Glossary

A
A ring One of three Saturnian rings visible from Earth. The A ring is farthest from the planet and is separated from the B ring by the Cassini division.
absolute brightness The apparent brightness a star would have if it were placed at a standard distance of 10 parsecs from Earth.
absolute magnitude The apparent magnitude a star would have if it were placed at a standard distance of 10 parsecs from Earth.
absorption line Dark line in an otherwise continuous bright spectrum, where light within one narrow frequency range has been removed.
acceleration The rate of change of velocity of a moving object.
accretion Gradual growth of bodies, such as stars or planets, by the accumulation of gas or other, smaller, bodies.
accretion disk Flat disk of matter spiraling down onto the surface of a neutron star or black hole. Often, the matter originated on the surface of a companion star in a binary system.
active galaxies The most energetic galaxies, which can emit hundreds or thousands of times more energy per second than the Milky Way.
active optics Collection of techniques now being used to increase the resolution of ground-based telescopes. Minute modifications are made to the overall configuration of an instrument as its temperature and orientation change, to maintain the best possible focus at all times.
active region Region of the photosphere of the Sun surrounding a sunspot group, which can erupt violently and unpredictably. During sunspot maximum, the number of active regions is also a maximum.
active Sun The unpredictable aspects of the Sun's behavior, such as sudden explosive outbursts of radiation in the form of prominences and flares.
adaptive optics Technique used to increase the resolution of a telescope by deforming the shape of the mirror's surface under computer control while a measurement is being taken, to undo the effects of atmospheric turbulence.
amino acids Organic molecules which form the basis for building the proteins that direct metabolism in living creatures.
amplitude The maximum deviation of a wave above or below the zero point.
angular momentum problem The fact that the Sun, which contains nearly all of the mass of the solar system, accounts for just 0.3 percent of the total angular momentum of the solar system. This is an aspect of the solar system that any acceptable formation theory must address.
angular resolution The ability of a telescope to distinguish between adjacent objects in the sky.
annular eclipse annular eclipse Solar eclipse occurring at a time when the Moon is far enough away from the Earth that it fails to cover the disk of the Sun completely, leaving a ring of sunlight visible around its edge.
aphelion The point on the elliptical path of an object in orbit about the Sun that is most distant from the Sun
Apollo asteroid See Earth-crossing asteroid.
apparent brightness The brightness that a star appears to have, as measured by an observer on Earth.
apparent magnitude The apparent brightness of a star, expressed using the magnitude scale.
arc degree Unit of angular measure. There are 360 arc degrees in one complete circle.
association Small grouping of (typically 100 or less) stars, spanning up to a few tens of parsecs across, usually rich in very young stars.
asteroid One of thousands of very small members of the solar system orbiting the Sun between the orbits of Mars and Jupiter. Asteroids are often referred to as "minor planets."
asteroid belt Region of the solar system, between the orbits of Mars and Jupiter, in which most asteroids are found.
asthenosphere Layer of Earth's interior, just below the lithosphere, over which the surface plates slide.
astronomical unit (A.U.) The average distance of Earth from the Sun. Precise radar measurements yield a value for the A.U. of 149,603,500 km.
astronomy Branch of science dedicated to the study of everything in the universe that lies above Earth's atmosphere.
asymptotic giant branch Path on the H-R diagram corresponding to the changes that a star undergoes after helium burning ceases in the core. At this stage, the carbon core shrinks and drives the expansion of the envelope, and the star becomes a swollen red giant for a second time.
atmosphere 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.
aurora Event which occurs when atmospheric molecules are excited by incoming charged particles from the solar wind, then emit energy as they fall back to their ground states. Aurorae generally occur at high latitudes, near the north and south magnetic poles.
autumnal equinox Date on which the Sun crosses the celestial equator moving southward, occurring on or near September 22.
B
B ring One of three Saturnian rings visible from Earth. The B ring is the brightest of the three, and lies just within the Cassini division, closer to the planet than the A ring.
barred-spiral galaxy Spiral galaxy in which a bar of material passes through the center of the galaxy, with the spiral arms beginning near the ends of the bar.
baseline The distance between two observing locations used for the purposes of triangulation measurements. The larger the baseline, the better the resolution attainable.
belt Dark, low-pressure region in the atmosphere of a jovian planet where gas flows downward.
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.
blackbody curve The characteristic way in which the intensity of radiation emitted by a hot object depends on frequency. The frequency at which the emitted intensity is highest is an indication of the temperature of the radiating object. Also referred to as the Planck curve.
black dwarf The end-point of the evolution of an isolated, low-mass star. After the white dwarf stage, the star cools to the point where it is a dark clinker in interstellar space.
black hole A region of space where the pull of gravity is so great that nothing-not even light-can escape. A possible outcome of the evolution of a very massive star.
blue giant Large, hot, bright star at the upper left end of the main sequence on the Hertzsprung–Russell diagram. Its name comes from its color and size.
blue shift Motion-induced changed 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 at the uppermost left end of the main sequence on the Hertzsprung–Russell diagram.
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 for the electron, 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 Remnant of a fragment of collapsing gas and dust that did not contain enough mass to initiate core nuclear fusion. Such objects are frozen somewhere along their pre-main-sequence contraction phase, continually cooling into compact dark objects. Because of their small sizes and low temperatures they are extremely difficult to detect observationally.
brown oval Feature of Jupiter's atmosphere that appears only at latitudes near 20 degrees N, this structure is a long-lived hole in the clouds that allows us to look down into Jupiter's lower atmosphere.
C
C ring One of three Saturnian rings visible from Earth. The C ring lies closest to the planet and is relatively thin compared to the A and B rings.
carbon-detonation supernova See type-I supernova.
Cassegrain telescope A type of reflecting telescope in which incoming light hits the primary mirror and is then reflected upward toward the prime focus, where a secondary mirror reflects the light back down through a small hole in the main mirror, into a detector or eyepiece.
Cassini Division A relatively empty gap in Saturn's ring system between the A and B rings, discovered in 1675 by Giovanni Cassini. It is now known to contain a number of thin ringlets.
catastrophic theory A theory that invokes statistically unlikely accidental events to account for observations.
celestial&151;coordinates Pair of quantities—right ascension and declination—similar to longitude and latitude on Earth, used to pinpoint locations of objects on the celestial sphere.
celestial equator The projection of the Earth's equator onto the celestial sphere.
celestial poles
celestial sphere Imaginary sphere surrounding the Earth, to which all objects in the sky were once considered to be attached.
center of mass The "average" position in space of a collection of massive bodies, taking their masses into account. 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.
chaotic rotation Unpredictable tumbling motion that non-spherical bodies in eccentric orbits, such as Saturn's satellite Hyperion, can exhibit. No amount of observation of an object rotating chaotically will ever show a well-defined period.
charge-coupled device (CCD) Electronic device used for data acquisition, composed of many tiny pixels, each of which records a buildup of charge to measure the amount of light striking it.
chromatic aberration The tendency for a lens to focus red and blue light differently, causing images to become blurred.
chromosphere The Sun's lower atmosphere, lying just above the visible photosphere.
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 Class of dark-matter candidates made up of very heavy particles, such as supersymmetric relics.
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.
color index A convenient method of quantifying a star's color by comparing its apparent brightness as measured through different filters. If the star's radiation is well described by a black-body spectrum, the ratio of its blue intensity (B) to its visual intensity (V) is a measure of the object's surface temperature.
color-magnitude diagram A way of plotting stellar properties, in which absolute magnitude is plotted against color index.
coma An effect occurring during the formation of an off-axis image in a telescope. Stars whose light enters the telescope at a large angle acquire comet-like tails on their images. The brightest part of a comet, often referred to as the "head."
comet A small body, composed mainly of ice and dust, in an elliptical orbit about the Sun. As it comes close to the Sun, some of its material is vaporized to form a gaseous head and extended tail.
comparative planetology The systematic study of the similarities and differences among the planets, with the goal of obtaining deeper insight into how the solar system formed and has evolved in time.
condensation nuclei Dust grains in the interstellar medium which act as seeds around which other material can coagulate. The presence of dust was very important in causing matter to clump during the formation of the solar system.
condensation theory Currently favored model of solar system formation which combines features of the old nebular theory with new information about interstellar dust grains, which acted as condensation nuclei.
conservation of mass and energy A fundamental law of modern physics which states that the sum of mass and energy must always remain constant in any physical process. In fusion reactions, the lost mass is converted into energy, primarily in the form of electromagnetic radiation.
constellation A human grouping of stars in the night sky into a recognizable pattern.
contact binary A binary star system in which both stars have expanded to fill their Roche lobes and the surfaces of the two stars merge. The binary system now consists of two nuclear burning stellar cores surrounded by a continuous common envelope.
continuous spectrum Spectrum in which the radiation is distributed over all frequencies, not just a few specific frequency ranges. A prime example is the black-body radiation emitted by a hot, dense body.
convection Churning motion resulting from the constant upwelling of warm fluid and the concurrent downward flow of cooler material to take its place.
convection zone Region of the Sun's interior, lying just below the surface, where the material of the Sun is in constant convective motion. This region extends into the solar interior to a depth of about 200,000 km.
Copernican revolution The realization toward the end of the sixteenth century that Earth is not at the center of the universe.
core The central region of Earth, surrounded by the mantle. The central region of the Sun.
core-collapse supernova See type-II supernova.
core hydrogen burning The energy burning stage for main sequence stars, in which the helium is produced by hydrogen fusion in the central region of the star. A typical star spends up to 90% of its lifetime in hydrostatic equilibrium brought about by the balance between gravity and the energy generated by core hydrogen burning.
corona One of numerous large, roughly circular regions on the surface of Venus, thought to have been caused by upwelling mantle material causing the planet's crust to bulge outward.
corona The tenuous outer atmosphere of the Sun, which lies just above the chromosphere, and at great distances turns into the solar wind.
coronal hole Vast regions of the Sun's atmosphere where the density of matter is about 10 times lower than average. The gas there streams freely into space at high speeds, escaping the Sun completely.
cosmic distance scale Collection of indirect distance-measurement techniques that astronomers use to measure the scale of the universe.
cosmic evolution The collection of the seven major phases of the history of the universe, namely galactic, stellar, planetary, chemical, biological, cultural, and future evolution.
cosmic microwave background The almost perfectly isotropic radio signal that is the electro-magnetic remnant of the Big Bang.
cosmological constant
cosmological principle Two assumptions which make up the basis of cosmology, namely that the universe is homogeneous and isotropic on sufficiently large scales.
cosmological redshift The component of the redshift of an object which is due only to the Hubble flow of the universe.
cosmology The study of the structure and evolution of the entire universe.
crater Bowl-shaped depression on the surface of a planet or moon, resulting from a collision with interplanetary debris.
critical density The cosmic density corresponding to the dividing line between a universe that recollapses and one that expands forever.
critical universe Geometry that the universe would have if the density of matter is exactly the critical density. The universe is infinite in extent, and has zero curvature. The expansion will continue forever, but approach an expansion speed of zero.
crust Layer of the Earth which contains the solid continents and the seafloor.
D
D ring Collection of very faint, thin rings, extending from the inner edge of the C ring down nearly to the cloud tops of Saturn. This region contains so few particles that it is completely invisible from Earth.
dark dust cloud A large cloud, often many parsecs across, which contains gas and dust in a ratio of about 10¹² gas atoms for every dust particle. Typical densities are a few tens or hundreds of millions of particles per cubic meter.
dark energy
dark halo Region of a galaxy beyond the visible halo where dark matter is believed to reside.
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 at any electromagnetic wavelength.
declination Celestial coordinate used to measure latitude above or below the celestial equator on the celestial sphere.
decoupling Event in the early universe when atoms first formed, and after which photons could propagate freely through space.
deferent A construct of the geocentric model of the solar system which was needed to explain observed planetary motions. A deferent is a large circle encircling the Earth, on which an epicycle moves.
degree See arc degree.
density A measure of the compactness of the matter within an object, computed by dividing the mass by the volume of the object. Units are kilograms per cubic meter (kg/m³), or grams per cubic centimeter (g/cm³).
deuteron An isotope of hydrogen in which there is a neutron bound to the proton in the nucleus. Often called “heavy hydrogen” because of the extra mass of the neutron.
differential rotation The tendency for a gaseous sphere, such as a jovian planet or the Sun, to rotate at a different rate at the equator than at the poles or for the rotation rate to vary with depth. For a galaxy or other object, a condition where the angular speed varies with location within the object.
differentiation Variation with depth in the density and composition of a body, such as Earth, with low-density material on the surface and higher density material in the core.
diffraction The tendency of waves to bend around corners. The diffraction of light establishes its nature as a wave.
Doppler effect Any motion-induced change in the observed wavelength (or frequency) of a wave.
Drake Equation Expression that gives an estimate of the probability that intelligence exists elsewhere in the galaxy, based on a number of supposedly necessary conditions for intelligent life to develop.
dust grain An interstellar dust particle, roughly 10&150;8 m in size, comparable to the wavelength of visible light.
dust lane A lane of dark, obscuring interstellar dust in an emission nebula or galaxy.
dust tail The component of a comet's tail that is composed of dust particles.
dwarf Any star with radius comparable to, or smaller than, that of the Sun (including the Sun itself).
dynamo theory Theory that explains planetary and stellar magnetic fields in terms of rotating, conducting material flowing in an object's interior.
E
E ring A faint ring, well outside the main ring system of Saturn, which was discovered by Voyager and is believed to be associated with volcanism on the moon Enceladus.
Earth-crossing asteroid An asteroid whose orbit crosses that of the Earth. Earth-crossing asteroids are also called Apollo asteroids, after the first of the type discovered.
earthquake A sudden dislocation of rocky material near the Earth's surface.
eccentricity A measure of the flatness of an ellipse, equal to the distance between the two foci divided by the length of the major axis.
eclipse Event during which one body passes in front of another, so that the light from the occulted body is blocked.
eclipse season Times of the year when the Moon lies in the same plane as the Earth and Sun, so that eclipses are possible.
eclipsing binary Rare binary-star system that is aligned in such a way that from Earth we periodically observe one star pass in front of the other, eclipsing the other star.
ecliptic The apparent path of the Sun, relative to the stars on the celestial sphere, over the course of a year.
electric field A field extending outward in all directions from a charged particle, such as a proton or an electron. The electric field determines the electric force exerted by the particle on all other charged particles in the universe; the strength of the electric field decreases with increasing distance from the charge according to an inverse-square law.
electromagnetic radiation Another term for light, electromagnetic radiation transfers energy and information from one place to another, even through the vacuum of empty space.
electromagnetic spectrum The complete range of electromagnetic radiation, from radio waves to gamma rays, including the visible spectrum. All types of electromagnetic radiation are basically the same phenomenon, differing only by wavelength, and all move at the speed of light.
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, one of the components of the atom.
electron degeneracy pressure The pressure produced by the resistance of electrons to compression once they are squeezed to the point of contact.
element Matter made up of one particular atom. The number of protons in the nucleus of the atom determines which element it represents.
ellipse Geometric figure resembling an elongated circle. An ellipse is characterized by its degree of flatness, or eccentricity, and the length of its long axis. In general, bound orbits of objects moving under gravity are elliptical.
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.
emission line Bright line in a specific location of the spectrum of radiating material, corresponding to emission of light at a certain frequency. A heated gas in a glass container produces emission lines in its spectrum.
emission nebula A glowing cloud of hot interstellar gas. The gas glows as a result of a nearby young star which is ionizing the gas. Since this gas is mostly hydrogen, the emitted radiation falls predominantly in the red region of the spectrum, because of a dominant hydrogen emission line.
emission spectrum The pattern of spectral emission lines produced by an element. Each element has its own unique emission spectrum.
Encke Gap A small gap in Saturn's A ring.
epicycle A construct of the geocentric model of the solar system which was necessary to explain observed planetary motions. Each planet rides on a small epicycle whose center in turn rides on a larger circle (the deferent).
epoch of inflation
equinox See autumnal equinox and vernal equinox.
escape speed 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 speed will never return.
event horizon Imaginary spherical surface surrounding a collapsing star, with radius equal to the Schwarzschild radius, within which no event can be seen, heard, or known about by an outside observer.
evolutionary theory A theory which explains observations in a series of gradual steps, explainable in terms of well-established physical principles.
evolutionary track A graphical representation of a star's life as a path on the Hertzsprung&151;Russell diagram.
excited state State of an atom when one of its electrons is in a higher energy orbital than the ground state. Atoms can become excited by absorbing a photon of a specific energy, or by colliding with a nearby atom.
extinction The dimming of starlight as it passes through the interstellar medium.
extrasolar planets
F
F ring Faint narrow outer ring of Saturn, discovered by Pioneer in 1979. The F ring lies just inside the Roche limit of Saturn, and was shown by Voyager to be made up of several ring strands apparently braided together.
flare Explosive event occurring in or near an active region on the Sun.
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.
fluidized ejecta The ejecta blankets around some Martian craters, which apparently indicate that the ejected material was liquid at the time the crater formed.
focus One of two special points within an ellipse, whose separation from each other indicate the eccentricity. In a bound orbit, objects move in ellipses about one focus.
forbidden line A spectral line seen in emission nebulae but not seen in laboratory experiments, because under laboratory conditions, collisions kick the electron in question into some other state before emission can occur.
force Action on an object that causes its momentum to change. The rate at which the momentum changes is numerically equal to the force.
fragmentation The breaking up of a large object into many smaller pieces (for example, as the result of high-speed collisions between planetesimals and protoplanets in the early solar system).
Fraunhofer lines The collection of over 600 absorption lines in the spectrum of the Sun, first categorized by Joseph Fraunhofer in 1812.
frequency The number of wave crests passing any given point per unit of time.
full Moon Phase of the Moon in which it appears as a complete circular disk in the sky.
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.
G
G ring
galactic bulge Thick distribution of warm gas and stars around the galactic center.
galactic cannibalism A galaxy merger in which a larger galaxy consumes a smaller one.
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 of 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 A collection of galaxies held together by their mutual gravitational attraction.
Galilean satellites The four brightest and largest moons of Jupiter (Io, Europa, Ganymede, Callisto), named after Galileo Galilei, the 17th century astronomer who first observed them.
gamma ray Region of the electromagnetic spectrum, far beyond the visible spectrum, corresponding to radiation of very high frequency and very short wavelength.
gamma-ray burst Object that radiates tremendous amounts of energy in the form of gamma rays, possibly due to the collision and merger of two neutron stars initially in orbit around one another.
general theory of relativity Einstein's theory of gravity, in which the force of gravity is reinterpreted as a curvature of spacetime in the vicinity of a massive object.
geocentric model A model of the solar system which holds that the Earth is at the center of the universe and all other bodies are in orbit around it. The earliest theories of the solar system were geocentric.
giant 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 50 parsecs. Globular clusters are distributed in the halos around the Milky Way and other galaxies.
Grand Unified Theories Class of theories describing the behavior of the single force that results from unification of the strong, weak, and electromagnetic forces in the early universe.
granulation Mottled appearance of the solar surface, caused by rising (hot) and falling (cool) material in convective cells just below the photosphere.
gravitational field Field created by any object with mass, extending outward 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 The effect induced on the image of a distant object by a massive foreground object. Light from the distant object is bent into two or more separate images.
gravitational red shift A prediction of Einstein's general theory of relativity. Photons lose energy as they escape the gravitational field of a massive object. Because a photon's energy is proportional to its frequency, a photon that loses energy suffers a decrease in frequency, which corresponds to an increase, or redshift, in wavelength.
gravity The attractive effect that any massive object has on all other massive objects. The greater the mass of the object, the stronger its gravitational pull.
Great Dark Spot Prominent storm system in the atmosphere of Neptune, located near the equator of the planet. The system is comparable in size to the Earth.
Great Red Spot A large, high-pressure, long-lived storm system visible in the atmosphere of Jupiter. The Red Spot is roughly twice the size of the Earth.
greenhouse effect The partial trapping of solar radiation by a planetary atmosphere, similar to the trapping of heat in a greenhouse.
ground state The lowest energy state that an electron can have within an atom.
H
heliocentric model A mode of the solar system which is centered on the Sun, with the Earth in motion about the Sun.
helioseismology The study of conditions far below the Sun's surface through the analysis of internal "sound" waves that repeatedly cross the solar interior.
helium capture The formation of heavy elements by the capture of a helium nucleus. For example, carbon can form heavier elements by fusion with other carbon nuclei, but it is much more likely to occur by helium capture, which requires less energy.
helium flash An explosive event in the post-main-sequence evolution of a low-mass star. When helium fusion begins in a dense stellar core, the burning is explosive in nature. It continues until the energy released is enough to expand the core, at which point the star achieves stable equilibrium again.
helium precipitation Mechanism responsible for the low abundance of helium of Saturn's atmosphere. Helium condenses in the upper layers to form a mist, which rains down toward Saturn's interior, just as water vapor forms into rain in the atmosphere of Earth.
Hertzsprung–Russell (H—R) diagram A plot of luminosity against temperature (or spectral class) for a group of stars.
high-energy telescope Telescope designed to detect radiation in X-rays and gamma rays.
highlands Relatively light-colored regions on the surface of the Moon which are elevated several kilometers above the maria. Also called terrae.
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.
horizontal branch Region of the Hertzsprung–Russell diagram where post–main sequence stars again reach hydrostatic equilibrium. At this point, the star is burning helium in its core, and hydrogen in a shell surrounding the core.
hot dark matter A class of candidates for the dark matter in the universe, composed of lightweight particles, such as neutrinos, much less massive than the electron.
Hubble classification scheme Method of classifying galaxies according to their appearance, developed by Edwin Hubble.
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 directly proportional to its distance away.
hydrogen envelope An invisible region engulfing the coma of a comet, usually distorted by the solar wind, and extending across millions of kilometers of space.
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.
hydrosphere Layer of the Earth which contains the liquid oceans and accounts for roughly 70 percent of Earth's total surface area.
I
image The optical representation of an object produced when light from the object is reflected or refracted by a mirror or lens.
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 unchecked cosmic expansion early in the history of the universe. During inflation, the universe swelled in size by a factor of about 1050.
infrared Region of the electromagnetic spectrum just outside the visible range, corresponding to light of a slightly longer wavelength than red light.
infrared telescope Telescope designed to detect infrared radiation. Many such telescopes are designed to be lightweight so that they can be carried above most of Earth's atmosphere by balloons, airplanes, or satellites.
inner core The central part of Earth's core, believed to be solid, and composed mainly of nickel and iron.
intensity A basic property of electromagnetic radiation that specifies the amount or strength of the radiation.
intercrater plains Regions on the surface of Mercury that do not show extensive cratering, but are relatively smooth.
interference The ability of two or more waves to interact in such a way that they either reinforce or cancel each other.
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 telescopes.
interferometry Technique in widespread use to dramatically improve the resolution of radio and infrared maps. Several telescopes observe an object simultaneously, and a computer analyzes how the signals interfere with one another to reconstruct a detailed image of the field of view.
interplanetary space The space between the objects in the solar system.
interstellar dust Microscopic dust grains that populate the space between stars, having their origins in the ejected matter of long-dead stars.
interstellar medium The matter between stars, composed of two components, gas and dust, intermixed throughout all of space.
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.
Io plasma torus Doughnut-shaped region of energetic ionized particles, emitted by the volcanoes on Jupiter's moon Io, and swept up by Jupiter's magnetic field.
ion An atom that has lost one or more electrons.
ion tail Thin stream of ionized gas that is pushed away from the head of a comet by the solar wind. It extends directly away from the Sun. Often referred to as a plasma tail.
ionized State of an atom that has had at least one of its electrons removed.
ionosphere Layer in Earth's atmosphere above about 100 km where the atmosphere is significantly ionized, and conducts electricity.
irregular galaxy A galaxy which does not fit into any of the other major categories in the Hubble classification scheme.
isotopes Nuclei containing the same number of protons but different numbers of neutrons. Most elements can exist in several isotopic forms. A common example of an isotope is deuterium, which differs from normal hydrogen by the presence of an extra neutron in the nucleus.
isotropic Assumed property of the universe such that the universe looks the same in every direction.
isotropy Assumed property of the universe such that the universe looks the same in every direction.
J
jovian planet One of the four giant outer planets of the solar system, resembling Jupiter in physical and chemical composition.
K
Kepler's Laws of Planetary Motion Three laws, based on precise observations of the motions of the planets by Tycho Brahe, which summarize the motions of the planets about the Sun.
Kirchhoff's laws Three rules governing the formation of different types of spectra.
Kirkwood gaps Gaps in the spacings of semi-major axes of orbits of asteroids in the asteroid belt, produced by dynamical resonances with nearby planets, especially Jupiter.
Kuiper belt A region in the plane of the solar system outside the orbit of Neptune where most short-period comets are thought to originate.
L
Lagrangian point One of five special points in the plane of two massive bodies orbiting one another, where a third body of negligible mass can remain in equilibrium.
law of conservation of mass and energy A fundamental law of modern physics which states that the sum of mass and energy must always remain constant in any physical process. In fusion reactions, the lost mass is converted into energy, primarily in the form of electromagnetic radiation.
laws of planetary motion Three laws, based on precise observations of the motions of the planets by Tycho Brahe, which summarize the motions of the planets about the Sun.
lava dome Volcanic formations formed when lava oozes out of fissures in a planet's surface, creating the dome, and then withdraws, causing the crust to crack and subside.
light See electromagnetic radiation.
light curve A plot of the variation in brightness of a star with time.
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. The period of the pulses is the star's rotation period.
light year The distance that light, moving at a constant speed of 300,000 km/s, travels in one year. One light year is about 10 trillion kilometers.
line of nodes The line of intersection of the Moon's orbit with the ecliptic plane.
lithosphere Earth's crust and a small portion of the upper mantle that make up Earth's plates. This layer of the Earth undergoes tectonic activity.
Local Group The small galaxy cluster that includes the Milky Way Galaxy.
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.
lunar eclipse Celestial event during which the Moon passes through the shadow of the Earth, temporarily darkening its surface.
lunar phases The appearance of the moon at different points along its orbit.
M
Magellanic Clouds Two small irregular galaxies that are gravitationally bound to the Milky Way Galaxy.
magnetic field Field which accompanies any changing electric field, and governs the influence of magnetized objects on one another.
magnetosphere A zone of charged particles trapped by a planet's magnetic field, lying above the atmosphere.
magnitude scale A system of ranking stars by apparent brightness, developed by the Greek astronomer Hipparchus. Originally, the brightest stars in the sky were categorized as being of first magnitude, while the faintest stars visible to the naked eye were classified as sixth magnitude. The scheme has since been extended to cover stars and galaxies too faint to be seen by the unaided eye. Increasing magnitude means fainter stars, and a difference of 5 magnitudes corresponds to a factor of 100 in apparent brightness.
main sequence Well-defined band on the Hertzsprung—Russell diagram, on which most stars are found, running from the top left of the diagram to the bottom right.
main-sequence turnoff Special point on the Hertzsprung—Russell diagram for a cluster, indicative of the cluster's age. If all the stars in the cluster are plotted, the lower mass stars will trace out the main sequence up to the point where stars begin to evolve off the main sequence toward the red giant branch. The point where stars are just beginning to evolve off is the main-sequence turnoff.
mantle Layer of the Earth just interior to the crust.
mare Relatively dark-colored and smooth region on the surface of the Moon.
maria
mass A measure of the total amount of matter contained within an object.
mass-luminosity relation The dependence of the luminosity of a main-sequence star on its mass. The luminosity increases roughly as the mass raised to the third power.
mass-radius relation The dependence of the radius of a main sequence star on its mass. The radius rises roughly in proportion to the mass.
mass-transfer binaries See semi-detached binary.
matter-dominated universe A universe in which the density of matter exceeds the density of radiation. The present-day universe is matter-dominated.
mesosphere Region of Earth's atmosphere lying between the stratosphere and the ionosphere, 50-80 km above Earth's surface.
meteor Bright streak in the sky, often referred to as a "shooting star," resulting from a small piece of interplanetary debris entering Earth's atmosphere and heating air molecules, which emit light as they return to their ground states.
meteor shower Event during which many meteors can be seen each hour, caused by the yearly passage of the Earth through the debris spread along the orbit of a comet.
meteorite Any part of a meteoroid that survives passage through the atmosphere and lands on the surface of Earth.
meteoroid Chunk of interplanetary debris prior to encountering Earth's atmosphere.
meteoroid swarm Pebble-sized cometary fragments dislodged from the main body, moving in nearly the same orbit as the parent comet.
micrometeoroids Relatively small chunks of interplanetary debris ranging from dust particle size to pebble-sized fragments.
Milky Way Galaxy The spiral galaxy in which the Sun resides. The disk of our Galaxy is visible in the night sky as the faint band of light known as the Milky Way.
millisecond pulsar A pulsar whose period indicates that the neutron star is rotating nearly 1000 times each second. The most likely explanation for these rapid rotators is that the neutron star has been spun up by drawing in matter from a companion star.
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.
molecular cloud complex Collection of molecular clouds that spans as much as 50 parsecs and may contain enough material to make millions of Sun-sized stars.
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.
moon A small body in orbit about a planet.
N
nebula General term used for any "fuzzy" patch on the sky, either light or dark.
nebular theory One of the earliest models of solar system formation, dating back to Descartes, in which a large cloud of gas began to collapse under its own gravity to form the Sun and planets.
neutrino Virtually massless and chargeless particle that is one of the products of fusion reactions in the Sun. Neutrinos move at close to the speed of light, and interact with matter hardly at all.
neutrino oscillations Possible solution to the solar neutrino problem, in which the neutrino has a very tiny mass. In this case, the correct number of neutrinos can be produced in the solar core, but on their way to Earth, some can "oscillate," or become transformed into other particles, and thus go undetected.
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 capture The primary mechanism by which very massive nuclei are formed in the violent aftermath of a supernova. Instead of fusion of like nuclei, heavy elements are created by the addition of more and more neutrons to existing nuclei.
neutron star A dense ball of neutrons that remains at the core of a star after a supernova explosion has destroyed the rest of the star. Typical neutron stars are about 20 km across, and contain more mass than the Sun.
new Moon Phase of the moon during which none of the lunar disk is visible.
Newtonian mechanics The basic laws of motion, postulated by Newton, which are sufficient to explain and quantify virtually all of the complex dynamical behavior found on Earth and elsewhere in the universe.
Newtonian telescope A reflecting telescope in which incoming light is intercepted before it reaches the prime focus and is deflected into an eyepiece at the side of the instrument.
north celestial pole Point on the celestial sphere directly above the Earth's north pole.
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 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.
nucleotide base An organic molecule, the building block of genes that pass on hereditary characteristics from one generation of living creatures to the next.
nucleus Dense, central region of an atom, containing both protons and neutrons, and orbited by one or more electrons. The solid region of ice and dust that composes the central region of the head of a comet.
O
Olbers's paradox A thought experiment suggesting that if the universe were homogeneous, infinite, and unchanging, the entire night sky would be as bright as the surface of the Sun.
Oort Cloud Spherical halo of material surrounding the solar system, out to a distance of about 50,000 A.U., where most comets originate.
opacity A quantity that measures a material's ability to block electromagnetic radiation. Opacity is the opposite of transparency.
open cluster Loosely bound collection of tens to hundreds of stars, a few parsecs across, generally found in the plane of the Milky Way.
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.
outer core The outermost part of Earth's core, believed to be liquid, and composed mainly of nickel and iron.
outflow channel Surface features on Mars, evidence that liquid water once existed there in great quantity, believed to be the relics of catastrophic flooding about 3 billion years ago. Found only in the equatorial regions of the planet.
ozone layer Layer of the Earth's atmosphere at an altitude of 20 to 50 km where incoming ultraviolet solar radiation is absorbed by oxygen, ozone, and nitrogen in the atmosphere.
P
parallax The apparent motion of a relatively close object with respect to a more distant background as the location of the observer changes.
parsec The distance at which a star must lie in order that its measured parallax is exactly 1 arc second, equal to 206,000 A.U.
pair production The process in which two photons of electromagnetic radiation 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.
penumbra Portion of the shadow cast by an eclipsing object in which the eclipse is seen as partial.
perihelion The closest approach to the Sun of any object in orbit about it.
period The time needed for an orbiting body to complete one revolution about another body.
period-luminosity relation A relation between the pulsation period of a Cepheid variable and its absolute brightness. Measurement of the pulsation period allows the distance of the star to be determined.
permafrost Layer of permanently frozen water ice believed to lie just under the surface of Mars.
phases
photoelectric effect Experiment concerning the detection of electrons from a metal surface, whose speed off the surface was dependent on the frequency of light striking the surface. The theoretical explanation rests on viewing light as made up of photons, or individual bullets of energy.
photometer A device that measures the total amount of light received in all or part of the image.
photometry Branch of observational astronomy in which intensity measurements are made through each of a set of standard filters.
photon Individual packet of electromagnetic energy that makes up electromagnetic radiation.
photosphere The visible surface of the Sun, lying just above the uppermost layer of the Sun's interior, and just below the chromosphere.
pixel One of many tiny picture elements, organized into an array, making up a digital image.
Planck curve see blackbody curve
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.
planetary ring system Material organized into thin, flat rings encircling a giant planet, such as Saturn.
planetesimal Term given to objects in the early solar system that had reached the size of small moons, at which point their gravitational fields were strong enough to begin to influence their neighbors.
plate tectonics The motions of regions of Earth's crust, which drift with respect to one another. Also known as continental drift.
polarization The alignment of the electric fields of emitted photons, which are generally emitted with random orientations.
positron Atomic particle with properties identical to those of a negatively charged electron, except for its positive charge. The positron is the antiparticle of the electron. Positrons and electrons annihilate one another when they meet, producing pure energy in the form of gamma rays.
precession The slow change in the direction of the axis of a spinning object, caused by some external influence.
primary atmosphere The chemical components that would have surrounded Earth just after it formed.
prime focus The point in a reflecting telescope where the mirror focuses incoming light to a point.
primordial nucleosynthesis The production of elements heavier than hydrogen by nuclear fusion in the high temperatures and densities which existed in the early universe.
Principle of Cosmic Censorship A proposition to separate the unexplained physics near a singularity from the rest of the well-behaved universe. The principle states that nature always hides any singularity, such as a black hole, inside an event horizon, which insulates the rest of the universe from seeing it.
prominence Loop or sheet of glowing gas ejected from an active region on the solar surface, which then moves through the inner parts of the corona under the influence of the Sun's magnetic field.
proper motion The angular movement of a star across the sky, as seen from Earth, measured in seconds of arc per year. This movement is a result of the star's actual motion through space.
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 most main-sequence stars.
protoplanet Clump of material, formed in the early stages of solar system formation, that was the forerunner of the planets we see today.
protostar Stage in star formation when the interior of a collapsing fragment of gas is sufficiently hot and dense that it becomes opaque to its own radiation. The protostar is the dense region at the center of the fragment.
protosun The central accumulation of material in the early stages of solar system formations, the forerunner of the present-day Sun.
Ptolemaic model Geocentric solar system model, developed by the second century astronomer Claudius Ptolemy. It predicted with great accuracy the positions of the then known planets.
pulsar Object that emits radiation in the form of rapid pulses with a characteristic pulse period and duration. Charged particles, accelerated by the magnetic field of a rapidly rotating neutron star, flow along the magnetic field lines, producing radiation that beams outward as the star spins on its axis.
pulsating variable star A star whose luminosity varies in a predictable, periodic way.
Q
quantized The fact that light and matter on small scales behave in a discontinuous manner, and manifest themselves in the form of tiny "packets" of energy, called quanta.
quarter Moon Lunar phase in which the moon appears as a half disk.
quasar Star-like radio source with an observed redshift that indicates extremely large distance from Earth.
quasi-stellar object (QSO) See quasar.
quiet Sun The underlying predictable elements of the Sun's behavior, such as its average photospheric temperature, which do not change in time.
R
radar Acronym for RAdio Detection And Ranging. Radio waves are bounced off an object, and the time at which the echo is received indicates its distance.
radial motion Motion along a particular line of sight, which induces apparent changes in the wavelength (or frequency) of radiation received.
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 darkening The effect of chemical reactions that result when high-energy particles strike the icy surfaces of objects in the outer solar system. The reactions lead to a build-up of a dark layer of material.
radiation-dominated universe Early epoch in the universe, when the density of radiation in the cosmos exceeded the density of matter.
radiation zone Region of the Sun's interior where extremely high temperatures guarantee that the gas is completely ionized. Photons are only occasionally diverted by electrons, and travel through this region with relative ease.
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 lobe Roundish region of radio-emitting gas, lying well beyond the center of a radio galaxy.
radio telescope Large instrument designed to detect radiation from space in radio wavelengths.
radio waves
radioactivity The release of energy by rare, heavy elements when their nuclei decay into lighter nuclei.
radius-luminosity-temperature relation A mathematical proportionality, arising from simple geometry and Stefan's law, which allows astronomers to indirectly determine the radius of a star once its luminosity and temperature are known.
red dwarf Small, cool faint star at the lower-right end of the main sequence on the Hertzsprung–Russell diagram.
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 corresponding to intense 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 giant region The upper-right-hand corner of the Hertzsprung–Russell diagram, where red-giant stars are found.
red shift Motion-induced 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.
red supergiant An extremely luminous red star. Often found on the asymptotic giant branch of the H-R diagram.
reddening Dimming of starlight by interstellar matter, which tends to scatter higher-frequency (blue) components of the radiation more efficiently than the lower-frequency (red) components.
reflecting telescope A telescope which uses a mirror to gather and focus light from a distant object.
refracting telescope A telescope which uses a lens to gather and focus light from a distant object.
refraction The tendency of a wave to bend as it passes from one transparent medium to another.
residual cap Portion of Martian polar ice caps that remains permanently frozen, undergoing no seasonal variations.
resonance
retrograde motion Backward, westward loop traced out by a planet with respect to the fixed stars.
revolution Orbital motion of one body about another, such as the Earth about the Sun.
revolving
right ascension Celestial coordinate used to measure longitude on the celestial sphere. The zero point is the position of the Sun on the vernal equinox.
rille A ditch on the surface of the Moon where molten lava flowed in the past.
ringlet Narrow region in Saturn's planetary ring system where the density of ring particles is high. Voyager discovered that the rings visible from Earth are actually composed of tens of thousands of ringlets.
Roche limit Often called the tidal stability limit, the Roche limit gives the distance from a planet at which the tidal force, due to the planet, between adjacent objects exceeds their mutual attraction. Objects within this limit are unlikely to accumulate into larger objects. The rings of Saturn occupy the region within Saturn's Roche limit.
Roche lobes An imaginary surface around a star. Each star in a binary system can be pictured as being surrounded by a tear-shaped zone of gravitational influence, the Roche lobe. Any material within the Roche lobe of a star can be considered to be part of that star. During evolution, one member of the binary star can expand so that it overflows its own Roche lobe, and begins to transfer matter onto the other star.
rotation Spinning motion of a body about an axis.
rotation curve Plot of the orbital speed of disk material in a galaxy against its distance from the galactic center. Analysis of rotation curves of spiral galaxies indicates the existence of dark matter.
RR Lyrae star Variable star whose luminosity changes in a characteristic way. All RR Lyrae stars have more or less the same period.
runaway greenhouse effect A process in which the heating of a planet leads to an increase in its atmosphere's ability to retain heat and thus to further heating, quickly causing extreme changes in the temperature of the surface and the composition of the atmosphere.
runoff channel River-like surface feature on Mars, evidence that liquid water once existed there in great quantities. Runoff channels are found in the southern highlands, and are thought to have been formed by water that flowed nearly 4 billion years ago.
S
S0 galaxy Galaxy which shows evidence of a thin disk and a bulge, but which has no spiral arms and contains little or no gas.
SB0 galaxy S0-type galaxy whose disk shows evidence of a bar.
scarp Surface feature on Mercury believed to be the result of cooling and shrinking of the crust, forming a wrinkle on the face of the planet.
Schwarzschild radius The distance from the center of an object such that, if all the mass 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.
scientific method The set of rules used to guide science, based on the idea that scientific laws be continually tested, and modified or replaced if found inadequate.
seasonal cap Portion of Martian polar ice caps that is subject to seasonal variations, growing and shrinking once each Martian year.
seasons Changes in average temperature and length of day that result from the tilt of Earth's (or any planet's) axis with respect to the plane of its orbit.
secondary atmosphere The chemicals that composed Earth's atmosphere after the planet's formation, once volcanic activity outgassed chemicals from the interior.
seeing A term used to describe the ease with which good telescopic observations can be made from Earth's surface, given the blurring effects of atmospheric turbulence.
seeing disk Roughly circular region on a detector over which a star's pointlike images is spread, due to atmospheric turbulence.
seismic wave A wave that travels outward from the site of an earthquake through the Earth.
semi-major axis One half of the major axis of an ellipse. The semi-major axis is the way in which the size of an ellipse is usually quantified.
semi-major axisx Type of active galaxy whose emission comes from a very small region within the nucleus of an otherwise normal-looking spiral system.
Seyfert galaxy Type of active galaxy whose emission comes from a very small region within the nucleus of an otherwise normal-looking sprial system.
shepherd satellite Satellite whose gravitational effect on a ring helps preserve the ring's shape. Examples are two satellites of Saturn, Prometheus and Pandora, whose orbits lie on either side of the F ring.
shield volcano A volcano produced by repeated nonexplosive eruptions of lava, creating a gradually sloping, shield-shaped low dome. Often contains a caldera at its summit.
shock wave Wave of matter, which may be generated by a star, which pushes material outward into the surrounding molecular cloud. The material tends to pile up, forming a rapidly-expanding shell of dense gas.
sidereal day The time needed for a star on the celestial sphere to make one complete rotation in the sky.
sidereal month Time required for the Moon to complete one trip around the celestial sphere.
sidereal year The time required for the constellations to complete once cycle around the sky and return to their starting points, as seen from a given point on Earth.
singularity A point in the universe where the density of matter and the gravitational field are infinite, such as at the center of a black hole.
solar constant The amount of solar energy reaching Earth per unit area per unit time, approximately 1400 W/m².
solar core The region at the center of the Sun, with a radius of nearly 200,000 km, where powerful nuclear reactions generate the Sun's energy output.
solar cycle The 22-year period that is needed for both the average number of spots and the Sun's magnetic polarity to repeat themselves. The Sun's polarity reverses on each new 11-year sunspot cycle.
solar day The period of time between the instant when the Sun is directly overhead (i.e. at noon) to the next time it is directly overhead.
solar eclipse Celestial event during which the new Moon passes directly between the Earth and Sun, temporarily blocking the Sun's light.
solar interior The region of the Sun between the solar core and the photosphere.
solar maximum The starting point of the sunspot cycle, during which only a few spots are seen. They are generally confined to narrow regions, one in each hemisphere, at about 25-30 degrees latitude.
solar minimum The starting point of the sunspot cycle, during which only a few spots are seen. They are generally confined to narrow regions, one in each hemisphere, at about 25&151;30 degrees latitude.
solar nebula The swirling gas surrounding the early Sun during the epoch of solar system formation, also referred to as the primitive solar system.
solar neutrino problem The discrepancy between the theoretically predicted numbers of neutrinos streaming from the Sun as a result of fusion reactions in the core and the numbers actually observed. The observed number of neutrinos is only about half the predicted number.
solar system The Sun and all the bodies that orbit it—Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, their moons, the asteroids, and the comets.
solar wind An outward flow of fast-moving charged particles from the Sun.
south celestial pole Point on the celestial sphere directly above the Earth's south pole.
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 stars. Usually, a spectrograph records a spectrum on a photographic plate, or more recently, in electronic form on a computer.
spectroscope Instrument used to view a light source so that it is split into its component colors.
spectroscopic binary A binary-star system which from Earth appears as a single star, but whose spectral lines show back-and-forth Doppler shifts as two stars orbit one another.
spectroscopic parallax Method of determining the distance to a star by measuring its temperature and then determining its absolute brightness by comparing with a standard H—R diagram. The absolute and apparent brightnesses of the star give the star's distance from Earth.
spectroscopy The study of the way in which atoms absorb and emit 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.
spin-orbit resonance State that a body is said to be in if its rotation period and its orbital period are related in a simple way.
spiral arm Distribution of material in a galaxy in a pinwheel-shaped design apparently emanating from near the galactic center.
spiral density wave (i) A wave of matter formed in the plane of planetary rings, similar to ripples on the surface of a pond, which wrap around the rings forming spiral patterns similar to grooves in a record disk. Spiral density waves can lead to the appearance of ringlets. (ii) A proposed explanation for the existence of galactic spiral arms, in which coiled waves of gas compression move through the galactic disk, triggering star formation.
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.
Standard Solar Model A self-consistent picture of the Sun, developed by incorporating the important physical processes that are believed to be important in determining the Sun's internal structure, into a computer program. The results of the program are then compared with observations of the Sun, and modifications are made to the model. The Standard Solar Model, which enjoys widespread acceptance, is the result of this process.
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 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 Earth.
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.
Stefan's law Relation that gives the total energy emitted per square centimeter of its surface per second by an object of a given temperature. Stefan's law shows that the energy emitted increases rapidly with an increase in temperature, proportional to the temperature raised to the fourth power.
stellar nucleosynthesis The formation of heavy elements by the fusion of lighter nucleii in the hearts of stars. Except for hydrogen and helium, all other elements in our universe result from stellar nucleosynthesis.
stellar occultation The dimming of starlight produced when a solar system object such as a planet, moon, or ring, passes directly in front of a star.
stratosphere The portion of Earth's atmosphere lying above the troposphere, extending up to an altitude of 40 to 50 km.
strong nuclear force Short-range force responsible for binding atomic nuclei together. The strongest of the four fundamental forces of nature.
subgiant branch The section of the evolutionary track of a star that corresponds to changes that occur just after hydrogen is depleted in the core, and core hydrogen burning ceases. Shell hydrogen burning heats the outer layers of the star, which causes a general expansion of the stellar envelope.
summer solstice Point on the ecliptic where the Sun is at its northernmost point above the celestial equator, occurring on or near June 21.
sunspot An Earth-sized dark blemish found on the surface of the Sun. The dark color of the sunspot indicates that it is a region of lower temperature than its surroundings.
sunspot cycle The fairly regular pattern that the number and distribution of sunspots follows, in which the average number of spots reaches a maximum every 11 or so years, then falls off to almost zero.
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.
supergranulation Large-scale flow pattern on the surface of the Sun, consisting of cells measuring up to 30,000 km across, believed to be the imprint of large convective cells deep in the solar interior.
supernova Explosive death of a star, caused by the sudden onset of nuclear burning (type I), or 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 scattered glowing remains from a supernova that occurred in the past. The Crab Nebula is one of the best-studied supernova remnants.
synchrotron radiation Type of nonthermal radiation caused by high-speed charged particles, such as electrons, as they are accelerated in a strong magnetic field.
synchronous orbit State of an object when its period of rotation is exactly equal to its average orbital period. The Moon is in a synchronous orbit, and so presents the same face toward Earth at all times.
synodic month Time required for the Moon to complete a full cycle of phases.
T
T Tauri star Protostar in the late stages of formation, often exhibiting violent surface activity. T Tauri stars have been observed to brighten noticeably in a short period of time, consistent with the idea of rapid evolution during this final phase of stellar formation.
tail Component of a comet that consists of material streaming away from the main body, sometimes spanning hundreds of millions of kilometers. May be composed of dust or ionized gases.
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.
terrae See highlands.
terrestrial planet The four innermost planets of the solar system, resembling the Earth in general physical and chemical properties.
theories of relativity Einstein's theories, on which much of modern physics rests. Two essential facts of the theory are that nothing can travel faster than the speed of light, and that everything, including light, is affected by gravity.
thick disk Region of a spiral galaxy where an intermediate population of stars resides, younger than the halo stars, but older than stars in the disk.
tidal bulge Elongation of the Earth caused by the difference between the gravitational force on the side nearest the Moon and the force on the side farthest from the Moon. The long axis of the tidal bulge points toward the Moon. More generally, the deformation of any body produced by the tidal effect of a nearby gravitating object.
tidal force The variation in one body's gravitational force from place to place across another body–for example, the variation of the Moon's gravity across the Earth.
tides Rising and falling motion that bodies of water follow, exhibiting daily, monthly, and yearly cycles. Ocean tides on Earth are caused by the competing gravitational pull of the Moon and Sun on different regions of the Earth.
time dilation A prediction of the theory of relativity, closely related to the gravitational reshift. 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 view by another.
total solar eclipse Celestial event during which one body is completely blocked from view by another.
transition zone The region of rapid temperature increase that separates the Sun's chromosphere from the corona.
transverse motion Motion perpendicular to a particular line of sight, which does not result in Doppler shift in radiation received.
triangulation Method of determining distance based on the principles of geometry. A distant object is sighted from two well-separated locations. The distance between the two locations and the angle between the line joining them and the line to the distant object are all that are necessary to ascertain the object's distance.
triple-alpha process The generation of Carbon-12 from the fusion of three helium-4 nuclei (alpha particles). Helium-burning stars occupy a region of the H-R diagram known as the horizontal branch.
Trojan asteroids One of two groups of asteroids which orbit at the same distance from the Sun as Jupiter, 60 degrees ahead and behind the planet.
tropical year The time interval between one vernal equinox and the next.
troposphere The portion of Earth's atmosphere from the surface to about 15 km.
Tully-Fisher relation A relation used to determine the absolute luminosity of a spiral galaxy. The rotational velocity, measured from the broadening of spectral lines, is related to the total mass, and hence the total luminosity.
21-centimeter radiation Radio radiation emitted when an electron in the ground state of a hydrogen atom flips its spin to become parallel to the spin of the proton in the nucleus.
Type I supernova One possible explosive death of a star. A white dwarf in a binary system can accrete enough 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 results.
Type II supernova One possible explosive death of a star, in which the highly evolved stellar core rapidly implodes and then explodes, destroying the surrounding star.
U
ultraviolet Region of the electromagnetic spectrum, just outside the visible range, corresponding to wavelengths slightly shorter than blue light.
ultraviolet telescope A telescope that is designed to collect radiation in the ultraviolet part of the spectrum. The Earth's atmosphere is partially opaque to these wavelengths, so ultraviolet telescopes are put on rockets, balloons, or satellites to get high above most or all of the atmosphere.
umbra Central region of the shadow cast by an eclipsing body. The central region of a sunspot, which is its darkest and coolest part.
unbound An orbit which does not stay in a specific region of space, but where an object escapes the gravitational field of another. Typical unbound orbits are hyperbolic in shape.
universe The totality of all space, time, matter, and energy.
V
Van Allen belts At least two doughnut-shaped regions of magnetically trapped charged particles high above Earth's atmosphere.
variable star A star whose luminosity changes with time.
vernal equinox Date on which the Sun crosses the celestial equator moving northward, occurring on or near March 21.
visible 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.
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.
visible spectrum The small range of the electromagnetic spectrum that human eyes percieve as light. The visible spectrum ranges from about 4000 to 7000 angstroms, 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 and "walls" of galaxies are organized.
volcano Upwelling of hot lava from below Earth's crust to the planet's surface.
W
water hole The radio interval between 18 cm and 21 cm, the wavelengths at which hydroxyl (OH) and hydrogen (H) radiate, respectively, in which intelligent civilizations might conceivably send their communication signals.
wave A pattern that repeats itself cyclically in both time and space. Waves are characterized by the velocity with which they move, their frequency, and their wavelength.
wave period The amount of time required for a wave to repeat itself at a specific point in space.
wavelength The length from one point on a wave to the point where it is repeated exactly in space, at a given time.
weak nuclear force Short-range force, weaker than both electromagnetism and the strong force, but much stronger than gravity, responsible for certain nuclear reactions and radioactive decays.
weird terrain A region on the surface of Mercury of oddly rippled features. This feature is thought to be the result of a strong impact which occurred on the other side of the planet, and sent seismic waves traveling around the planet, converging in the weird region.
white dwarf A dwarf star with a surface temperature that is hot, so that the object glows white.
white dwarf region The bottom left-hand corner of the Hertzsprung–Russell diagram, where white dwarf stars are found.
white light Visible light that contains approximately equal proportions of all colors.
white oval Light-colored region near the Great Red Spot in Jupiter's atmosphere. Like the red spot, such regions are apparently rotating storm systems.
Wien's law Relation which gives the connection between the wavelength at which a black-body curve peaks and the temperature of the emitter. The temperature is inversely proportional to the peak wavelength, so the hotter the object, the bluer its radiation.
winter solstice Point on the ecliptic where the Sun is at its southernmost point below the celestial equator, occurring on or near December 21.
X
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.
Z
zero-age main sequence The region on the Hertzsprung—Russell diagram, as predicted by theoretical models, where stars are located at the onset of nuclear burning in their cores.
zodiac The twelve constellations through which the Sun moves as it follows its path on the ecliptic.
zonal flow Alternating regions of westward and eastward flow, roughly symmetrical about the equator, associated with the belts and zones in the atmosphere of a jovian planet.
zone Bright, high-pressure region, in the atmosphere of a jovian planet, where gas flows upward.