aberration The small angular displacement of a star’s apparent position due to the Earth’s orbital motion and the finite speed of light. The tiny displacement oscillates with a period of one year.
absolute zero The lowest possible temperature (–273ºC), at which all molecular motion and thermal radiation ceases.
accretion The collection of material together to form a star, planet, or moon, usually mediated by a rotating disk.
active galaxy A galaxy with an unusually strong output of energy, thought to be powered by a supermassive black hole in its core.
ammonia A molecule composed of one nitrogen atom and three hydrogen atoms (NH3).
Andromeda Galaxy The nearest large spiral galaxy to the Milky Way, about 2.5 million light-years away. Also known as M31.
angular momentum The momentum a body possesses due to its rotation or its motion in a curve, such as orbital motion, defined mathematically as the product of mass, speed, and distance from a central axis.
antimatter Matter made of particles with the same mass as the corresponding particles of conventional matter, but with an opposite electrical charge.
arc A measure of angular separation in the sky. The angle from the horizon to the zenith (directly overhead) is 90 degrees of arc. Both the Moon and the Sun span one-half degree of arc. One degree = 60 minutes and one minute = 60 seconds of arc.
archaea The third major domain of life, after bacteria and eukaria. They are single-celled microbes without a nucleus, like bacteria, but genetically distinct from them. Many archaea thrive in high temperature environments.
asteroid A small rocky or metallic body that orbits a star.
asteroid belt The region of the solar system where most of the asteroids orbit. It lies between the orbits of Mars and Jupiter.
astrobiology The study of extraterrestrial life.
astrometry The branch of astronomy that deals with the precision measurement of positions and motions of stars.
astronomical unit (AU) The average distance between the Earth and the Sun (149.6 million kilometers), used as a convenient “yardstick” for distances in the solar system.
astronomy The scientific study of the universe.
astrophysics The branch of astronomy that deals with the physics of astronomical objects and phenomena.
atmosphere The gaseous envelope surrounding a star, planet, or satellite, and bound to it by gravity.
atom The smallest unit of matter that retains the properties of a chemical element. An atom consists of a positively-charged nucleus surrounded by at least one electron.
ballistic trajectory The path of a projectile determined by the force of gravity alone.
barycenter The center of mass of any system of bodies moving under their mutual gravity.
baryonic Ordinary matter, consisting of protons and neutrons.
Big Bang The fiery birth of the observable universe in an explosion of space itself, which occurred at some time between 12 and 15 billion years ago. According to the prevailing theory, the Big Bang launched the observed expansion of the universe that continues to this day.
binary star system Two stars orbiting their common center of gravity.
biochemistry The chemical processes of living organisms; the scientific study of those processes.
biosphere The part of a planet or moon (its atmosphere, waters, soil, and crustal rock) in which living organisms exist.
bipolar Having or related to two poles.
black hole A region in space where gravity is so strong that space closes back on itself, allowing nothing, not even light, to escape.
blueshift The shortening of light waves due to the motion through space of a light source and an observer toward each other (the Doppler effect). It is so named because blue light is at the short-wavelength end of the visible spectrum.
blue straggler A star that forms in a globular cluster from the collision or merger of two stars. It is hotter and bluer than its non-coalesced counterparts.
brown dwarf An astronomical object with mass in the range between a planet and a star (greater than 1.3 percent and less than 8 percent the mass of the Sun). Brown dwarfs have a brief phase of weak nuclear fusion of deuterium (heavy hydrogen), but never become hot enough to fuse hydrogen, as do stars.
carbon An element with six protons, formed during the red giant phase of stars. Carbon is the chemical basis of all terrestrial life.
carbon dioxide A molecule consisting of one carbon atom and two oxygen atoms (CO2). Its gas is denser than air. Its solid form is called “dry ice.”
carbon monoxide A molecule consisting of one carbon and one oxygen atom (CO). One of the most common molecules in the universe.
Cartesian One who follows the philosophy of René Descartes regarding his logical analysis or interpretation of nature.
centrifugal force The apparent force that seems to push an object moving in a curved path away from the axis of rotation or center of orbital motion.
circumstellar disk Dust and gas forming a disk in orbit around a star. Some circumstellar disks may contain planetary systems.
comet A small solar system body made of ice and dust that moves in an elliptical orbit around the Sun. A typical comet has a solid nucleus a few kilometers in diameter. When it nears the inner solar system, ices evaporate and form an extended and diffuse atmosphere, which is blown away from the Sun by the solar wind and radiation pressure to form a prominent tail of gas and dust.
cosmic Of or relating to the universe as a whole.
cosmic background radiation The microwave energy observed from all directions in the sky, at an equivalent temperature of 2.7 degrees above absolute zero, and interpreted as the residual glow from the Big Bang.
cosmic horizon The apparent edge of the observable universe, at the distance from us in all directions that light has traveled in the age of the universe.
cosmic rays Fast-moving, high energy, subatomic particles, mainly protons, that permeate the galaxy.
cosmic time The absolute time common to all observers in our universe who are not moving through space; time in the world map (see definition).
cosmological constant A theoretical long-range repulsive pressure that may accelerate the expansion of the universe. Originally, a term introduced by Albert Einstein in his equations of General Relativity to allow for a model of the universe that neither expanded nor contracted.
cosmology The astrophysical study of the universe as a whole, including its origin, evolution, structure, and dynamics.
cosmos The universe regarded as a whole, including all matter, energy, and space.
crater A bowl-shaped depression on a planet or moon created from above by the impact of an extraterrestrial body or from below by a volcanic eruption.
Cretaceous The geological Period from 144 to 65 million years ago. The so-called K/T boundary between the Cretaceous and the subsequent Tertiary Period marks the extinction of the dinosaurs.
critical density The average density of matter that would just allow the universe to expand forever.
crust The outermost solid layer of the Earth or of similar bodies.
cryogenic Relating to processes at extremely low temperatures or to the technology used to produce such temperatures.
crystal carbon lattice The highly compressed and crystalline form of carbon that comprises a white dwarf star.
dark matter Unseen matter that is detected only by its gravitational pull on visible matter. Most of the universe is evidently made of dark matter. Its nature is yet to be determined.
dark matter halo A roughly spherical halo of dark matter that surrounds a galaxy, including the Milky Way, and extends far beyond the region of luminous stars.
deep sea vent Seafloor vent that releases hot, mineral-rich water from fissures at a mid-ocean ridge. Deep sea vents often create sulfide chimneys that rise up from the ocean floor like stalagmites and support biological communities based on chemical energy.
density The amount of matter in a prescribed volume of material.
detritus Any loose matter derived directly from an older source by disintegration or erosion and transported from its place of origin.
Doppler effect The shift of the observed wavelengths of light, relative to the emitted wavelengths, due to the motion through space of the light source relative to the observer. A similar effect occurs for sound.
dwarf galaxy The smallest and most common kind of galaxy.
eclipse The obscuration of one celestial body caused by its passage behind or through the shadow cast by another body.
electromagnetic spectrum The complete array of electromagnetic radiation (light). In order of increasing wavelength (decreasing frequency and energy), the spectrum ranges from gamma rays through X-rays, ultraviolet light, visible light, infrared radiation, microwaves to radio waves.
electron A small subatomic particle with a negative electric charge. Electrons bound to the nucleus of an atom are responsible for its chemical properties.
element A substance composed of atoms having the same number of protons in each nucleus.
elliptical galaxy A galaxy that appears round or elliptical and lacks a disk with spiral arms. Such galaxies have little dust and gas, and show few signs of ongoing star formation.
energy Any attribute of matter or electromagnetic radiation that can be converted into motion. Energy can be converted among its various forms (motion, light, mass, etc.) but the total amount of energy remains constant.
escape velocity The speed required to escape the gravitational hold of a celestial body.
Euclidean Characterizing the “flat” geometry developed by the ancient Greek mathematician Euclid, in which parallel lines never cross and the angles of a triangle sum to 180 degrees. This is the geometry of familiar experience, in which space is not curved.
exobiology The study of evidence relevant to life on other worlds (synonymous with astrobiology).
exponential A process that changes at an accelerating rate, for example with a constant doubling time.
extrasolar Located or occurring outside the solar system, usually in other star systems.
extraterrestrial Located or originating outside the Earth and its atmosphere.
field A distribution throughout space of some physical quantity, such as magnetism or gravity.
fluorescence The absorption of light by a substance at some wavelength and its re-emission at one or more longer wavelengths.
fusion See nuclear fusion.
galactic disk The flat disk of a spiral galaxy, which includes young stars and the gas and dust clouds from which they are formed.
galaxy A massive, gravitationally-bound assembly of stars, interstellar clouds, and dark matter.
gamma rays Highly energetic photons, having the shortest wavelengths and the highest frequencies in the electromagnetic spectrum.
gas-giant planet A giant planet with a massive and deep atmosphere that surrounds a relatively small rocky core.
general relativity The modern theory of gravity, introduced by Albert Einstein in 1916; it describes gravity as the curvature or warping of space due to the presence of matter.
giant star A highly luminous star, approaching the end of its life, with an extended, tenuous atmosphere surrounding a hot core depleted in hydrogen.
globular cluster A dense spherical cluster of hundreds of thousands of stars bound together by gravity.
gravitational field The distribution through space of the attractive influence of the mass of a body.
gravitational lensing The deflection, distortion, or magnification of light from a distant source, due to its passage through the gravitational field of a foreground mass.
gravitational waves Disturbances or ripples in the fabric of space produced by violent events in the cosmos.
gravity The force of attraction acting between any two masses (according to Isaac Newton); the curvature of space by matter (according to Albert Einstein).
greenhouse effect The warming of a planet that occurs when visible sunlight is absorbed by the surface and re--radiated as infrared light which is then trapped by gases in the atmosphere.
helium A lightweight, chemically-inert element created in the Big Bang and in stars by the fusion of hydrogen.
Hubble Deep Field An image produced by pointing the Hubble Space Telescope at one area of the sky for 150 consecutive orbits, resulting in the “deepest-ever” optical image of the universe.
Hubble diagram A diagram that plots the redshifts or recession velocities of galaxies versus their distances or apparent magnitudes.
Hubble law The law stating that distant galaxies are moving away from us at rates proportional to their distances.
hydrocarbon A chemical compound containing only hydrogen and carbon atoms.
hydrogen The lightest, simplest, and the most abundant element in the universe.
hypersonic Related to or capable of speeds equal to or greater than five times the speed of sound.
hyperthermophiles Single-celled bacteria and archaea that live at temperatures exceeding about 80ºC. The term means “extreme heat lovers.”
inflation A modification of Big Bang theory which proposes that the infant universe went through a very brief period of extremely rapid exponential expansion. Inflation suggests that the universe is vastly larger than its observable part.
infrared Invisible electromagnetic radiation (light) with wavelengths longer than red light and shorter than microwaves. Infrared light occupies the spectral band extending from 0.75 to about 200 micrometers.
interferometric telescope An observatory that combines the electromagnetic waves gathered by two or more separate telescopes so that the waves cancel (interfere) when out of phase. It can identify sources separated by extremely small angles.
intergalactic Occurring or existing in the space between the galaxies.
interplanetary Occuring or existing in the space between planets.
interstellar Occurring or existing in the space between the stars.
ion An atom with a net positive or negative electric charge, due to an unequal number of protons and electrons.
ionized gas A superheated gas partially or totally composed of ions.
kilometer (abbreviated km) A unit of length equal to 1,000 meters, or 0.62 miles.
kiloton The explosive energy equivalent of 1,000 tons of TNT (or 4.2 x 1019 ergs).
kinetic energy The energy inherent in a body due to its motion.
Kuiper Belt A donut-shaped region of comets in orbit beyond Neptune, assumed to be the oldest surviving remnant of the original solar nebula and the source of short-period comets.
Kuiper Belt objects (KBOs) The comets that populate the Kuiper Belt.
law of gravitation The law stating that any two bodies attract each other with a force that increases in proportion to their masses and decreases in proportion to the square of the distance between them (discovered by Isaac Newton).
laws of orbital motion Three laws discovered by Johannes Kepler: (1) A planet follows an elliptical orbit with the Sun at one focus. (2) The line from a planet to the Sun sweeps out equal areas in equal times, so the planet moves faster the closer it is to the Sun. (3) The square of a planet’s orbital period is proportional to the cube of its average distance from the Sun, so distant planets have longer periods than close ones. These laws apply to all bodies in gravitational orbits, not just planets.
light-year The distance that light travels in one year (63,000 astronomical units, or 9.46 trillion kilometers), a convenient unit of measurement for interstellar distances.
Local Group A small group of about two dozen galaxies, including its two largest members, the Milky Way and the Andromeda Galaxy.
long-period comet A comet with an orbital period exceeding about 200 years. Such long-period comets have very elongated elliptical orbits, and can have periods of more than a million years. They originate from the Oort cloud in the outermost reaches of our solar system.
lookback time How long ago a distant object emitted the light we now see. For example, when we see the Andromeda Galaxy, which is 2.5 million light-years away, we are looking back 2.5 million years in time.
luminosity The power output of a star or other luminous body.
macho (Massive Compact Halo Object) An unseen stellar or planetary body that may contribute to the dark matter in galaxies.
magellanic clouds Two irregular satellite galaxies of the Milky Way, visible even to the naked eye in the southern skies.
magnetic field The region of space influenced by a magnetized body. Magnetic fields, due to motion involving charged or magnetic material, are found within the universe at every scale—from the atomic to the galactic.
magnetometer An instrument used to measure a magnetic field.
magnetosphere The volume of space around a star or a planet in which the global magnetic field influences the motion of charged particles.
magnitude A measure of the brightness of an astronomical object on a logarithmic scale, with increasing numbers corresponding to decreasing brightness.
main sequence The diagonal sequence of stars on a plot of stellar luminosity versus surface temperature, ranging from very luminous hot stars to relatively cool stars of low luminosity. Stars on the main sequence are in their stable mid-life phase of hydrogen fusion. More than ninety percent of all stars are found on the main sequence.
mantle The part of the Earth (or other rocky body) lying between the outer crust and the central core.
mean motion The average angular speed of a body in its orbit.
megaparsec (mpc) 3.26 million light-years.
megaton The energy equivalent of a million tons of TNT.
meteor A bright streak of light produced by a small fragment of rock or metal that burns up as it enters the atmosphere.
meteorite A fragment of rock or metal that has landed on the Earth from interplanetary space. Most meteorites come from the asteroids, but a few are from other planets or satellites.
meter The principal unit of length in the metric system, equal to 3.28 feet.
methane The simplest hydrocarbon molecule, made of one carbon and four hydrogen atoms (CH4). Methane is the principal constituent of natural gas.
microbe (also microorganism) A microscopic single-celled living organism.
micrometer (or micron) A unit of length equal to one-millionth of a meter.
microwave Electromagnetic radiation with wavelengths between about one millimeter and thirty centimeters, intermediate between infrared light and radio.
Milky Way The faint band of light stretching across the sky, due to myriad faint stars and nebulas; the name of the spiral galaxy containing our solar system.
molecular cloud A large interstellar cloud of gas and dust with temperatures low enough for atoms to combine as molecules. Giant molecular clouds are the main regions of star and planet formation in galaxies.
molecule The smallest unit of matter that retains the properties of a chemical compound. It consists of two or more atoms held together by chemical bonds between the outer electrons.
momentum The measure of the motion possessed by a body, the product of its mass and velocity.
nanometer One-billionth of a meter.
natural selection The mechanism of biological evolution. The differential selection of traits most suitable for survival in reproducing entities (usually living organisms) subject to mutation.
nebula (pl. nebulae or nebulas) An immense cloud-like mass of interstellar gas and dust, generally in the spiral arms of a galaxy.
neutrino A subatomic particle with no electric charge and little or no mass that travels at nearly the speed of light. Neutrinos are produced in large quantities by nuclear reactions in stars.
neutron An electrically neutral subatomic particle found in the nucleus of all atoms except ordinary hydrogen.
neutron star An extremely dense collapsed star consisting mainly of neutrons. A neutron star is what often remains after the supernova explosion of a massive star.
nuclear Of or relating to the atomic nucleus.
nuclear fusion The energy-producing process in stars and the source of all the heavier elements; the combining of two atomic nuclei at high temperature to form a heavier nucleus..
nuclear physics The study of the forces, behavior, and internal structures of atomic nuclei.
nucleus (pl. nuclei) The “core” of an atom, containing the atom’s protons and neutrons.
observable universe The part of the universe lying within our cosmic horizon.
observational selection The tendency to record those objects or phenomena that are most readily observed with the available tools or techniques, and to overlook the others.
Olbers’ paradox The puzzle of why the sky is dark at night in an infinite universe of stars. The paradox, attributed to Heinrich Olbers in 1823, was first recognized by Johannes Kepler in 1610. Solutions to the paradox invoke the cosmic horizon, the expanding universe, and the fact that the stars do not live long enough to fill up space with light.
Oort cloud A spherical cloud of trillions of comets extending about halfway to the nearest stars and weakly bound by the Sun’s gravity. Long-period comets originate from the Oort cloud.
orbit The path of one celestial body moving around another under the force of gravity.
orbital period The time interval for a body to complete one orbit around another.
order of magnitude A factor or “power” of ten. Two orders of magnitude is a hundred, three is a thousand, etc.
organic molecules Molecules formed by chemical bonds mainly between carbon atoms. Life on Earth is based on organic molecules.
Orion Nebula A large interstellar cloud of gas and dust giving birth to stars in the constellation of Orion, about 1,500 light-years away.
oxygen An element consisting of atoms with eight protons. Two oxygen atoms combine to make molecular oxygen (O2) and three make ozone (O3). The Earth’s atmosphere is twenty-one percent molecular oxygen.
ozone A molecule consisting of three oxygen atoms (O3). Ozone in the Earth’s stratosphere absorbs most of the potentially lethal solar ultraviolet radiation, preventing it from reaching the surface.
parabola A curve that describes both the cross-section of a reflector that focuses rays to a point and the orbit of a body having escape velocity.
parallax The difference in the direction of an object when viewed from two different locations. If the object is a star and the two locations are at opposite ends of the Earth’s orbit, the measured angle of parallax allows one to calculate the distance to the star.
Pauli exclusion principle A basic law of quantum mechanics named for Wolfgang Pauli stating that no two electrons may occupy the same small volume of space simultaneously. This law accounts for the properties of the chemical elements and the pressure that supports white dwarf stars.
perturbation A slight disturbance in the orbit of one celestial body around another caused by the gravitational attraction of a third body
photodetector A device to detect and measure the intensity of light.
photometric Relating to the measurement of light.
photon A particle or wave packet of light.
photosynthesis The chemical process by which green plants use solar energy to synthesize carbohydrates from carbon dioxide and water, releasing molecular oxygen as a byproduct.
physics The study of matter and energy, and the forces and fields by which they interact in space and time.
planet An astronomical body with enough mass for its gravity to make it spherical but not enough to generate nuclear energy. Planets have non-intersecting orbits around stars or drift freely in space.
planetary nebula An expanding shell of luminous gas that surrounds a small white dwarf star. The ionized shell receives ultraviolet light from the hot white dwarf and re-emits it as colorful visible light by fluorescence. (Planetary nebula have nothing to do with planets, the term is a historical misnomer.)
planetesimal One of the family of asteroid-sized bodies that first condensed out of the disk of the solar nebula and later collided to form the planets.
plasma A hot gas consisting of ionized atoms and free electrons. A fourth state of matter (as distinct from solids, liquids, and normal gases) at high temperatures.
plume An upwelling of matter moving away from a source of heat.
plutino A subclass of Kuiper Belt objects which, like Pluto, orbit the Sun twice during every three orbits of Neptune.
proton A positively-charged subatomic particle. Every atomic nucleus contains one or more protons.
protoplanetary disk A disk of dust, gas, and perhaps developing planets orbiting a young star. A transitional stage between a solar nebula and a solar system.
protostar A gravitationally contracting gas cloud in the early stage of star formation, before fusion begins in its core.
pulsar A rapidly spinning neutron star that emits radio energy at regular intervals and is thereby observed on Earth as a pulsating radio source.
quantum mechanics The branch of physics that describes the interaction of matter and radiation at the atomic and subatomic levels, based on the fact that energy is observed in discontinuous units or quanta rather than being continuously divisible.
quasar The highly luminous core of a remote galaxy, thought to be powered by a supermassive black hole. Quasars look like stars on an ordinary photograph but have very different spectra.
radiation The emission of energy by waves (including light) or particles.
radioactivity The emission of energetic subatomic particles and/or gamma rays from the decay of unstable atomic nuclei.
radio astronomy The study of the universe using observations of the natural radio emissions from celestial objects.
radio telescope A radio antenna or dish-shaped reflector used to collect and detect radio emissions from astronomical sources.
radio waves Low energy electromagnetic radiation, with long wavelengths and low frequencies.
rarefied gas A gas with extremely low density.
red giant A large, highly luminous and relatively cool (red) star at a late stage of its life, once it has exhausted its core hydrogen fuel.
redshift The lengthening of light waves due to the motion through space of a light source and an observer away from each other (the Doppler red shift), or due to the expansion of space itself (the cosmic redshift). It is so-named because red light is at the long wavelength end of the visible spectrum.
refraction A change in direction or the bending of light rays as they pass at an angle through different transparent substances, such as glass, water, or air.
resolution The ability of a telescope to distinguish adjacent objects or of a spectrograph to distinguish adjacent wavelengths; the clarity of an image.
resonance One of the natural states of oscillation in a physical system, such as the periodic swing of a pendulum or vibration of a spring.
right ascension A measure of longitude in hours along the celestial equator.
rogue planet A planet-sized body that escaped its host planetary system and is not gravitationally bound to a star.
satellite A body that orbits around a larger body.
seismometer An instrument that detects vibrations in the ground from distant earthquakes.
SETI An acronym for the search for extraterrestrial intelligence, currently conducted with radio telescopes and optical detectors.
shock wave An abrupt wave (with higher pressure, temperature, and density) that passes through a medium when a disturbance, such as an explosion, generates motions that exceed the speed of sound in the medium.
short-period comet A comet with an orbital period less than about 200 years, the most famous example being Halley’s Comet, which appears every seventy-six years. Short-period comets come from the Kuiper Belt and typically orbit the Sun in the same direction as the planets.
sidereal Of or related to the stars.
solar mass The amount of mass in the Sun, a convenient unit for expressing the mass of other stars.
solar nebula The cloud of gas and dust that formed the young Sun and the surrounding planets.
solar system The Sun and all the objects bound to it by gravity (planets, satellites, asteroids, comets).
spallation The process in which small quantities of rock near the surface around a meteorite impact are ejected at high speed with only slight impact shattering.
spectroscope An optical instrument designed to spread out light into the spectrum of its component colors.
spectrum (pl. spectra) The range of electromagnetic radiation (light) expressed in terms of frequency or wavelength. A rainbow displays the spectrum of visible light.
spiral galaxy A system of billions of stars with a central bulge of older stars surrounded by a flat disk with spiral arms of gas and dust nebulas and young stars. The Milky Way is a spiral galaxy.
standard candle A class of celestial objects of known luminosity, which can be used to find the distance to associated objects. For example, a supernova of known luminosity can be used as a standard candle to calculate the distance to the galaxy in which it occurs.
star A self-luminous body held together by gravity and with a central temperature sufficient to liberate energy by nuclear fusion.
stellar Relating to or consisting of stars.
stellar sarallax The minute shift in the apparent position of a star, relative to the pattern of more distant stars, when observed from opposite sides of the Earth’s orbit.
stellar wind The outflow of charged particles that a star emits into interstellar space.
strong force The force that holds the atomic nucleus together.
subduction The descent of a slab of crustal rock into the interior of a planet where two tectonic plates converge.
supercluster A congregation of clusters of galaxies.
supergiant An extremely luminous star with an extended tenuous atmosphere.
supernova (pl. supernovae or supernovas) The catastrophic explosion of a star, which blows off most of its mass, increasing in brightness by as much as a billion times. A Type I supernova is due to the thermonuclear detonation of a compact white dwarf star which becomes unstable by accreting mass from an orbiting companion star. A Type II supernova results from the gravitational collapse of a massive star that has exhausted its nuclear fuel.
supersonic Relating to velocities greater than the speed of sound in a given medium.
tectonic Relating to the internal processes that deform the outer layers of a terrestrial planet or moon.
telescope An instrument designed to gather and focus electromagnetic radiation (light) to study celestial objects and events.
terraform The process of generating an Earth-like environment on celestial bodies that are currently inhospitable.
terrestrial Having to do with the Earth or other rocky/metallic planets.
thermal Relating to heat or heat transfer.
thermonuclear Relating to the fusion of light atomic nuclei at high temperatures and the associated release of nuclear energy.
tidal friction The dissipation of energy in a planet or satellite due to the friction from tides. The periodic deformation of the solid body and/or any oceans by tidal gravitational forces produces heat.
topography The surface elevation of land and its variations.
trajectory The curving path of a body in motion through space.
tsunami A very large ocean wave that can be produced by an underwater earthquake, landslide, volcanic eruption, or the rare impact of a comet or asteroid in the ocean.
T Tauri star A class of young star with variable luminosity, thought to be in the process of gravitational contraction before its arrival at the main sequence where it will begin to fuse hydrogen into helium.
ultracentrifuge A laboratory device to produce centrifugal forces thousands of times stronger than gravity.
ultraviolet radiation (UV) Invisible electromagnetic radiation (light) with wavelengths shorter than violet light and longer than X-rays. Most of the UV light from the Sun is absorbed by the Earth’s atmospheric ozone layer before it reaches the surface.
universe The physical system that encompasses all matter, energy, and space that exists (see observable universe).
vacuum In classical physics, a region of space devoid of matter. In quantum theory, the vacuum is seething with subatomic “virtual particles” of matter and antimatter that rapidly materialize and vanish again.
velocity The speed and direction of an object’s motion.
viscous Characterized by resistance to flow due to internal friction within a fluid.
visible light The portion of the electromagnetic spectrum corresponding to the visible colors, with wavelengths longer than ultraviolet light and shorter than infrared radiation.
volatile Able to vaporize at relatively low temperature. Ices of water, methane, carbon dioxide, and ammonia are volatile.
weak force The force that mediates radioactive decay in the atomic nucleus. It is weaker than the strong force that holds the nucleus together and the force of electromagnetism, but stronger than gravity.
white dwarf The hot, collapsed core of a red giant star after it has expelled its outer layers and ceased to produce energy by fusion. A white dwarf has a mass comparable to that of the Sun but is no larger than the Earth.
wimp (Weakly Interacting Massive Particle) Hypothetical subatomic particle proposed as a candidate for dark matter.
world map The universe as it actually exists at a given instant of cosmic time, with all parts depicted at the identical age. The world map represents the universe as it would appear if the speed of light were infinite. It has no cosmic horizon.
world picture The universe as actually observed, with its more remote parts appearing younger than regions closer to the observer, due to the finite speed of light. The world picture is bounded by a cosmic horizon.
X-ray Electromagnetic radiation with wavelengths shorter than ultraviolet light but longer than gamma rays.
X-ray astronomy The branch of astronomy that studies celestial objects by examining the X-rays they emit.
