(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 1, LIVE FROM A BLACK HOLE.


accretion disk Flat disk of matter spiraling down onto the surface of a star or into a black hole. Often, the matter originated on the surface of a companion star in a binary system. (See Binary star)
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 momentum The momentum of rotation about a fixed axis.
astronomy Branch of science dedicated to the study of everything in the universe that lies above Earth's atmosphere.
atmosphere A layer of gas confined close to a planet's surface by the force of gravity.
atom Building block of matter, composed of positively charged protons and neutral neutrons in the nucleus, surrounded by negatively charged electrons.


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. Stellar-mass black holes are thought to result from the collapse of certain very massive stars at the ends of their evolution. (See also supermassive black holes)
blue giant Large, hot, bright star at the upper left end of the main sequence on the H-R (Hertzprung-Russell, q.v.) diagram. Its name comes from its color and size.
blue shift Motion-induced change in the observed wavelength from a source 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.


Chandrasekhar limit The upper limit to the mass of a white dwarf (equals 1.4 times the mass of the Sun).
Chandra X-ray Observatory (CXO) Formerly called AXAF, Chandra was launched July 23, 1999, and is the largest and most sophisticated X-ray observatory to date.
charge-coupled device (CCD) An electronic device used to detect photons, composed of many tiny pixels, each of which records a buildup of charge to measure the amount of light striking it.
clusters of galaxies Galaxies can swarm together to form groups and clusters of galaxies held together by their mutual gravitational attraction. X-ray observations show that these enormous systems of galaxies are filled with colossal clouds of hot gas. These clouds have temperatures as high as a hundred million degrees and contain as much mass as all the stars in the galaxies in the cluster.
constellation A grouping of stars in the night sky into a recognizable pattern. Most of the constellations get their name from the Latin translation of one of the ancient Greek star patterns that lies within it. In more recent times, a number of additional groups were introduced by contemporary astronomers, and there are now 88 standard configurations recognized.
core The central region of a planet, star, or galaxy.


dark dust cloud (dark nebula) A region of interstellar space containing a rich concentration of gas and dust in an irregular but well-defined cloud that obscures the light from stars beyond it.
density A measure of the compactness of the matter within an object, computed by dividing the mass by the volume of the object.
diffraction The ability that waves have to bend around corners. The diffraction of light establishes its nature as a wave.
Doppler Effect Apparent change in wavelength of the radiation from a source due to its relative motion away from or towards the observer.
dwarf Any star with a radius comparable to, or smaller than that of the Sun (including the Sun itself).


electromagnetic radiation Consists of massless packets of pure energy called photons produced by changes in the energy of charged particles, usually electrons. Photons travel through space at the speed of light. When the changes of energy are small, streams of photons can be described as waves of changing electric and magnetic fields, called electromagnetic waves. The most familiar type of electromagnetic radiation is visible light, but the full spectrum of electromagnetic radiation includes radio, microwave, infrared, ultraviolet, X rays and gamma rays.
electromagnetic spectrum The entire range of electromagnetic waves, named in order of increasing frequency or energy, ranges from radio waves, to microwave, to infrared, to visible or optical, to ultraviolet, to X rays, to gamma rays.
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 degeneracy pressure The pressure produced by the resistance of electrons to compression once they are squeezed to the point where quantum effects become important. (The astronomer Subrahmanyan Chandrasekhar used this concept to calculate that there was an upper limit to the mass of a white dwarf star, 1.4 times that of the Sun.,)
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.
elliptical galaxy Category of galaxy in which the stars are distributed in an elliptical shape on the sky, ranging from highly elongated to nearly circular in appearance.
escape velocity The speed necessary for an object to escape the gravitational pull of an object. Anything that moves away from the object with more than the escape velocity will never return.
event horizon Imaginary spherical surface surrounding a black hole, with radius equal to the Schwarzschild radius, within which no event can be seen heard, or known about by an outside observer.


force Action of an object that causes momentum to change. The rate at which the momentum changes is numerically equal to the force.
frequency The number of wave crests passing any given point in a given period of time.
fusion Mechanism of energy generation in the core of the Sun in which light nuclei are combined, or fused, into heavier ones, releasing energy in the process.


galactic center The center of the Milky Way, or any other, galaxy. The point about which the disk of a spiral galaxy rotates.
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.
gamma ray Region of the electromagnetic spectrum, beyond x-rays, corresponding to radiation of very high frequency and very short wavelength.
gamma-ray burst An outburst that radiates tremendous amounts of energy equal to or greater than a supernova, in the form of gamma rays, in a few minutes.
giant star A star with a radius between 10 and 100 times that of the Sun.
globular cluster Tightly bound, roughly spherical collection of hundreds of thousands, and sometimes millions, of stars spanning about 100 light years. Globular clusters are distributed in the halos around the Milky Way and other galaxies.
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.


H-R (Hertzprung-Russell) diagram A plot of luminosity against temperature (or spectral class) for a group of stars.
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.
hydrogen shell burning Fusion of hydrogen in a shell that is driven by contraction and heating of the helium core. Once hydrogen is depleted in the core of a star, hydrogen burning stops and the core contracts due to gravity, causing the temperature to rise, heating the surrounding layers of hydrogen in the star, and increasing the burning rate there.


image The representation of an object produced when light from the object is reflected or refracted by a mirror or a lens.
infrared Region of the electromagnetic spectrum just outside the visible range, corresponding to light of a slightly longer wavelength than red light.
infrared telescope Telescopes designed to detect infrared radiation.
inverse-square law The law that a field follows if its strength decreases with the square of the distance. Fields that follow the inverse square law rapidly decrease in strength as the distance increases, but never quite reach zero.


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


Kelvin temperature scale, K 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.
kilometer A unit of distance equal to 0.6214 mile.
kinetic energy The energy of an object due to its motion.


lighthouse model The leading explanation for pulsars. A small region of the neutron star, near one of the magnetic poles, emits a steady stream of radiation which sweeps past Earth each time the star rotates. Thus the period of the pulses is just the star's rotation period.
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 Galaxy, and about 20 smaller galaxies.
luminosity One of the basic properties used to characterize stars, luminosity is defined as the total energy radiated by a star each second, at all wavelengths.
luminosity class A classification scheme which groups stars according to the width of their spectral lines. For a group of stars with the same temperature, luminosity class differentiates between supergiants, giants, main-sequence stars and subdwarfs.


Magellanic Clouds Two small irregular galaxies that orbit our Milky Way Galaxy.
main sequence A well-defined band on an H-R diagram on which most stars tend to be found, running from the top left of the diagram to the bottom right.
mass A measure of the total amount of matter contained within an object.
microwave radiation Radiation between radio and infrared wavelengths, having a wavelength between about 0.1 and 10 cm.
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.
millisecond pulsar A pulsar whose period indicates that the neutron star is rotating nearly 1,000 times each second.
molecular cloud A cold, dense interstellar cloud which contains a high fraction of molecules. It is widely believed that the relatively high density of dust particles in these clouds plays an important role in the formation and protection of the molecules.
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.


nebula General term used for any "fuzzy" patch on the sky, either light or dark; a cloud of interstellar gas and dust.
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.
nova A star that suddenly increases in brightness, often by a factor of as much as 10,000, then slowly fades back to its original luminosity. A nova is the result of an explosion on the surface of a white dwarf star, caused by matter falling onto its surface from the atmosphere of a binary companion.
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.
nuclear fusion A nuclear process that releases energy when lightweight nuclei combine to form heavy-weight nuclei.
nucleus Dense, central region of an atom, containing both protons and neutrons, and orbited by one or more electrons.



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.
pixel One of many tiny picture elements, organized into an array, making up a digital image.
planet One of nine major bodies that orbit the Sun, visible to us by reflected sunlight.
planetary nebula The ejected envelope of a red giant star, spread over a volume roughly the size of our solar system when it collapses to form a white dwarf. Such nebulae/nebulas, in fact, have nothing to do with planets!
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.
pulsar Object that emits radiation in the form of rapid pulses with a characteristic pulse period and duration. Generally used to describe the pulsed radiation from a rotating neutron star.


quasars Originally, a distant, highly luminous object that looks like a star. Strong evidence now exists that a quasar is produced by gas falling into a supermassive black hole in the center of a galaxy.


radiation A way in which energy is transferred from place to place in the form of a wave. Visible light is a form of electromagnetic radiation.
radio Region of the electromagnetic spectrum corresponding to radiation of the longest wavelengths.
radio galaxy Type of active galaxy that emits most of its energy in the form of long-wavelength radiation.
radio telescope Large instrument designed to detect radiation from space in radio wavelengths.
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-giant branch The section of the evolutionary track of a star that corresponds to continued heating from rapid hydrogen shell burning, which drives a steady expansion and cooling of the outer envelope of the star. As the star gets larger in radius and its surface temperature cools, it becomes a red giant.
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.
relativity, general theory A theory formulated by Einstein that describes how a gravitational field can by replaced by a curvature of space-time.
relativity, special theory 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.
resolution In astronomy, "resolution" or "resolving power" refers to the ability of a telescope to distinguish details.
   "Angular resolution" refers to the ability to distinguish details in an image. For example, Chandra can distinguish details that are only half an arc second apart. If your eyes had similar resolving power, you could read the letters on a stop sign at a distance of 12 miles!
   "Energy resolution" refers to the ability to distinguish the energies or wavelengths of photons. In visible light, this amounts to the ability to distinguish different colors. When Chandra makes an observation with the transmission gratings in place, it can distinguish thousands of different X-ray energies or colors.
revolution Orbital motion of one body about another, such as the Earth about the Sun.
rotation Spinning motion of a body about an axis.


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.
Schwarzschild radius The distance from the center of a non-rotating black hole such that, if all the mass were compressed within that region, the escape velocity would equal the speed of light. Once a stellar remnant collapses within this radius, light cannot escape and the object is no longer visible. See event horizon.
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.
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 4th. Great Observatory dedicated to infrared astronomy and scheduled for launch in 2001.
spacetime A synthesis of the three dimensions of space and of a fourth dimension, time; a hallmark of relativity theory.
spectral class Classification scheme, based on the strength of stellar spectral lines, which is an indication of the temperature of a star.
spectrometer Instrument used to produce detailed spectra of cosmic objects. Usually, a spectrometer records a spectrum in electronic form on a computer.
spectrum See electromagnetic spectrum.
speed of light The fastest possible speed of anything in the Universe, 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.
star A glowing ball of gas held together by its own gravity and powered by nuclear fusion in its core.
starburst galaxy Galaxy in which a violent event, such as near-collision, has caused a sudden, intense burst of star formation in the recent past.
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."
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.
transmission grating A device that disperses light into a spectrum of wavelengths when it passes through a finely constructed grating.
type-I supernova One possible explosive death of a star. A white dwarf in a binary system can accrete so much mass that it cannot support its own weight. The star collapses and temperatures become high enough for carbon fusion to occur. Fusion begins throughout the white dwarf almost simultaneously and an explosion occurs.
type-II supernova One possible explosive death of a star, in which the massive highly evolved stellar core rapidly implodes and then explodes, destroying the surrounding star.


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 4,000 to 7,000 angstroms, corresponding to blue through red light.
visible light The relatively 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.


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.
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.


XMM/Newton The European Space Agency's large x-ray observatory, launched on Dec. 10, 1999, which is capable of sensitive x-ray spectroscopic observations.
X-ray Region of the electromagnetic spectrum corresponding to radiation of high frequency and short wavelengths, far outside the visible spectrum.
X-ray burster X-ray source that radiates thousands of times more energy than our Sun, in short bursts that last only a few seconds. A neutron star in a binary system accretes matter onto its surface until temperatures reach the level needed for hydrogen fusion to occur. The result is a sudden period of rapid nuclear burning and release of energy.



zodiac The twelve constellations through which the Sun moves as it follows its path on the ecliptic.