(Adapted, with thanks, from the full Chandra X-ray Center Glossary)

The following words, definitions and acronyms relate to concepts and sequences appearing in PASSPORT TO THE UNIVERSE program 2, LIVE FROM THE EDGE OF SPACE AND TIME.


absolute brightness The apparent brightness a star would have if it were placed at a standard distance of 10 parsecs from Earth.
absolute zero The lowest point on any temperature scale, the temperature at which all (non-quantum mechanical) motion ceases. Absolute zero occurs at zero degrees in the Kelvin scale, -273 degrees on the centigrade (Celsius) scale and -459.7 degrees on the Fahrenheit scale.
abundances of the elements The amounts of each chemical element relative to hydrogen in the Universe, or a star or in some particular object.
acceleration The rate of change of velocity of a moving object.
Active Galactic Nucleus (AGN) The central region of a galaxy that shows unusual energetic activity.
active galaxy A galaxy which shows explosive activity and can emit large amounts of energy, especially from its central regions.
Advanced X-ray Astrophysics Facility (AXAF) Renamed the Chandra X-ray Observatory and launched on July 23, 1999, the largest and most sophisticated X-ray observatory to date.
angular resolution The ability of a telescope to distinguish two adjacent objects on the sky, or to study the fine details on the surface of some object; often synonymous with "clarity" or sharpness."
antimatter Matter that has the same gravitational properties as ordinary matter, but an opposite electric charge and nuclear force charge.
apparent brightness The brightness of an object as it naturally appears in the sky.
astronomy Branch of science dedicated to the study of everything in the Universe that lies above Earth's atmosphere.
atom Building block of matter, composed of positively charged protons and neutral neutrons in the nucleus, surrounded by negatively charged electrons.


Baryons Heavy subatomic particles, such as protons and neutrons, that make up the nuclei of atoms.
Big Bang Event that cosmologists consider the beginning of the Universe, in which all matter and radiation in the entire Universe came into being.
binary star system A system which consists of two stars in orbit about their common center of mass, held together by their mutual gravitational attraction. Most stars are found in binary star systems.
Black Hole A dense, compact object whose gravitational pull is so strong that-within a certain distance of it-nothing can escape, not even light. Black holes are thought to result from the collapse of certain very massive stars at the ends of their evolution.
blue giant Large, hot, bright star at the upper left end of the main sequence on the H-R diagram. Its name comes from its color and size.
blue shift Motion-induced change in the observed wavelength from a source(of light or sound) that is moving toward us. Relative approaching motion between the object and the observer causes the wavelength to appear shorter (and hence bluer) than if there were no motion at all.
blue supergiant The very largest of the large, hot, bright, stars.
Bohr model First theory of the hydrogen atom to explain the observed spectral lines. This model rests on three ideas: that there is a state of lowest energy, that there is a maximum energy, beyond which the electron is no longer bound to the nucleus, and that within these two energies the electron can only exist in certain energy levels.
brown dwarf Clouds of collapsing gas and dust that did not contain enough mass to initiate core nuclear fusion. Such objects are then frozen somewhere along their pre-main sequence contraction phase, continually cooling into compact dark objects. Because of their small size and low temperature, they are extremely difficult to detect observationally.


center of mass The "average" position in space of a collection of massive bodies, weighted by their masses. In an isolated system this point moves with constant velocity, according to Newtonian mechanics.
Cepheid variable Star whose luminosity varies in a characteristic way, with a rapid rise in brightness followed by a slower decline. The period of a Cepheid variable star is related to its luminosity, so a determination of this period can be used to obtain an estimate of the star's distance.
closed Universe Geometry that the Universe as a whole would have if the density of matter is above the critical value. A closed Universe is finite in extent, and has no edge, like the surface of a sphere. It has enough mass to stop the present expansion, and will eventually collapse.
cold dark matter (CDM) Hypothetical class of dark-matter candidates made up of slow-moving subatomic particles, such as supersymmetric relics of a very early stage of the Big Bang.
collecting area The total area of a telescope that is capable of capturing incoming radiation. The larger the telescope, the greater its collecting area, and the fainter the objects it can detect.
conservation of energy A fundamental law of modern physics which states that the sum of the various forms of energy must always remain constant in any physical process.
Copernican revolution The realization toward the end of the sixteenth century that Earth is not at the center of the Universe.
cosmic abundances A standard listing of the relative numbers of the various elements, determined by studies of the spectral lines in astronomical objects and averaged for many stars in our cosmic neighborhood.
cosmic microwave background radiation (CMB) The microwave radiation coming from all directions that is believed to be the redshifted glow of the Big Bang.
cosmic rays Atomic nuclei (mostly protons) that are observed to strike the Earth's atmosphere with exceedingly high energies.
cosmological constant A modification of the equations of general relativity that represents a possible repulsive force in the Universe. The cosmological constant could be due to the energy density of the vacuum.
cosmology The study of the origin and evolution of the Universe as a whole.
critical mass density of Universe The cosmic density corresponding to the dividing line between a Universe that collapses back on itself and one that expands forever. The Universe is infinite in extent, and has zero curvature. The expansion will continue forever, but approach an expansion speed of zero.


dark halo A large envelope of dark matter around a galaxy that is postulated to explain the rapid rotation of galaxies and other observations.
dark matter Term used to describe the mass in galaxies and clusters whose existence we infer from rotation curves and other techniques, but which has not been confirmed by observations on any electromagnetic wavelength.
decoupling Event in the early Universe when atoms first formed, and after which photons could propagate freely through space.
Deep Space Network (DSN) NASA's three ground stations (in Spain, Australia, and California) that are used to relay commands to an orbiting spacecraft, such as the Chandra X-ray Observatory.
density A measure of the compactness of the matter within an object, computed by dividing the mass by the volume of the object.
Doppler Effect Apparent change in wavelength of the radiation from a source due to its relative motion away from or towards the observer.


eclipse Event during which one body passes in front of another, so that the light from the occulted body is blocked.
electromagnetism The union of electricity and magnetism, which do not exist as independent quantities, but are in reality two aspects of a single physical phenomenon.
electron An elementary particle with a negative electric charge. Electrons orbit the nucleus of an atom. They can be torn away from an atom by collisions with other particles or photons.
electron volt (eV) The energy gained by an electron accelerated by a potential of 1 volt.
element Matter made up of one particular atom. The number of protons in the nucleus of the atom determines which element it represents.
elementary particle One of the basic particles of matter. The most familiar of the elementary particles are the proton, neutron, and electron.
evolutionary theory A theory which explains observations in a series of gradual steps, explainable in terms of well-established physical principles.


flatness problem One of two conceptual problems with the Standard Big Bang model, which is that there is no natural way to explain why the density of the Universe is so close to the critical density.
force Action of an object that causes momentum to change. The rate at which the momentum changes is numerically equal to the force.


galactic bulge Thick distribution of warm gas and stars around the galactic center.
galactic center The center of the Milky Way, or any other, galaxy. The point about which the disk of a spiral galaxy rotates.
galactic disk Flattened region of gas and dust that bisects the galactic halo in a spiral galaxy. This is the region of active star formation.
galactic halo Region of a galaxy extending far above and below the galactic disk, where globular clusters and other old stars reside.
galactic nucleus Small central high-density region of a galaxy. Nearly all the radiation from an active galaxy is emitted from the nucleus.
Galaxy Gravitationally bound collection of a large number of stars. The Sun is a star in the Milky Way Galaxy.
galaxy cluster See cluster of galaxies.
Grand Unified Theories Theories which describe the behavior of the single force that results from unification of the strong, weak, and electromagnetic forces in the early Universe.
gravitational field Field created by an object with mass, extending out in all directions, which determines the influence of that object on all others. The strength of the gravitational field decreases as the square of the distance.
gravitational lensing Banding of light from a distant object by a massive foreground object.
gravity (gravitational force) The attractive effect that any massive object has on all other massive objects. The greater the mass of the object, the stronger is its gravitational pull.
gravitational wave The gravitational analog of an electromagnetic wave whereby gravitational radiation is emitted at the speed of light from any mass that undergoes rapid acceleration.


homogeneity Assumed property of the Universe such that the number of galaxies in an imaginary large cube of the Universe is the same no matter where in the Universe the cube is placed.
horizon problem One of two conceptual problems with the standard Big Bang model, which is that some regions of the Universe which have very similar properties are too far apart to have exchanged information in the age of the Universe.
hot dark matter A class of candidates for the dark matter in the Universe, composed of lightweight, rapidly moving particles, such as neutrinos, much less massive than the electron.
Hubble's Constant The constant of proportionality which gives the relation between recessional velocity and distance in Hubble's Law.
Hubble's Law Law that relates the observed velocity of recession of a galaxy to its distance from us. The velocity of recession of a galaxy is proportional to its distance.
Hubble Space Telescope The first large optical telescope launched above the Earth's atmosphere and carrying instruments sensitive to visible and ultraviolet light. The telescope was built by NASA with major contributions from the European Space Agency, and was launched in April, 1990.


inertia The tendency of an object to continue in motion at the same speed and in the same direction, unless acted upon by a force.
inflation Short period of extremely rapid cosmic expansion early in the history of the Universe. During inflation, the Universe swelled in size by a factor of 10 to the 50th power.
Interferometer Collection of two or more telescopes working together as a team, observing the same object at the same time and at the same wavelength. The effective diameter of an interferometer is equal to the distance between its outermost dishes.
interferometry Technique in widespread use to dramatically improve the resolution of telescopes, especially radio telescopes. Several radio telescopes observe the object simultaneously, and a computer analyzes how the signals interfere with each other.
intergalactic matter Matter in the space between galaxies.
intergalactic space The space between galaxies.
interplanetary matter Matter in the space between planets.
interstellar dust Microscopic dust grains that populate the space between stars, having their origins in the ejected matter of long-dead stars.
ion An atom with one or more electrons removed (or added), giving it a positive (or negative) charge.
ionization The process by which ions are produced, typically by collisions of electrons, ions, or photons.
irregular galaxy A strangely shaped galaxy, often rich in interstellar matter, but apparently not a member of any of the major classes of spiral or elliptical galaxies.
isotropy Assumed property of the Universe such that the Universe looks the same way in every direction.


jet A highly directed flow of matter, or radiation that comes from such a flow.


Kelvin temperature scale An internationally agreed upon temperature scale, equal to the Celsius (or centigrade) scale plus 273 degrees; hence water freezes at 273 kelvins and boils at 373 kelvins.
Kepler's Laws of Planetary Motion Three laws which summarize the motions of the planets about the Sun, or more generally, the motion of one body around another under the influence of gravity.
kiloelectron volt A unit used to describe the energy of X-rays, equal to a thousand electron volts.
kilometer A unit of distance equal to 0.6214 mile.


light-year The distance that light, moving at a constant speed of 300,000km/s, travels in one year. One light year is about 10 trillion kilometers.
Local Group The small galaxy group that includes the Milky Way Galaxy, the Andromeda Nebula, and about 20 smaller galaxies.


magnetic field Field which accompanies any electric current or changing electric field, and governs the influence of magnetized objects on one another.
matter-dominated Universe A Universe in which the density of matter exceeds the density of radiation. The present-day Universe is matter-dominated.
matter-antimatter annihilation A highly efficient process in which equal amounts of matter and anti-matter collide and destroy each other, thus producing a burst of energy, primarily in the form of gamma rays.
microwave radiation Radiation between radio and infrared wavelengths, having a wavelength between about 0.1 and 10 cm.
microwave background radiation See cosmic microwave background radiation.
Milky Way Galaxy The specific galaxy to which the Sun belongs, so named because most of its visible stars appear overhead on a clear, dark night as a milky band of light extending across the sky.
molecule A tightly bound collection of atoms held together by the electromagnetic fields of the atoms. Molecules, like atoms, emit and absorb photons at specific wavelengths.
momentum A measure of the state of motion of a body; the momentum of a body is the product of its mass and velocity. In the absence of a force, momentum is conserved.


neutrino An electrically neutral elementary particle that is one of the products of nuclear fusion reactions. Neutrinos have little or no mass, move at close to the speed of light, and interact very weakly with matter.
neutron An elementary particle with roughly the same mass as a proton, but which is electrically neutral. Along with protons, neutrons form the nuclei of atoms.
neutron star A dense ball of neutrons that remains after a supernova has destroyed the rest of the star. Typically neutron stars are about 20km across, and contain more mass than the Sun.
Newtonian mechanics The basic laws of motion, postulated by Newton, which are sufficient to explain and quantify virtually all of the complex dynamical system behavior found on Earth and elsewhere in the Universe.
nuclear force The force that binds protons and neutrons within atomic nuclei, and which is effective only at distances less than about 10 -13 centimeter.
nucleosynthesis The building up of heavy elements from lighter ones by nuclear fusion.
nucleus Dense, central region of an atom, containing both protons and neutrons, and orbited by one or more electrons.


omega symbol used to denote the ratio of the mass density of the Universe to the critical mass density.
opacity A quantity that measures a material's ability to block electromagnetic radiation. Opacity is the opposite to transparency.
open Universe Geometry that the Universe would have if the density of matter were less than the critical value. In an open Universe there is not enough matter to halt the expansion of the Universe. An open Universe is infinite in extent.


pair production Process in which a very high energy photon interacts with another photon or a strong magnetic field to give rise to a particle-anti-particle pair.
partial eclipse Celestial event during which only a part of the occulted body is blocked from view.
period The time needed for a complete cycle of repetitive motion; for example the time for an orbiting body to complete one revolution about another body, or the time needed for a pendulum to make a complete swing.
photon Individual packet of electromagnetic energy that makes up electromagnetic radiation.
positron Atomic particle with properties identical to those of a negatively charged electron, except for its positive charge. This positron is the antiparticle of the electron. Positrons and electrons annihilate each other when they meet, producing pure energy in the form of gamma rays.
primordial nucleosynthesis The production of elements heavier than hydrogen by nuclear fusion in the high temperatures and densities which existed in the early Universe.
proton An elementary particle, carrying a positive electric charge, a component of all atomic nuclei. The number of protons in the nucleus of an atom dictates what type of atom it is.
proton-proton chain The chain of fusion reactions, leading from hydrogen to helium, that powers main-sequence stars.


quantum A discrete physical unit into which something can be divided, for example a photon, or the difference in energy states in an atom.
quantum mechanics The framework of laws governing the behavior of particles on atomic and subatomic levels. It starts with the assumption that energy is not infinitely divisible, but comes in quanta.
quark A fractionally charged, basic building block of protons, neutrons, and other elementary particles.


radiation A way in which energy is transferred from place to place in the form of a wave. Light is a form of electromagnetic radiation.
radiation-dominated Universe Early epoch in the Universe, when the density of radiation in the cosmos exceeded the density of matter.
radio galaxy Type of active galaxy that emits most of its energy in the form of long-wavelength radiation.
red dwarfs Small, cool faint stars at the lower-right end of the main sequence on the H-R diagram, whose color and size give them their name.
red giant A giant star whose surface temperature is relatively low, so that it glows with a red color.
red shift Change in the wavelength of light emitted from a source moving away from us. The relative recessional motion causes the wave to have an observed wavelength longer (and hence redder) than it would if it were not moving. The cosmological redshift is caused by the stretching of space as the Universe expands.
red supergiant An extremely luminous and large red star.
relativistic particle A particle moving at nearly the speed of light.
relativity, general theory of A theory formulated by Einstein that describes how a gravitational field can by replaced by a curvature of space-time.
relativity, special theory of A theory formulated by Einstein that describes the relations between measurements of physical phenomena by two different observers who are in relative motion at constant velocity.


satellite A body that orbits around a planet, such as the Moon (a natural satellite) or the Chandra X-ray Observatory (an artificial telescope), both of which orbit around the Earth.
scientific method The set of rules used to guide science, based on the idea that scientific "laws" be continuously tested, and replaced if found inadequate.
Seyfert galaxy Type of active galaxy that exhibits intense energetic activity from a small region within the nucleus of an otherwise normal-looking spiral galaxy.
singularity A point in the Universe where the density of matter and the gravitational field are infinite, such as the center of a black hole.
(SIRTF) Space Infrared Telescope Facility NASA's planned Great Observatory for infrared astronomy, scheduled to orbit Earth around the turn of the century.
spacetime A synthesis of the three dimensions of space and of a fourth dimension, time; a hallmark of relativity theory.
spectroscopy The study of the way in which atoms absorb and emit light electromagnetic radiation Spectroscopy allows astronomers to determine the chemical composition of stars.
speed of light The fastest possible speed, according to the currently known laws of physics. Electromagnetic radiation exists in the form of waves or photons moving at the speed of light.
spiral arm Distribution of material in a galaxy in a pinwheel-shaped design apparently emanating from near the galactic center.
spiral galaxy Galaxy composed of a flattened, star-forming disk component which may have spiral arms and a large central galactic bulge.
"standard candle" Any object with an easily recognizable appearance and known luminosity, which can be used in estimating distances. Supernovae, which all have the same peak luminosity (depending on type) are good examples of standard candles and are used to determine distances to other galaxies, see also Cepheid variable.
star A glowing ball of gas held together by its own gravity and powered by nuclear fusion in its core.
star cluster A grouping of anywhere from a dozen to a million of stars which formed at the same time from the same cloud of interstellar gas. Stars in clusters are useful to aid our understanding of stellar evolution because they are all roughly the same age and chemical composition, and lie at roughly the same distance from the Earth.
stellar evolution The changes experienced by stars as they originate, mature, and grow old.
stellar nucelosynthesis The formation of heavy elements by the fusion of lighter nuclei in the hearts of stars. Except for hydrogen and helium, all other elements in our Universe result from stellar nucleosynthesis.
strong force See "nuclear force."
supercluster Grouping of several clusters of galaxies into a larger, but not necessarily gravitationally bound, unit.
supergiant A star with a radius between 100 and 1000 times that of the Sun.
supermassive black hole A black hole with a mass much greater than the most massive stars (100 solar masses). The central regions of virtually every galaxy are thought to contain a supermassive black hole of a million solar masses or more.
supernova Explosive death of a star, caused by the sudden onset of nuclear burning (type I), or gravitational collapse followed by an enormously energetic shock wave (type II). One of the most energetic events of the Universe, a supernova may temporarily outshine the rest of the galaxy in which it resides.
supernova remnant The expanding glowing remains from a supernova.


telescope Instrument used to capture as many photons as possible from a given region of the sky and concentrate them into a focused beam for analysis.
temperature A measure of the amount of heat in an object, and an indication of the speed of the particles that comprise it.
time dilation A prediction of the theory of relativity, closely related to the gravitational redshift. To an outside observer, a clock lowered into a strong gravitational field will appear to run slow.
total eclipse Celestial event during which one body is completely blocked from one view by another.


ultraviolet Region of the electromagnetic spectrum, just outside the visible range, corresponding to wavelengths slightly shorter than blue light.
Universe The totality of all matter, radiation and space; everything accessible to our observations.


visible spectrum The small range of the electromagnetic spectrum that human eyes perceive as light. The visible spectrum ranges from about 4000 to 7000 angstroms, corresponding to blue through red light.
visible light The small range of the electromagnetic spectrum that human eyes perceive as light. The visible spectrum ranges from about 400 to 700 nm, corresponding to blue through red light.
visual binary A binary star system in which both members are resolvable from Earth.
void Large, relatively empty region of the Universe around which superclusters of galaxies are organized.


wave A pattern that repeats itself cyclically in time and space. Waves are characterized by the velocity with which they move, their frequency, and their wavelength.
wavelength The distance between successive crests of a wave.
weak force The nuclear force involved in radioactive decay. The weak force is characterized by the slow rate of certain nuclear reactions-such as the decay of the neutron, which occur with a half-life of 11 min.
white dwarf A star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size; such a star is near its final stage of life.
white light Visible light that contains approximately equal proportions of all colors.


X-ray Region of the electromagnetic spectrum corresponding to radiation of high frequency and short wavelengths, far outside the visible spectrum.



zero-age main sequence The region on the H-R diagram, as predicted by theoretical models, where stars are located at the onset of nuclear burning in their cores.