carbon, nitrogen, and iron are formed out of the fusion of lighter elements, such as hydrogen, during the normal evolution of stars, during supernova explosions, and in the Big Bang.
The study of astronomical objects using light waves with wavelengths from about 1 to 0.3 µm. The human eye is sensitive to most of these wavelengths. See electromagnetic spectrum.
The apparent shift in position of a nearby object relative to a more distant object, as the observer changes position. Using basic trigonometry, it is possible to derive the distance of a star from its parallax as observed from opposite points on the earth's orbit. See astrometry.
A unit of astronomical distance equal to 3.3 light-years.
The smallest element of a digital image. A typical image from the Hubble Space Telescope is a square with 1,600 × 1,600 discrete pixels.
Planetary debris disk—
A cloud of solid material (dust) orbiting a star; possibly the solid material left over from the formation of a planetary system.
Galaxies are thought to have formed fairly early in the history of the universe, by the collapse of giant clouds of gas. During this process, a first generation of stars formed, and these should be observable with the telescopes discussed in this report.
Protoplanetary or protostellar disk—
A disk of gas and dust surrounding a young star or protostar out of which planets may form.
The earliest phase in the evolution of a star, in which most of its energy comes from the infall of material, or accretion, onto the growing star. A protostellar disk probably forms around the star at this time.
An extremely compact, luminous source of energy found in the cores of certain galaxies. A quasar may outshine its parent galaxy by a factor of 1,000 yet be no larger than our own solar system. The accretion of gas onto a black hole may power the quasar. Active galaxies are probably less luminous, more-nearby versions of quasars.
The study of astronomical objects using radio waves with wavelengths generally longer than 0.5 to 1 mm. See electromagnetic spectrum.
Radiation from an approaching object is shifted to higher frequencies (to the blue), while radiation from a receding object is shifted to lower frequencies (to the red). A similar effect raises the pitch of an ambulance siren as it approaches. The expansion of the universe makes objects recede so that the light from distant galaxies is redshifted. The redshift is often denoted by z, where z = v/c and v is the velocity and c the speed of light. The wavelength shift is then given by the factor (1 + z).
Spatial resolution describes the ability of an instrument to separate features at small details; see diffraction limit and interferometer. Spectral resolution describes the ability of an instrument to discern small shifts in wavelength; see spectroscopy.
Space Exploration Initiative—
A plan proposed by the President for the manned exploration of the moon and Mars.
A technique whereby the light from astronomical objects is broken up into its constituent colors. Radiation from the different chemical elements that make up the object can be distinguished, giving information about the abundances of these elements and their physical state.
Certain galaxies, particularly those perturbed by a close encounter or collision with another galaxy, often form stars at a rate hundreds of times greater than in our galaxy. Such galaxies are bright sources of infrared radiation.
Electromagnetic radiation with wavelengths between about 0.1 and 1 mm intermediate between radio and infrared radiation.
An astrophysical effect whereby the distribution of wavelengths of radiation seen through the gas in a distant cluster of galaxies is subtly modified. Measurement of this effect can be used to determine the distance to the cluster.
At any one time, the fastest, most powerful computer available at any price.
A star that, due to accretion of matter from a companion star or exhaustion of its own fuel supply, can no longer support itself against its own weight and collapses, throwing off its outer layers in a burst of energy that outshines an entire galaxy. In 1987 a star in the Large Magellanic Cloud was observed as a dramatic supernova called Supernova 1987A.
One trillion (1012) bytes. A unit of information used to measure quantities of data. All the images taken with the Hubble Space Telescope in a given year will comprise a few terabytes.
Ultraviolet (UV) astronomy—
The study of astronomical objects using short-wavelength radiation, from 0.3 µ to 0.01 µm (10 nm), to which the
atmosphere is opaque and the human eye insensitive. See electromagnetic spectrum.
V, visual magnitude—
A National Research Council report (NRC, 1964) on astronomy in the 1960s chaired by A.E. Whitford.
The unrefereed reports (NRC, 1991) of 14 of the 15 panels constituted as part of this survey of astronomy and astrophysics.
The study of astronomical objects using x-rays having wavelengths shorter than about 10 nm to which the atmosphere is opaque. X-rays are emitted by extremely energetic objects having temperatures of millions of degrees. See electromagnetic spectrum.
The sky as observed at x-ray wavelengths is not completely dark, but glows faintly. The origin of this diffuse emission remains puzzling: possibly the radiation from countless galaxies, possibly the radiation from gas filling the space between galaxies. AXAF and other space missions may discover the secret in the 1990s.
Advanced Composition Explorer. A space mission to study cosmic rays.
The proposed Astrometric Interferometry Mission (AIM) would use small telescopes in space separated by up to 100 m to measure the positions of stars with 3- to 30-millionths-of-an-arcsecond precision.
Astronomical Image Processing System. A set of programs developed to process astronomical data from the Very Large Array (VLA) and other radio wavelength interferometers.
A collection of telescopes that flew on the Space Shuttle in December 1990 to measure ultraviolet and x-ray radiation.
A Japanese-built x-ray telescope to be flown in 1993 in collaboration with U.S. astronomers.
Astronomical Unit. A basic unit of distance equal to the separation between the earth and the sun, about 150 million km.
The Advanced X-ray Astrophysics Facility. A telescope, now under construction, designed to observe x-rays. The third of NASA's Great Observatories.
Chicago Airshower Array for the study of gamma rays.
See charge-coupled device.
See carbon monoxide.
The Cosmic Background Explorer. A NASA mission launched in 1989 to study the cosmic background radiation from the Big Bang.
The Caltech Submillimeter Observatory, a 10-m telescope operating on Mauna Kea, Hawaii. The telescope is used for observations of millimeter and submillimeter wavelength radiation.
Department of Defense.
Department of Energy.
The European Space Agency. The European equivalent of NASA.
The European Southern Observatory.
The Extreme Ultraviolet Explorer. A NASA mission planned for the 1990s.
Flexible Image Transport System. A worldwide format for transferring astronomical images and other information between computers.
The Far Ultraviolet Spectroscopy Explorer. A NASA mission planned for the 1990s.
The Green Bank Telescope.
The Giant Meter Wave Radio Telescope in India.
Global Oscillations Network Group. A worldwide network of telescopes designed to study vibrations in the sun. See helioseismology.
The Gamma Ray Observatory. A telescope to be launched in 1991 to study highly energetic gamma rays from astronomical sources. NASA 's second Great Observatory.
The High-Energy Astronomical Observatory. A series of three telescopes launched in the 1980s to study x-rays and gamma rays. HEAO-2 was also called the Einstein Observatory.
The High-Energy Transient Experiment. An experiment to be launched from the Space Shuttle to look for the origins of mysterious bursts of x-rays and gamma rays.
HI, HII, H2—
Hubble Space Telescope.
Image Reduction and Analysis Facility. A set of computer programs for working with astronomical images.
The Infrared Astronomical Satellite. A NASA Explorer satellite launched in 1983 that surveyed the entire sky in four infrared wavelength bands using a helium-cooled telescope.
The Infrared Telescope Facility. A 3-m telescope located on Mauna Kea, Hawaii, and operated by NASA to study planets and other astronomical objects.
The Infrared Space Observatory. A European mission planned for launch in 1994 to study infrared radiation.
The International Ultraviolet Explorer. A joint NASA-ESA orbiting telescope to study ultraviolet radiation.
See Kuiper Airborne Observatory.
The Large Earth-based Solar Telescope.
The Millimeter Array. A proposed instrument that would link 40 telescopes together as an interferometer to study millimeter wavelength radiation.
Max-Planck-Institute for Radio Astronomy.
National Astronomy and Ionosphere Center.
National Academy of Sciences.
National Aeronautics and Space Administration.
The Near-Infrared Camera and Multi-Objective Spectrometer. A second-generation instrument for infrared imaging and spectroscopy planned for the Hubble Space Telescope.
National Optical Astronomy Observatories.
National Radio Astronomy Observatory.
National Research Council.
National Science Foundation.
National Solar Observatory.
NASA's Office of Space Science and Applications.
A proposed Soviet mission to fly a radio telescope in space as part of an earth-space interferometer.
The Roentgen Satellite, an orbiting x-ray telescope launched in 1990, is named after the German scientist W. Röntgen, the discoverer of x-rays. ROSAT is a German-U.S.-U.K. collaboration.
Search for extraterrestrial intelligence.
The Space Infrared Telescope Facility. NASA's fourth Great Observatory will study infrared radiation.
Small Explorer. A NASA program to fly small, inexpensive satellites on a rapid time scale.
The Stratospheric Observatory for Far-Infrared Astronomy. A 2.5-m telescope flown above most of the earth's water vapor in a modified 747 aircraft to study infrared and submillimeter radiation.
The Solar-Heliospheric Observatory, a European Space Agency mission that will be launched around 1995 to study the sun.
The Space Telescope Imaging Spectrometer. A second-generation instrument for ultraviolet imaging and spectroscopy planned for the Hubble Space Telescope.
The Submillimeter Wave Astronomy Satellite. A Small Explorer (SMEX) payload to study submillimeter emission from water and oxygen.
The Very Large Array. A radio interferometer consisting of 27 antennae spread out over 35 km and operating with 0.1-arcsecond spatial resolution.
The Very Long Baseline Array. An array of radio telescopes operating as an interferometer with a transcontinental baseline and spatial resolution less than a thousandth of an arcsecond.
Very long baseline interferometry. A technique whereby a network of radio telescopes can operate as an interferometer with baselines that can be as large as the entire earth, or even larger when satellites are used.
The Very Large Telescope. A European project to build four 8-m telescopes.
A Japanese mission to fly a radio telescope in space to operate as part of an earth-space interferometer.
The Wide-Field/Planetary Camera. The main imaging instrument on the Hubble Space Telescope. A modified version will be installed in 1993 to correct for distortions in the HST mirror.
University of Wisconsin, Indiana University, Yale University, and NOAO 3.5-m telescope.
The X-ray Timing Explorer. A NASA mission to study x-ray radiation.