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.
Active galaxy—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 supermassive black hole. Active galaxies can emit strongly across the entire electromagnetic spectrum, from radio waves to gamma rays. See quasar.
Active optics—A technique to reduce the effects 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.
Adaptive optics—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 Earth’s atmosphere.
Anisotropy—Dependence of the properties of a system on the orientation or the direction of observation. The distribution of galaxies in space is not uniform, whereas the intensity of the cosmic background radiation from the Big Bang is highly uniform in all directions—i.e., it is almost isotropic. Astronomers are using sensitive telescopes to study the small anisotropies in the cosmic background radiation that should be present given the non-uniform distribution of galaxies.
Arcminute—A unit of angle corresponding to 1/60th of a degree. The full moon is 30 arcminutes in diameter.
Arcsecond—A unit of angle corresponding to 1/3600th of a degree; 1/60th of an arcminute. An arcsecond is approximately the size of a dime viewed from a distance of 1 mile.
Array—There are two examples of arrays in common use in astronomy: (1) A group, or array, 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.
Astrometry—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 astronomers’ ability to determine distances to stars and galaxies. See parallax.
Bahcall report—The National Research Council decadal survey report, The Decade of Discovery in Astronomy and Astrophysics (1991), on astronomy and astrophysics for the 1990s, chaired by J.N. Bahcall.
Baseline—The separation between telescopes in an interferometer. The largest baseline determines the finest detail that can be discerned with an interferometer.
Big Bang—Most astronomers believe that the universe began in a giant explosion called the Big Bang about 14 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.
Black hole—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 are probably the end point in the evolution of some types of stars and are probably located at the centers of some active galaxies and quasars.
Blackbody radiation—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.
Brown dwarf—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 dark matter.
Byte—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.
Carbon monoxide—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.
Charge-coupled device, or CCD—An electronic detector used for lowlight-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. See also array.
Cosmic microwave 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 2.7 degrees above absolute zero (2.7 K). Blackbody radiation from the cosmic background is observed at radio, millimeter, and submillimeter wavelengths.
Cosmic rays—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 Earth or from satellites.
Dark energy—An as yet unknown form of energy that pervades the universe. Its presence was inferred from the discovery that the expansion of the universe is accelerating, and these observations suggest that about 70 percent of the total density of matter plus energy is in this form. Such an acceleration would be predicted if the cosmological constant that Einstein included in his general theory of relativity were non-zero.
Dark matter—Approximately 80 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.
Diffraction limit—The finest detail that can be discerned with a telescope. The physical principle of diffraction limits this level of detail to a value proportional to the wavelength of the light observed divided by the diameter of the telescope.
Electromagnetic spectrum—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 scientists’ current understanding of the expanding universe. See redshift.
Extragalactic—Objects outside our galaxy, more than about 50,000 light-years away, are referred to as extragalactic.
Fly’s Eye—A cosmic-ray telescope used to monitor gamma rays from astronomical sources.
Galaxy—An isolated grouping of tens to hundreds of billions of stars ranging in size from 5,000 to 150,000 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.
Gamma-ray astronomy—The study of astronomical objects using the most energetic form of electromagnetic radiation.
General relativity—Einstein’s theory of gravity in which the gravity is the curved geometry of space and time.
Gigabyte—One billion (109) bytes. A unit of information used to describe quantities of data or the storage capacity of computers.
Gravitational lens—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.
Great Observatories—A NASA program to launch four major observa-
tories to cover the optical (HST), gamma-ray (CGRO), x-ray (Chandra), and infrared (SIRTF) portions of the electromagnetic spectrum.
Halo (of a galaxy)—The roughly spherical distribution of dark matter and thinly scattered stars, star clusters, and gas that surround a spiral galaxy.
Helioseismology—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 space telescope designed to study visible, ultraviolet, and infrared radiation; the first of NASA’s Great Observatories.
Hydrogen—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) and molecular (H2) and ionized (HII) forms.
Infrared astronomy—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 1,000 µm. Objects with temperatures around room temperature or lower emit most of their radiation in the infrared.
Interferometer, interferometry—A spatial interferometer combines beams of light from different telescopes to synthesize the aperture of a single large telescope; see array. Spatial interferometry is the main technique used by astronomers to map sources at high resolution and to measure their positions with high precision. A different form of interferometer can be used on a single telescope to break up the light into its constituent colors; see spectroscopy.
Light-year—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 25,000 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.
Magnetohydrodynamics—The study of the motion of gases in the presence of magnetic fields.
Magnitude—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 of magnitude 15 is one-millionth as bright as the half-dozen brightest stars of magnitude 0. Stars as faint as magnitude 28 can be seen with powerful terrestrial or spaceborne telescopes.
Massive compact halo object (MACHO)—An object of roughly stellar mass in the halo of our galaxy that is too faint to be detected by its own emission. MACHOs are indirectly detected via gravitational microlensing of more distant stars.
Megabyte—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.
Microlensing—Gravitational lensing due to a stellar mass object. This lensing phenomenon is called microlensing because the mass of the lens is so small compared with that of a galaxy. Microlensing of distant stars by intervening faint stars can reveal planets in orbit around the lensing star.
Milky Way—Our Sun is located in the Milky Way Galaxy, a spiral galaxy consisting of some 100 billion stars spread in a disk more than 80,000 light-years across and hundreds of light-years thick. The central disk of the Milky Way is the wide path of faint light that stretches across the night sky.
Neutrino—One of a family of subatomic particles with little or no mass. These particles are generated in nuclear reactions on 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.
Nucleosynthesis—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.
Optical astronomy—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.
Parallax—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 Earth’s orbit. See astrometry.
Parsec—A unit of astronomical distance equal to 3.2616 light-years.
Pixel—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.
Protogalaxy—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.
Protostar—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.
Quasar—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 supermassive black hole probably powers the quasar. Active galaxies are probably less luminous and less distant versions of quasars.
Radio astronomy—The study of astronomical objects using radio waves with wavelengths generally longer than 0.5 to 1 mm. See electromagnetic spectrum.
Redshift—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 parameterized by z, where the wavelength shift is given by the factor (1 + z) times the wavelength.
Resolution—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.
Spectroscopy—A technique whereby the light from astronomical objects is broken up into its constituent colors. Radiation from the different chemical elements that make up an object can be distinguished, giving information about the abundances of these elements and their physical state.
Starburst galaxy—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 that evident in our galaxy. Such galaxies are bright sources of infrared radiation.
Submillimeter radiation—Electromagnetic radiation with wavelengths between about 0.1 and 1 mm intermediate between radio and infrared radiation.
Sunyaev-Zeldovich effect—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.
Supermassive black hole—A black hole that is much more massive than the Sun. Supermassive black holes with masses exceeding a million solar masses are found in the nuclei of most galaxies.
Supernova—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 thus 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.
Terabyte—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.
Triad—An experimental low-altitude spacecraft launched in 1972 for gravitational physics tests.
Ultraviolet (UV) astronomy—The study of astronomical objects using short-wavelength radiation, from 0.3 µm to 0.01 µm (10 nm), to which the atmosphere is opaque and the human eye insensitive. See electromagnetic spectrum.
X-ray astronomy—The study of astronomical objects using x rays with wavelengths shorter than about 10 nm, to which the atmosphere is
opaque. X rays are emitted by extremely energetic objects that have temperatures of millions of degrees. See electromagnetic spectrum.
ABBREVIATIONS AND ACRONYMS
The Advanced Cosmic-ray Composition Experiment for the Space Station, a cosmic-ray experiment on the International Space Station.
Astronomical Image Processing System. A set of programs developed to process astronomical data from the Very Large Array (VLA) and other radio wavelength interferometers.
The Atacama Large Millimeter Array.
See adaptive optics.
The Advanced Radio Interferometry between Space and Earth, an orbiting antenna that will combine with the ground-based VLBA.
The Japanese Advanced Satellite for Cosmology and Astrophysics mission.
The Advanced Solar Telescope; also, the NSF’s Division of Astronomical Sciences.
Australia Telescope Compact Array. An interferometric radio array consisting of six antennae.
Australia Telescope National Facility. The main national organization supporting and undertaking research in radio astronomy.
Astronomical unit. A basic unit of distance equal to the separation between Earth and the Sun, about 150 million km.
The Advanced X-ray Astrophysics Facility; now called Chandra, after astrophysicist S. Chandrasekhar.
The Burst and Transient Spectrometer Experiment aboard CGRO, an all-sky gamma-ray-burst monitor.
Berkeley-Illinois-Maryland Association. A consortium that operates a millimeter-wave radio interferometer at Hat Creek, California.
Balloon Observations of Millimetric Extragalactic Radiation and Geophysics, an extremely sensitive microwave telescope flown under a stratospheric balloon that circumnavigated Antarctica over 11 days in late 1998 and early 1999.
The Combined Array for Research in Millimeter-wave Astronomy, a millimeter-wave array in the Northern Hemisphere.
See charge-coupled device.
The Compton Gamma Ray Observatory. A telescope launched in 1991 to study highly energetic gamma rays from astronomical sources. NASA’s second Great Observatory. It was safely deorbited and reentered Earth’s atmosphere in June 2000.
An x-ray observatory launched in 1999. NASA’s third Great Observatory.
See cosmic microwave background radiation.
See carbon monoxide.
The Cosmic Background Explorer. A NASA mission launched in 1989 to study the cosmic background radiation from the Big Bang.
The Constellation X-ray Observatory, a suite of four powerful x-ray telescopes.
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 Energetic Gamma Ray Experiment aboard the Compton Gamma Ray Observatory.
European Space Agency. The European equivalent of NASA.
The European Southern Observatory.
The Extreme Ultraviolet Explorer NASA mission.
The Expanded Very Large Array.
The Energetic X-ray Imaging Survey Telescope, which will be attached to the ISS.
The Frequency Agile Solar Radio telescope.
The Five College Radio Astronomy Observatory.
The European Far Infrared Space Telescope.
The Green Bank Telescope.
The Gamma-ray Large Area Space Telescope, a joint NASA-DOE mission.
The Giant Metrewave Radio Telescope in India.
Global Oscillations Network Group. A worldwide network of telescopes designed to study vibrations in the Sun. See helioseismology.
Goddard Space Flight Center.
The Giant Segmented Mirror Telescope, a 30-m-class, ground-based telescope.
An 8-m radio telescope satellite and the key component of Japan’s VLBI Space Observatory Program.
The High-Energy Astronomical Observatory. A series of three telescopes launched in the late 1970s and early 1980s to study x rays and gamma rays. HEAO-2 was also called the Einstein Observatory and was the first fully imaging x-ray telescope put into space.
The High Energy Solar Spectroscopic Imager.
The Hobby-Eberly Telescope.
The High-Energy Transient Explorer, a small mission launched to look for the origins of mysterious bursts of x rays and gamma rays.
HI, HII, H2—
Respectively, atomic hydrogen, ionized hydrogen, and molecular hydrogen.
The Hubble Space Telescope.
Image Reduction and Analysis Facility. A set of computer programs for working with astronomical images.
Institut de Radioastronomie Millimétrique. An international (French, German, and Spanish) institute for research in millimeter astronomy.
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.
International Space Station.
The International Ultraviolet Explorer. A joint NASA-ESA orbiting telescope to study ultraviolet radiation.
The James Clerk Maxwell Telescope.
The Large Binocular Telescope.
The Large Hadron Collider. A particle accelerator under construction at the European Laboratory for Particle Physics.
The Laser Interferometer Gravitational-wave Observatory, an NSF-sponsored project to build and operate two 4-km laser interferometers to detect gravitational waves.
The Laser Interferometer Space Antenna.
The Large Millimeter Telescope.
The Low Frequency Array, a joint Dutch-U.S. initiative to study radio wavelengths longer than 2 m.
The Large-aperture Synoptic Survey Telescope, a 6.5-m-class optical telescope.
Long-Term Space Astrophysics research program sponsored by NASA’s Office of Space Science.
See massive compact halo object.
The Microwave Anisotropy Probe mission.
Millimeter Anisotropy experiment Imaging Array, a balloon-borne millimeter-wave telescope designed to measure the angular power spectrum of fluctuations in the CMB over a wide range of angular scales.
The Multi-Element Radio Linked Interferometer Network, an array of radio telescopes distributed around Great Britain and operated by the Jodrell Bank Observatory.
Mid-size Explorer mission.
The Millimeter Array, now part of ALMA.
The Multiple Mirror Telescope.
NASA Mission Operations and Data Analysis.
National Astronomy and Ionosphere Center.
National Academy of Sciences.
National Aeronautics and Space Administration.
The Next Generation Space Telescope, an 8-m-class infrared space telescope.
The National Optical Astronomy Observatories.
The National Radio Astronomy Observatory.
National Research Council.
National Science Foundation.
The National Solar Observatory.
The National Virtual Observatory, a “virtual sky” based on enormous data sets.
One Hectare Telescope.
NASA’s Office of Space Science.
Owens Valley Radio Observatory.
An ESA-led space mission to image anisotropies in the CMB.
The DOE’s Relativistic Heavy Ion Collider at Brookhaven National Laboratory.
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.
Robotic Optical Transient Search Experiment.
The Rossi X-ray Timing Explorer, a NASA mission.
The Single Aperture Far Infrared Observatory, an 8-m-class space-based telescope.
The DOE and NSF’s Scientific Assessment Group for Experiments in Non-Accelerator Physics.
The Southern African Large Telescope.
The Solar Dynamics Observer, a successor to the pathbreaking SOHO mission.
Search for extraterrestrial intelligence.
The Space Interferometry Mission.
The Space Infrared Telescope Facility. NASA’s fourth Great Observatory will study infrared radiation.
The Square Kilometer Array, an international centimeter-wave radio telescope.
The Submillimeter Array.
Small Explorer. A NASA program to fly small, inexpensive satellites on a rapid timetable.
The Stratospheric Observatory for Infrared Astronomy. A 2.5-m telescope flown above most of Earth’s water vapor in a modified 747 aircraft to study infrared and submillimeter radiation.
The joint ESA-NASA Solar and Heliospheric Observatory.
The SOHO-Michelson Doppler Imager.
The Synoptic Optical Long-term Investigation of the Sun, a ground-based facility for solar observations that is funded by the NSF and designed and built by NSO with planned operations beginning in 2001.
The South Pole Submillimeter-wave Telescope.
NASA’s Space Science Advisory Committee.
The Solar-Terrestrial Relations Observatory.
Space Telescope Science Institute.
The Terrestrial Planet Finder, designed to study terrestrial planets around nearby stars. It is currently envisaged as a free-flying infrared interferometer
Transition Region and Coronal Explorer, a current NASA mission.
The Telescope System Instrumentation Program.
Two Micron All Sky Survey, a current ground-based project funded by the NSF and NASA.
NASA’s Ultralong-Duration Balloon program.
The Very Energetic Radiation Imaging Telescope Array System, designed to study energetic gamma rays using ground-based telescopes.
The Very Large Array. A radio interferometer consisting of 27 antennas spread out over 35 km and operating with 0.1-arcsecond resolution.
The Very Long Baseline Array. An array of radio telescopes operating as an interferometer with a transcontinental baseline and resolution of 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 diameter of Earth, or even larger when satellites are used.
The Very Large Telescope. The European Southern Observatory’s four 8-m telescopes.
VLT Infrared Spectrometer and Array Camera.
Wide Field Camera 3, an imaging instrument planned to replace the existing imaging camera onboard HST.
European x-ray space mission launched in 1999.