Glossary of Terms


21-cm emission line
The important radio radiation at 21-cm wavelength from interstellar neutral atomic hydrogen.


aberration of starlight
The apparent displacement on the position of a star that results from the finite speed of light and the speed of the Earth's orbit.
absolute brightness
Any measure of the intrinsic brightness or luminosity of a celestial object.
The loss of photons as light passes through a medium. A photon is lost when it strikes an electron, and the photon's energy is consumed in knocking the electron to a higher energy level.
absorption lines
Narrow spectral features that represent a reduction in intensity over a small wavelength range. They are caused by the loss of photons that raise an atom from a particular energy state to a higher energy state.
absorption spectrum
Dark lines superimposed on a continous spectrum.
The rate of change of velocity with respect to time.
The building of a larger mass from smaller masses sticking together.
accretion disk
A disk of hot gas and dust surrounding a star, usually used to denote material that has been thrown off one star onto a companion. There is weaker observational evidence for very large accretion disks in the central regions of active galaxies and quasars.
The difference between a true value and a measurement. Established by taking multiplemeasurements.
active galaxy
Galaxy whose nucleus emits more energy than other, normal galaxies. Typical signatures of active galaxies are variable brightness, broad emission lines, and strong radio emission.
The structure, process, or behavior that helps an organism survive and pass its genes on to the next generation.
adenosine triphosphate (ATP)
An energy-carrying molecule, found in a cell, that contains three phosphate groups, the sugar ribose, and the base adenine.
A process that requires the presence of oxygen; e.g., respiration.
age of globular clusters
About 12-13 billion years.
age of stars
Time since star formation, typically billions of years for smaller stars, but less than a million years for some massive stars in recently formed clusters. Age is difficult to measure for individual stars, but possible to measure for clusters of stars.
age of the Earth
The period since the Earth's formation from planetesimals, measured to be 4.6 billion years.
Age of the Universe
About 13 billion years, as determined by big bang model and age of oldest stars.
Ages of Groups of Stars
Determined by H-R diagrams and stellar models; color and luminosity of stars leavingmain sequence is a measure of age.
Describes the fraction of sunlight reflected by a surface; albedo = 0 means no reflection at all (a perfectly black surface); albedo = 1 means all light is reflected (a perfectly white surface).
Alpha Centauri
(1) The nearest star system, composed of three members; (2) the brightest of these three.
alpha particle
A helium nucleus emitted during the alpha decay of a nucleus.
In the Altitude-Azimuth coordinate system, the angle vertically from the horizon in the celestial sphere.
amino acid
A complex organic molecule important in composing protein and called a "building block of life".
The maximum deviation of a wave above or below zero point.
An organism that does not depend on free oxygen for its metabolism.
Andromeda galaxy
The nearest spiral galaxy comparable to our own, about 670 kpc away.
Angular Measurement
The angle covered or subtended by something (measured in degrees, minutes of arc, seconds of arc), as opposed to linear size of something (measured in units of kilometers or parsecs).
angular momentum
The tendency for bodies to continue revolving or rotating due to its inertia.
angular seperation
The observed angular distance between two celestial objects, measured in degrees, minutes, and seconds of angular measure.
angular size
The angle subtended by an object at a given distance.
Something that is not the same in every direction.
Anthropic Principle
The idea that certain characteristics of the physical universe are carefully tuned to allow the existence of carbon-based lifeforms.
Material, with equivalent properties to matter, but with subatomic particles' quantum properties reversed; for example, particles' charges are opposite. Antimatter happens to be rare in our universe. Matter and antimatter annihilate on contact to produce gamma rays.
The direction in the sky toward which the Sun appears to be moving relative to local stars; located in the constellation Hercules.
apparent brightness
The brightness of an object as perceived by an observer at a specified location (but not measuring the object's intrinsic, or absolute, brightness).
The most ancient of the three major branches of life on the Earth; others are Eukarya and Bacteria.
The study of the astronomical and cultural practices of ancient cultures.
1/60 of a degree, shown by the symbol '.
1/60 of an arminute or 1/3600 of a degree, designated by the symbol ".
Arrow of Time
The sense that time moves in only one direction. The flow of time is related to the increase in entropy of all physical systems.
An especially noticeable star pattern in the sky, such as the Big Dipper.
A rocky or metallic interplanetary body (usually larger than 100 meters in diameter).
asteroid belt
The grouping of asteroids orbiting between Mars and Jupiter.
The study of life on Earth and beyond; emphasizes research into questions of the origin of life, the conditions under which life can survive, and the search for life beyond Earth.
The superstitious belief that human lives are influenced or controlled by the positions of planets and stars; this belief is rejected by modern astronomers and other scientists.
The study of positions and motions of the stars.
astronomical unit
(AU) The mean distance from the Earth to the Sun, about 150 million kilometers.
The study of all matter and energy in the universe.
Layer of gas confined close to the planet's surface by the force of gravity.
atmospheric absorption or extinction
The decrease in light caused by passage through the atmoshpere.
atmospheric escape
The process in which gases in the upper atmosphere of a planet with enough kinetic energy eventually escape the planet's gravity.
atmospheric pressure
The force per unit area exerted by a planet's atmosphere.
A particle of matter composed of a nucleus surrounded by orbiting electrons.
atomic nucleus
The dense core of every atom. Atomic nuclei are positively charged and contain most of the mass of each atom, in the form of protons and neutrons.
Glowing, often moving colored light forms seen near the north and south magnetic poles of the Earth; caused by radiation from high-altitude air molecules excited by particles from the Sun and Van Allen belts.
The best estimate of a quantity based on multiple measurements, given by sum of independent values divided by number of measurements made. Also called mean value.
average density of matter
In cosmology, the average density of all material substance in the universe.
The straight line through a body about which the body rotates.
Angular position on the celestial sphere along the horizon as measured counterclockwise from the north.


The smallest type of living organisms.
A type of volcanic rock, often formed in lava flows, which is common on the Moon and terrestrial planets.
base pair
One of four possible bonding combinations of the bases adenine, thymine, guanine, and cytosine on the DNA molecule: AT, TA, GC, and CG.
beta particle
The electron or positron given off by beta decay.
Big Bang
The tremendous release of energy in the beginning of the universe from which all matter derived.
Binary Star
Two stars in orbit due to their mutual gravity.
binary x-ray source
A binary system containing an x-ray emitter, which is usually a collapsed object surrounded by a hot accretion disk giving off x-rays.
binding energy
The energy required to seperate completely the constituent parts of an atomic nucleus.
biological evolution
The natural development over time from simple to complex organisms, generated by mutations that change the gene structure and directed by natural selection of those individuals best-adapted to the enviroment.
The branch of science devoted to the study of living systems.
The layer of soil, water, and air surrounding the Earth in which living organisms thrive.
black hole
Any object whose surface gravity is so strong that no radiation or matter can escape. A black hole is the end-state of any star with a core more than about three times the Sun's mass. Theoretically, black holes much less amssive and much more massive than stars can exist.
blackbody spectrum
The continous spectrum emitted by a blackbody; the flux at each wavelength is given by the formula known as Planck's law.
Bohr atom
A particular model of atom, invented by Niels Bohr, in which the electrons are described as revolving about the nucleus in circular orbits.
bolometric luminosity
The total amount of energy emitted per second by an astronomical source, in all forms and at all wavelengths.
brecciated meteorite
A meteorite formed from cemented fragments of one or more meteorite types.
Energy per unit area recieved from or emitted by an object; object's luminosity.
brown dwarf
A star-like object too small to achieve nuclear reactions in its center; any stellar object smaller than about 0.08 solar masses.
The central position of a spiral galaxy that is roughly spherical (or football shaped) and bulges above and below the plane of the galactic disk.


A tool for dividing time; in particular, a way of the counting the days in a solar year, or one Earth orbit of the Sun.
captured moons
Satellites that did not originate in orbit around a planet but were captured into orbit from interplanetary space.
A class of modular molecules made from carbon, hydrogen, and oxygen that form the solid structure of living things and play a central role in how living things acquire oxygen.
carbon cycle
A series of nuclear reactions in which hydrogen is converted to helium, releasing energy in stars more massive than about 1.5 solar masses. Carbon is used as a catalyst.
Carbon Dioxide (CO2) Cycle
The cycle of carbon dioxide between the atmosphere and Earth's crust.
carbonaceous chondrite
A type of carbon-rich and volatile-rich meteorite, believed to be a nearly unaltered example of some of the earliest-formed matter in the solar system.
carbonaceous material
Carbon-rich rocks and soil seen on comets and on the surfaces of some of the moons of the giant planets.
Cassini's division
The most prominent gap in Saturn's rings.
An early scientific school which held that most features of nature formed in sudden events, or catastrophes, instead of by slow processes.
cause of the seasons
The tilt (obliquity) of the Earth's axis to its orbit plane causes first the North Pole and later the South Pole to be tipped toward the Sun during the course of a year.
A charge-coupled device, which is an electronic instrument for detecting light or recording an image--and is much more sensitive than film.
celestial navigation
Navigation on the surface of the Earth accomplished by observations of the Sun and stars.
The smallest unit of life processes; highly organized chemical factories.
cell membrane
A structure, formed from bilayers of lipids, that seperates the inside of the cell from the outside, or seperates one part of a cell from another.
center of gravity
Center of mass.
Cepheid variable
Any of a group of luminous variable stars with periods of 5 to 30 days (depending on their population). The periods are correlated with luminosity, allowing distance estimates out to about 3 Mpc.
Chandrasekhar limit
A mass of about 1.4 solar masses, the maximum for white dwarfs; stars of greater mass have too great a central pressure, causing formation of a star type denser than a white dwarf.
A field of study modeling systems in nature that can be described in Newtonian terms but whose futures are, for all practical purposes, unpredicatable; e.g., the turbulent flow of water or the beating of a human heart.
Chaotic Orbits
Objects that suffer many small gravitational influences, have positions that become impossible to predict after many orbital periods. Also, an orbit where a small change in the initial conditions makes a large change in the eventual outcome.
chemical element
A chemical that cannot be broken down into more basic chemicals.
chemical enrichment
The process by which the abundance of heavy elements (heavier than helium) in the interstellar medium gradually increases over time as these elements are produced by stars and released into space.
chemical potential energy
The type of energy that is stored in the chemical bonds between atoms, such as the energy in flashlight batteries.
chemical reaction
Reaction between elements or compounds in which electron structures are altered; atoms may be moved from one molecule to another, but nuclei are not changed and thus no element is changed to another.
The study of the composition, structure, properties, and reactions of atoms and molecules.
The main energy transformation organelles in plant cells; places where the molecules of chlorophyll are found and photosynthesis occurs.
Stony meteorite containing chondrules, believed to be little altered since their formation 4.6 billion years ago.
BB-sized spherule in certain stony meteorites, believed among the earliest-formed solid materials in the solar system.
A long strand of the DNA double helix, with the strand wrapped around a series of protein cores.
One of the Sun's outer layers, visible for a few minutes as a spectacular halo during during a total eclipse of the Sun.
circular velocity
The velocity required to keep an object in a circular orbit around a much larger object, such as a satellite orbiting Earth.
The average weather conditions of a place or area over a period of years.
Climate Change on Mars
Various pieces of evidence that Mars was warmer and wetter and had a thicker atmospherein the past.
close binary
A binary star system in which the stars are seperated by a distance roughly comparable to their diameters.
cluster of galaxies
A relatively close grouping of galaxies, often with some members coorbiting or interacting with each other.
cluster of stars
A group of stars that formed together and that have remained together due to their mutual gravitational attraction.
Any group of objects where the average spacing is less than the average spacingof a randomly distributed population is said to be clustered. Gravity causes clusteringof both stars and galaxies.
CNO cycle
A series of nuclear reactions in which carbon is used as a catalyst to transform hydrogen into helium.
collecting area
The amount of area a telescope has that is capable of collecting electromagnetic radiation. Collecting area is important for a telescope's sensitivity: the more radiation it can collect (that is, the larger its collecting area), the more likely it is to detect dim objects.
An ice-rich interplanetary body that, when heated by the Sun in the inner solar system, releases gases that form a bright head and diffuse tail. (See also coma.)
comet head
The coma and nucleus regions of a comet.
comet nucleus
The brightest star-like object near the center of a comet's head; the physical body (believed to be icy and a few kilometers across) within a comet.
comet tail
Diffuse streamers of gas and dust released from a comet and blown in the direction away from the Sun by the solar wind.
comparative planetology
An interdisciplinary field of astronomy and geology attempting to discover and explain differences between planets in properties such as climate and interior structure.
A substance composed of two or more atoms bound by chemical forces.
condensation sequence
The sequence in which chemical compounds condense to form solid grains in a cooling, dense nebula.
One of three processes that transfers heat from hot to cold regions; occurs as fast-moving molecules in the hot region agitate adjacent molecules.
conservation law
Any statement that says that a quantity in nature does not change.
conservation of angular momentum
A useful physical rule which states that the total angular momentum in an isolated system remains constant.
conservation of energy
A law of nature stating that the total amount of energy in an isolated system always remains constant.
Imaginary pattern found among the stars, resembling animals, mythical heroes, and the like; different cultures map different constellations.
continuous spectrum
A spectrum made up of all wavelengths, without emission or absorption lines or bands.
One of three modes of transmission of heat (energy) from hot regions to cold regions; involves motions of masses of material.
Copernican revolution
The intellectual revolution associated with adopting Copernicus' model of the solar system, which displaced the Earth from the center of the universe.
The densest inner region of the Earth, probably of nickel-iron composition; in other planets, similar high-density central regions; in the Sun or stars, a dense central region where nuclear reactions occur; in galaxies, the densest, brightest central regions.
core hydrogen burning
The thermonuclear fusion of hydrogen at the center of the star.
The outermost atmosphere of the Sun, having a temperature of about 1 to 2 million Kelvin.
Cosmic Abundance
The average abundance in the universe of all the stable elements.
cosmic background radiation
The nearly uniform thermal radiation believed to be a relic of the hot big bang. It is observed in microwaves with a temperature of 2.7 K.
Cosmic Evolution
The inevitable fact that distant objects in the universe are observed as they were when they were younger, as a result of the large light-travel time.
cosmic nucleosynthesis
The fusion of light elements during the early hot phase of the big bang, to produce heavier elements. It resulted in nearly a quarter of the mass of the universe being turned from hydrogen into helium.
cosmic rays
High-energy atomic particles (85% protons) that enter the Earth's atmosphere from space. Many may originate in supernovae and pulsars.
cosmological constant
Term that can be added to the equations of general relativity to give a static solution.
cosmological principle
The assumption that the universe is homogeneous and isotropic. This assumption is crucial to modern cosmology, and it has been shown to be a good approximation to the observed state of the universe.
cosmological redshift
Any redward Doppler shift attributed to the mutual recession of galaxies or the expanding universe.
The study of the structure of the universe. The term is often broadened to include the origin of the universe as well.
The universe as an ordered system.
A circular depression on the surface of a planet or moon, usually left behind after a meteor impact or a volcanic eruption.
Cretaceous-Tertiary impact
The impact of one or more asteroids or comets, up to about 10 km in size, about 65 million years ago, which apparently led to extinction of most species of plants and animals living at that time, including dinosaurs.
critical density
The mass density needed to just halt the universal expansion.
The outermost, solid layer of a planet, with composition distinct from the mantle and differentiated by a seismic discontinuity.
The photosynthetic bacteria that produced the oxygen which built up in Earth's atmosphere.


dark matter
Mass that makes its presence felt by gravitational forces but does not emit light. It is usually detected by anomalously high orbital velocities in gravitational systems. Dark matter makes up about 90% of the mass of the universe, and its nature is still unknown.
Decimal System
The system of counting and arithmetic based on factors of ten (the binary systemis based on factors of two).
Angular distance north or south of the celestial equator. (Abbreviation: Dec.)
The point in the history of the universe when the big bang radiation is cool enough for stable atoms to form. At this point, radiation and matter decouple, and photons travel freely through the universe.
A logical method for combining ideas, observations, measurements, or numbers. In word form, deduction allows well-justified premises to be combined to draw a reliableconclusion (see also induction).
degenerate electron (neutron) pressure
The pressure exerted by degenerate electrons (neutrons).
The amount of mass per unit volume in a region of space.
density parameter
The ratio of the mean mass density of the local universe to the density required to just halt the universal expansion, given the symbol Omega(0).
Deoxyribonucleic Acid
Abbreviated as DNA, this long molecule in the shape of a double helix is the keyto life. DNA carries the genetic code in a four letter chemical alphabet.
A "heavy"; form of hydrogen, in which the nucleus of each atom consists of one proton and one neutron.
Any process that tends to separate different chemicals from their original mixed state and concentrate them in different regions.
differentiation (geological)
Gravitational seperation or segregation of materials of different density into layers in the interior of a planet or satellite.
Digital Information
Any form of information that comes in the form of (or can be converted into) discrete levels suitable for manipulation by a computer. The simplest form of digital informationis a binary code consisting of two levels, 0 and 1.
dirty iceberg model
A theoretical description of a comet nucleus as a large icy body with bits of silicate "dirt"; embedded in it.
The flattend part of a spiral galaxy outside of of the bulge.
Distance Indicator
Any property of a star or galaxy that can be used to measure distance; usually involves a local calibrator whose distance is known via a different technique.
distance scale
An overlapping set of techniques that are used to measure distances in the universe, starting with direct geometric methods such as parallax for nearby stars, and ending with global measures of galaxies. The errors in the distance scale increase with increasing distance from the Earth.
DNA (deoxyribonucleic acid)
A long, replicating molecule, shaped like a twisted ladder, that is the basis of the genetic code. Information for all of life's functions is coded in the sequence of base pairs that join the two strands. DNA is found in all living organisms.
Doppler effect
The shift in wavelength of light or sound as perceived by the observer of an approaching or receding body. For speeds well below that of light, the shift is given by the equation Original wavelength * radial velocity / velocity of light.
Drake equation
The statement that the fraction of stars harboring intelligent life equals the number of all stars times a sequence of fractions, such as the fraction of all stars having planets, the fraction of planets that are habitable, and so on. Named after radio astronomer Frank Drake.
dust scattering
Scattering of light by particles much bigger than the light's wavelength. Causes extinction and reddening of starlight.
dwarf elliptical galaxy
An ellipsoidal galaxy resembling a globular cluster but usually at least a few times larger.


earth-crossing asteroids
Asteroids that have orbits that can cross Earth's orbit of the Sun.
Vibration or rolling motion of the Earth's surface accompanying the fracture of underground rock.
The ratio of the length between the center and the focus to the semimajor axis of an ellipse.
An event in which the shadow of one body falls on another body.
(1) The plane of the Earth's orbit and its projection in the sky as seen from Earth; (2) approximately, the plane of the solar system.
electromagnetic force
One of the four fundamental forces of the universe.
electromagnetic radiation
Light, radio waves, X rays, and other forms of radiation that propagate as disturbances in electric and magnetic fields, travel at the speed of light, and combine to make up the electromagnetic spectrum.
electromagnetic spectrum
The full range of electromagnetic radiation from long to short wavelengths, or low to high frequencies. Only the narrow wavelength range of the visible spectrum can be detected by the eye. Radio waves, microwaves, and infrared radiation are at longer wavelengths; ultraviolet radiation, X rays, and gamma rays are at shorter wavelengths.
Negatively charged particle orbiting around the atomic nucleus, with mass 9.1×10-31 kg.
A chemical material with a specified number of protons in the nucleus of each atom. Atoms with one proton are hydrogen; with two protons, helium; and so on.
A closed, oval-shaped curve (generated by passing a plane through a cone) describing the shape of the orbit of one body around another.
elliptical galaxy
A galaxy that has rotational symmetry and an ellipical shape.
elliptical orbit
The general shape that describes the motion of one object bound by gravity to another object; a closed figure like a squashed circle.
Release of electromagnetic radiation from matter.
emission line
Very narrow wavelength intervals in which atoms emit light.
Derived from observation or experiment.
In physics, a specific quality equal to work or the ability to do work. Energy may appear in many forms, including electromagnetic radiation, heat, motion, and even mass (according to the theory of relativity).
energy level
The orbit of an electron around the nucleus of an atom.
English system
A nondecimal system of units using pounds, inches, and seconds. In scientific use, and in most countries, it is replaced by the more convenient metric system.
The measure of disorder in a physical system, which always tends to increase with time. Entropy is also related to the number of possible states of a system, since ordered states are less probable.
A protein that causes or accelerates chemical reactions at a lower temperature than those reactions would normally occur.
A small circular motion superimposed on a larger circular motion.
epicycle theory
An early theory by Ptolemy that the planets move around the Earth in epicycles.
A state in which there is no net change in a system.
The date when the Sun passes through the Earth's equatorial plane (occurs twice annually).
Removal of rock and soil by any natural process.
escape temperature
The temperature that particles in a planet's atmosphere will escape into space.
escape velocity
The minimum speed needed to allow a projectile to move away from a planet and never return to its point of launch. It equals 21/2 * circular velocity.
A rough calculation that is accurate to no more than one or two significant figures. Also called an order of magnitude calculation.
Cell with a nucleus, that is, with DNA contained by an interior membrane; a multicelled organism. Eukaryotes first appeared about 1.4 billion years ago.
event horizon
Imaginary surface at the distance from a black hole where the escape velocity is the speed of light. Matter and energy cannot escape from inside the event horizon.
Scientific evidence consists of quantitative observations or experimental results that can be confirmed by other investigators.
evidence for planets near other stars
Although planets the size of Jupiter or smaller near other stars are beyond our current detection capability, new techniques of imaging and astrometry will allow their detection within a few years, if they exist. Already some objects larger than Jupiter but smaller than stars have been found.
Changes caused by environmental pressures over time.
expanding universe
A term popularized by Eddington to describe the mutual recession of galaxies.
explosive nucleosynthesis
In the explosive expanding shell of a supernova, the formation of nuclei of heavy elements up to plutonium by means of rapid neutron capture.
The dimming of light as it passes through a medium, such as gas or dust.
extrasolar planet
A planet orbiting a star beyond the solar system.


false color
A technique scientists use to help them see details in images of objects. The colors the scientist picks for his or her image may have nothing to do with the color of the object. Sometimes the chosen colors represent the density, temperature, or other properties of parts of the object. Thus, the viewer of a false color image must always be told by the creator of the image what the colors mean.
A fracture along which displacement has occurred on the solid surface of a planet or other celestial body.
Entities dispersed in space but having a measurable value or magnitude that can be measured at any point in space. Examples are gravitational, electric, and magnetic fields.
An unusually bright meteor, which may yield meteorites.
first quarter
The phase of the Moon when it is one-fourth of the way around its orbit from new moon; the first quarter moon is seen in the evening sky with a straight terminator and half the disk illuminated.
One of the two interior points around which planets or stars move in an elliptical orbit.
In physics, a specific phenomenon producing acceleration of mass. Forces can be generated in many ways, such as by gravity, pressure, and radiation.
The remains of a living organism that have been turned to stone over a long period of time.
free-fall contraction
Contraction of a cloud or system of particles by gravity only, unresisted by any other force.
Number of electromagnetic oscillations per second corresponding to electromagnetic radiation of any given wavelength.
frost line
A region between 3 and 4 AU from the Sun in the asteroid belt. Asteroids beyond this region have water that is frozen and inactive.
full moon
The phase of the Moon when it is closest to 180° from the Sun and therefore fully illuminated.
fundamental forces
Four forces that account for all the physical behavior in the universe. Gravity and electromagnetism are long-range forces, and the strong and weak nuclear forces operate only within atoms.
A process where nuclei collide so fast they combine, overcoming the natural repulsion of the positively charged protons. In the center of most stars, hydrogen fuses together to form helium. Fusion is so powerful it supports the star's enormous mass from collapsing in on itself, and heats the star so high it glows as the bright object we see today.


galactic bulge
The spheroidal distribution of stars toward the center of the Milky Way that are intermediate in age between the disk stars and halo stars.
galactic cannibalism
The absorption of one galaxy by another during a collision, forming a new, larger galaxy.
galactic disk
The thin formation of gas and dust on circular orbits where most of the young stars in the Milky Way are found.
galactic halo
A spherical swarm of globular clusters above and below the galactic disk, centered on a point in the direction of the constellation Sagittarius.
galactic latitude
Angular distance around the galactic equator from the galaxy's center.
galactic morphology
The appearance of a galaxy in terms of its different components: disk, bulge, halo, bar, nucleus.
galactic rotation curve
The orbital speed of an object as a function of distance away from the center of a galaxy.
Any of the largest groupings of stars, usually of mass 108 to 1013 solar masses.
galaxy clusters
A concentration of galaxies held in one large region of space by gravity, with anywhere from a few dozen to thousands of members.
galaxy evolution
The gradual change in the overall properties of galaxies with time, due to the evolution of the stars they contain.
galaxy formation
The process in which enormous clouds of gas collapse by gravity into systems of stars. Most galaxy formation took place 5 to 10 billion years ago.
galaxy group
Between a few a few dozen galaxies held in on region of space by gravity.
galaxy interactions
In regions of the universe that are denser than average, the interactions of galaxies at a distance by the force of gravity. Collisions and mergers can also occur.
galaxy supercluster
An enormous concentration of galaxies that may contain thousands to tens of thousands of members.
Galilean satellites
The four large satellites of Jupiter, discovered by Galileo.
gamma-ray burster
The theoretical interpretation is unclear, but these sources display erratic bursts of high energy gamma-rays and no preference for the disk of the galaxy.
A low-density form of matter in which atoms or molecules can move freely and in which collisions are rare. Any substance can be turned into a gas, given a sufficient temperature.
gas giant planet
A planet made primarily of gaseous hydrogen and helium; in the solar system, the outer planets Jupiter, Saturn, Uranus, and Neptune.
gas laws
The mathematical relationships that govern the temperature, density, and pressure of gases.
The minimum amount of genetic material that expresses a characteristic of living organism; a sequence of several hundred bases along DNA molecule.
general theory of relativity
Einstein's theory that deals with accelerations caused by gravity and other forces. It begins to differ substantially from Newton's theory only when gravitational fields begin to get very strong. It predicts that the structure of space-time is affected by the mass-energy of the universe.
Earth-centered; The idea that everything in the cosmos is centered on the Earth.
geocentric cosmology
The description of a spherical universe with Earth stationary at the center.  As described by Artistotle, this cosmology was user for over 2000 years.
geological time scale
The sequence of events in the history of the Earth.
The study of the history, origin, and structure of Earth.  This term has been extended to include other planets.
giant star
Highly luminous star larger than the Sun. O and B main-sequence stars are sometimes called blue giants; evolved stars of extremely large radius are called red giants.
globular cluster X radiation
X-ray radiation from globular clusters, discovered unexpectedly in the 1970s; it indicates energetic environments somewhere within them.
globular star cluster
A dense spheroidal cluster of stars, usually old, with mass of 104 to 106 solar masses.
A rock type of modest density and high silica content, formed in association with differentiation processes and, therefore, found primarily on the Earth.
gravitational collapse
The contraction of matter due to its internal gravitational attraction.
gravitational contraction
Slow contraction of a cloud, star, or planet due to gravity, causing heat and radiation.
gravitational force
The force exerted by gravity on any object with mass.
gravitational lensing
The creation of a distorted image of a distant quasar or galaxy when its light is focused by the gravity of a galaxy between it and us.
gravitational microlensing
The amplification of the light of a background star due to the presence of an object between us and the star.
gravitational perturbation
A small deviation to an orbit that can lead to a very different eventual position.
gravitational potential energy
The energy an object with mass has when it is in a gravitational field.
The force by which all masses attract all other masses.
Great Red Spot
A large, reddish, oval, semipermanent cloud formation on Jupiter.
greenhouse effect
Heating of an atmosphere by absorption of outgoing infrared radiation.
greenhouse warming of venus
An extremely hot atmosphere resulting from a high concentration of carbon dioxide.
Gregorian calendar
Essentially the modern calendar system, introduced around A.D. 1600 under Pope Gregory XIII, and containing the modern system of reckoning leap years.


H-R diagram
A technique for representing stellar data by plotting spectral type (or color or temperature) against luminosity (or absolute magnitude), named after its early proponents, Hertzsprung and Russell.
In any phenomenon, the time during which the main variable changes by half its original value; often used loosely to indicate the characteristic time scale of a phenomenon. In radioactive decay, the time for half the atoms in a system to disintegrate.
Halley's comet
The most famous comet, which visits the inner solar system every 76 years, most recently in 1986 when close-up photos were made by space probes.
The spherical region of a spiral galaxy composed of diffuse gas and containing few stars and star clusters.
The energy of motion of particles in an object.
Heisenberg uncertainty principle
A fundamental limitation to the precision of physical measurements. It states that we cannot know with arbitrary accuracy both the position and momentum of a system, or the energy of the system at every instant of time.
Sun-centered; The idea that everything in the cosmos is centered around the Sun.
heliocentric cosmology
The description of the universe with the Sun at the center and the planets in orbit around the Sun; proposed by Copernicus and shown to be correct by Galileo.
helium burning
The fusion of helium into carbon through the triple-alpha process.
Helmholtz contraction
Slow contraction of a cloud or system of particles by the force of gravity, which is retarded by outward gas pressure and the limited rate at which radiation can escape.
The unit of frequency; equal to 1 cycle per second.
HI region
Interstellar region in which hydrogen is predominantly neutral.
HII region
Interstellar region in which hydrogen is predominantly ionized.
Uniform in composition throughout the volume considered.
The boundary of the observable universe, defined by the distance that light can travel in the age of the universe.
Hubble Constant (H)
A measure of the rate of expansion of the Universe; current estimates are about 70 km/s/megaparsec.
Hubble law
The observed linear relationship between the distance of a galaxy and its velocity of recession. In modern cosmology, this is interpreted as evidence of an expanding universe.
hubble relation
A linear correlation between redshift and distance for galaxies. More distant galaxies move away from the Milky Way faster, which is evidence for an expanding universe. The slope of the Hubble relation gives the current expansion rate. Also called the Hubble law.
hydrogen burning
The fusion of hydrogen into helium or heavier elements.
hydrostatic equilibrium
The balance that exists at every point in a stable star between the inward force of gravity and the outward pressure due to energy released from nuclear reactions.
A proposed explanation of an observed phenomenon or a proposal that a certain observable phenomenon occurs.


icy material
Frozen liquids and gases that form much of the material in gas giant planets and comets.
ideal gas law
The pressure of an ideal gas is directly proportional to the temperature and density of the gas.
igneous rock
Rock crystallized from molten material.
impact crater
A roughly circular depression of any size (known examples range from microscopic size to diameters greater than 1000 km) caused by a meteorite impact.
impact-trigger theory
The leading theory of the moon's origin, in which material was blasted off Earth's mantle and then reaccumulated to form the Moon.
impacts of interplanetary debris
Collisions of interplanetary rocky or icy bodies with Earth (or with other worlds). Small impacts are much more common than large impacts.
A logical method for drawing a broad conclusion from a limited set of observations or experiments. The truth of an induction argument cannot be guaranteed with certainty.
An object's resistance to any change in its motion.
inflationary universe
A modification of the standard big bang model; it says that the universe went through a brief early period of unusually rapid expansion just after the big bang itself. Inflation helps to explain the smoothness of the microwave background radiation and the fact that the geometry of the universe is close to flat.
Radiation of wavelength too long to see, usually about 1 to 100 µm.
infrared telescope
A telescope designed to detect infrared radiation.
Variable in density and not the same at all locations.
The capability for abstract thought, coupled with a mastery of tools or technology. In living organisms, intelligence is found only in animals with large and complex brains.
The strength of radiation. In the wave description of radiation, intensity is proportional to the square of the amplitude of the wave.
A system for obtaining high-resolution astronomical observations by linking several physically separated telescopes electronically, in effect creating a single, much larger telescope.
interstellar extinction
The absorption, of obscuration, of light by interstellar dust, causing distant objects to appear fainter.
interstellar medium
The gas and dust drifting between the stars in a galaxy.
interstellar obscuration
Absorption of starlight by interstellar dust, causing distant objects to appear fainter.
interstellar reddening
Loss of blue starlight due to interstellar dust, causing distant objects to appear redder and fainter.
inverse square law
The relation describing any entity, such as radiation or gravity, that varies as 1/r2, where r is the distance of the entity from the source.
Charged atom or molecule.
The process of knocking one or more electrons off a neutral atom or molecule.
ionized gas
A gas in which many of the atoms have lost at least one electron, thus becoming charged particles, or ions.
iron meteorite
Meteorite composed of a nearly pure nickel-iron alloy.
irregular galaxy
A galaxy of amorphous shape. Most have relatively low mass (108 to 1010 solar masses).
A variation of a chemical element with more neutrons in the atmoic nucleus; usually radioactive.
The same in all directions.


The largest planet in the solar system, containing just over two-thirds of the mass of all the planets.


Kelvin-Helmholtz contraction
The contraction of a gaseous body, such as a star or nebula, during which gravitational energy is transformed into thermal energy.
Kepler's first law
The statement that each planet moves around the Sun in an elliptical orbit with the Sun at one focus of the ellipse.
Kepler's law of planetary motion
The three laws of planetary motion that describe how the planets move in elliptical orbits of the Sun, and allow accurate prediction of planetary positions.
Kepler's second law
The statement that a planet sweeps out equal areas in equal times as it orbits the Sun; also called the law of equal areas.
Kepler's third law
A relationship between the period of an orbiting object and the semimajor axis of its elliptical orbit.
Keplerian motion
Orbital motion that follows kepler's laws.
kiloparsec (kpc)
1000 parsecs, or 3260 light years.
kinetic energy
The amount of energy an object has due only to its velocity.
Kirchhoff's laws of radiation
Laws governing radiation from hot objects. Every sufficiently hot gas will emit a continuous spectrum. A hot gas will emit a set of spectral lines characteristic of the elements in the gas. A cool gas in front of a hot continuum source will produce a set of spectral lines in absorption.
Kuiper belt
A group of comets lying in the plane of the solar system at a distance of 30 to 100 AU from the Sun, just beyond the orbits of Neptune and Pluto.


large-scale structure
The distribution of matter in the universe on the largest scales, from groups and clusters of galaxies to superclusters of galaxies.
The anglular distance north or south an object is as measured from the equator.
law of conservation of angular momentum
The fact that the amount of angular momentum in a system is constant. If the system shrinks, the rotation speed will increase; if the system expands, the rotation speed will decrease.
law of conservation of energy
The fact that the total amount of energy in a system is constant, even though energy may change forms.
laws of thermodynamics
Rules that govern the behavior of heat and other forms of energy. Energy can change forms, but the total amount of energy in the system is constant. As energy changes forms, the proportion of energy in the disordered form of heat increases. It is impossible to remove all the heat from a physical system.
The fundamental measure of size or distance, represented by units of meters in the metric system.
A self-contained set of chemical reactions involving carbon based molecules. Living things grow, evolve, reproduce, utilize energy, and respond to their environment.
life on Mars
Evidence pointing to fossil microbial life forms in a Martian meteorite that was recovered on Earth.
light curve
A plot of the brightness of a star (or any astronomical object) versus time.
light travel time
The time it takes light to travel a certain distance in the universe. Typical light travel times are hours within the solar system, years to the nearest stars, millions of years to the nearest galaxies, and billions of years to the most distant galaxies.
The distance light travels in 1 year, 9.5x 1012 km.
A high-density form of matter in which the atoms or molecules can move freely and take the shape of the container. Matter densities of liquids and solids are similar.
local group
The concentration of galaxies that contains the Milky Way, Andromeda (M31), and a few dozen small galaxies.
local supercluster
The large flattened concentration of galaxies that contains the Milky Way, the Local Group, the Virgo cluster, and thousands of other galaxies.
The total amount of energy emitted per second by an astronomical source.
lunar eclipse
The dimming of the Moon as it passes through Earth's shadow cast by the Sun.


Magellanic clouds
The two galaxies nearest the Milky Way, irregular in form and visible to the naked eye in the Southern Hemisphere.
A measurement of an object's brightness.
main asteroid belt
The largest group of asteroids, found on circular orbits between Mars and Jupiter.
main sequence
Stars that convert hydrogen to helium in their cores through the p-p or CNO cycles.
main-sequence fitting
A technique for measuring the relative distance of two groups of stars, based on the vertical offset between the two main sequences on an H-R diagram.
A region of intermediate density surrounding the core of Earth or any other planet.
The fourth planet form the Sun, most similar to Earth in terms of geology and surface features.
The fundamental measure of the amount of material, represented by units of kilograms in the metric system. Also, a measure of the resistance of an object to any change in its motion.
mass extinction
A brief interval, compared with the time scale of evolution, when a significant fraction of Earth's species become extinct. Some mass extinctions may be caused by catastrophic impacts from space.
mass loss
The rate stars lose mass through stellar wind.
mass-energy conversion
The conversion of mass into a much larger amount of equivalent energy, according to Einstein's equation E=mc2. Energy can be converted into mass according to the same equation.
mass-luminosity relation
The mathematical relationship between the mass of main-sequence stars and their total emission. More massive stars have far higher values of luminosity.
mass-to-light ratio
The ratio of the mass of an astronomical object to its luminosity, in solar units. The mass-to-light ratio of the Sun is one, by definition.
The study of numbers and their properties, and the symbols and operations that can apply to numbers.
A quantitative way to present scientific evidence, in the form of a number, an error or uncertainty, and a unit that represents the type of measurement.
One million parsecs, or 3.26 million light-years.
The small rocky planet closest to the Sun, similar in size to Earth's moon.
An imaginary line in the sky that runs form the direction of the South Pole, through the zenith, to the direction of the North Pole. Objects to the east of the meridian are said to be rising, and objects to the west of the meridian are said to be setting.
metamorphic rock
Rock formed when igneous or sedimentary rocks are modified by heat, pressure, or chemical reactions.
A small interplanetary body that burns up in Earth's atmosphere. Also called shooting star.
meteor shower
A concentrated group of meteors, seen when the Earth's orbit intersects debris from a comet.
A rocky or metallic interplanetary object large enough to reach Earth's surface.
A particle in space, generally smaller than a few meters across.
metric system
A system of measurement based on powers of ten, with fundamental units of kilograms for mass, meters for length, and seconds for time.
microwave background fluctuations
Tiny spatial variations in the temperature of the microwave background radiation; they form the seeds for eventual galaxy formation.
Milky Way galaxy
The spiral galaxy in which we live.
Miller-Urey experiments
A series of experiments in which amino acids and other organic molecules were created in laboratory conditions that simulate the conditions of the early Earth.
molecular cloud
An interstellar cloud of gas and dust, with greater than average density, dust content, and high concentration of molecules.
A substance made up of two or more bound atoms.
A measure of the motion of a body equal to the product of its mass and velocity.
morphology-density relation
The observation that the relative number of spiral and elliptical galaxies depends on the density of the region. High-density regions or clusters have a larger fraction of ellipticals than low-density regions.
multiple star system
A system of three or more stars orbiting around each other.
The fundamental mechanism for generating change in genetic material. Mutations alter the molecular structure of the DNA molecule in ways that can be helpful, harmful, or neutral to the organism.


A unit of length equal to 10-9 meters.
natural selection
The theory that states that those individuals best adapted to the ever-changing environment produce a greater number of offspring.
A cloud of dense gas and/or dust in interstellar space or surrounding a star.
negatively curved geometry
A geometry in which parallel lines diverge; sometimes called a hyperbolic geometry.
The outermost gas giant planet in the outer solar system.
A subatomic particle created in certain nuclear reactions inside stars. It can pass through most matter. Its mass is uncertain, being either zero or a tiny fraction of an electron's mass.
One of the two major particles constituting the atomic nucleus; it has zero charge and mass 1.6749×10-27 kg.
neutron star
A star with a core composed mostly of neutrons, with density 1016 to 1018 kg/m3. Many or most neutron stars are pulsars.
new moon
The phase of the moon when it is nearest the Earth-Sun line, hence invisible from Earth because of the Sun's glare.
A unit of force, equal to the product of an object's mass and acceleration.
Newton's laws of motion
Three rules describing motion and forces. Briefly, (1) a body remains in its state of motion unless a force acts on it; (2) force equals mass times acceleration; (3) for every action there is an equal and opposite reaction.
nonthermal radiation
Radiation not due to the heat of the source; for example, synchrotron radiation.
A type of suddenly brightening star (from the Latin for "new") resulting from explosive brightening when gas is dumped from one member of a binary star pair onto the other.
nuclear fission
The splitting of a massive atom into two or more lighter atoms to release energy.
nuclear fusion
The merging of light atomic nuclei into heavier atomic nuclei, with the release of particles and radiation.
nucleic acid
The large molecule on which genetic material is based.
The production of all the elements on the periodic table from the fusion of lighter elements to heavier ones.
(1) The matter at the center of an atom, composed of protons and neutrons; (2) the bright central core (or solid body) of a comet; (3) the bright central core of a galaxy.


The angle by which a planet's rotation axis is tipped to its orbit.
observable universe
The parts of the universe that can be detected by the light they emit.
observational error
The uncertainty in a measurement, often caused by limitations in the equipment or observation.  Every measurement has some degree of error.  Random errors can be reduced by multiple measurements, but systematic errors are caused by unrecognized flaws in the measuring apparatus.
The passage of an object of large angular size in front of a smaller object, such as the Moon in front of a distant star or the rings of Saturn in front of the Voyager spacecraft.
Olbers' paradox
The problem of why the sky is dark at night if the universe is filled with stars.
Oort cloud
The swarm of comets surrounding the solar system.
The extent to which gaseous (or other) material absorbs light.
open star cluster
A grouping of relatively young Population I stars (usually 10<sup>2</sup> to 10<sup>3</sup> solar masses), sometimes called a galactic cluster.
The manipulation of light by reflection or refraction, usually to make an image.
The path of a body that is in revolution about another body or point.
organic chemistry
Chemistry involving organic molecules.
organic molecule
Molecule based on the carbon atom, usually large and complex, but not necessarily part of living organisms.
An ordered living creature.
Orion star-forming region
The larger region in which stars are forming around the Orion Nebula.
A heavy molecule of oxygen that contains three atoms rather than the more normal two.
ozone hole
A region of the Earth's atmosphere over Antartica where the concentration of ozone is abnormally low.
ozone layer
An atmospheric layer rich in ozone (O<sub>3</sub>), created by the interaction between oxygen molecules (<span style="vertical-align: sub" class="Apple-style-span">O<sub>2</sub></span>) and solar radiation. On Earth, its altitude is about 20 to 60 km.


An angular shift in apparent position due to an observer's motion; more specifically, a small angular shift in a star's apparent position due to the Earth's motion around the Sun. Stellar parallax, used to measure stellar distance, is defined as the angle subtended by the radius of the Earth's orbit as seen from the star.
parallax distance
A direct and trigonometric measurement of distance for a star, based on the parallax shift observed from Earth.
parent body
The larger interplanetary object of which a meteorite is a fragment.
A distance of 206,265 AU, 3.26 ly, or 3.09×1013 km; defined as the distance corresponding to a parallax of 1 second of arc.
particlelike properties of light
Characteristics of light, such as concentration of energy and momentum in discrete microscopic packets (photons) that mimic the properties of particles.
period-luminosity relation
A correlation between the luminosity and the period of variation of Cepheid variable stars, which enables astronomers to measure the period of Cepheids in a remote galaxy and infer the distance of the galaxy.
periodic process
Any physical process that repeats in a regular way, with a fixed cycle of variations, such as waves, vibrations, and oscillations.
periodic table
A way of organizing the elements according to the number of outer electrons in each atom. Elements in a particular column of the periodic table share many chemical properties.
Semipermanent underground ice.
phases of the Moon
The 29.5-day cycle of changing illumination of the Moon as seen from Earth. The Moon is always half-lit by the Sun, but we see a different fraction of the lit face as the Moon orbits Earth.
Phobos and Diemos
The two small, irregularly shaped moons of Mars.
The measurement of the amount of light, either total or in different specified colors, coming from an object.
The quantum unit of light, having some properties of a wave. For each wavelength of radiation, the photon has a different energy.
The light-emitting surface layer of the Sun.
Process that converts sunlight into stored chemical energy, essential for the proliferation of advanced forms of life.
physical binary stars
Two stars orbiting around a common center of mass.
The study of the forces of nature and the laws that govern the way matter and radiation interact.
A solid (or partially liquid) body orbiting around a star but too small to generate energy by nuclear reactions.
planetary nebula
A type of circumstellar gas cloud that has spheroidal shape and often appears as a faint disk in telescopes; it has nothing to do with planets except for the rough resemblance to a planet's shape when seen telescopically.
One of the small bodies from which planets formed, usually ranging from micrometers to kilometers in diameter.
A high-temperature gas consisting entirely of ions, instead of neutral atoms or molecules. Because of the high temperature, the atoms strike each other hard enough to keep at least the outer electrons knocked off.
plate tectonics
Motions of a planet's lithosphere, causing fracturing of the surface into plates. Primary example occurs on the Earth.
plurality of worlds
The idea that Earth is just one among many worlds in space, including the idea that there may be applications of geology, chemistry, and biology beyond Earth.
Cataloged as the outermost and smallest planet in the solar system, but possibly one of many small worldlets on the fringe of the solar system.
polar ice caps of Mars
Regions of frozen carbon dioxide at eh poles of mars that grow and shrink with the Martian seasons. A smaller amount of frozen water remains at each pole year round.
Population I
Stars with a few percent heavy elements (heavier than helium), found in the disks of spiral galaxies and in irregular galaxies.
Population II
Stars composed of nearly pure hydrogen and helium, found in the halo and center of spiral galaxies, in elliptical galaxies, and to a limited extent in irregulars.
positively curved geometry
A geometry where parallel lines converge, sometimes called a spherical geometry.
potential energy
Stored energy, or energy with the potential to do work. Examples are the energy stored in chemical bonds, the energy stored in a raised object, and the mechanical energy in a coiled spring or rubber band.
pre-main-sequence star
Evolutionary state of stars prior to arrival on the main sequence, especially just before the main sequence is reached.
The wobble in the position of a planet's rotation axis caused by external forces. Also, the change in a coordinate system (tied to any planet) caused by such a wobble.
A measure of how finely a quantity can be specified, in terms of the number of significant figures in the measurement. Most observations in astronomy are quoted with a precision of no more than three significant figures.
prograde and retrograde satellite orbits
Satellite orbits in which motion is in the same or opposite direction, respectively, as the planet's rotation.
prograde rotation
Spinning on an axis from west to east, or counterclockwise as seen from the North Pole (as in the case of the Earth).
Cell that contains a single long strand of DNA but no nucleus. Prokaryotes were the first and simplest life forms on the Earth, dating back 3.8 billion years.
proper motion
The angular rate of motion of a star or other object across the sky. (Most stars have proper motions less than a few seconds of arc per year.)
Any of several types of complex organic molecules made from amino acids inside plants and animals, which are essential in living organisms.
One of the two basic particles in an atomic nucleus, with positive charge and mass 1.6726 = 10-27 kg.
proton-proton chain
A series of thermonuclear reactions that convert hydrogen nuclei to helium nuclei, converting a tiny amount of mass into energy.
A gravitationally stable cloud of stellar mass contracting in an early pre-main-sequence evolutionary state.
A rapidly rotating neutron star with a strong magnetic field, observed to emit pulses of radiation.


quantum fluctuations
Microscopic violations of the law of conservation of energy, when particles can momentarily come in and out of existence. Quantum fluctuations exist even in the vacuum of space; in the inflationary cosmology, they are magnified into the seed for galaxy formation.
quantum mechanics
The branch of physics dealing with the structure and behavior of atoms and their interaction with light.
quantum state
The quantum description of the arrangement of electrons in an atom; allowed quantum states are filled starting with those of lowest energy first.
quantum theory of radiation
The idea that energy is not smooth and continuous but comes in discrete packets called photons. Light is made up of these microscopic units of radiation.
An active galaxy seen at such a large distance that the bright nucleus overwhelms the surrounding galaxy, and the image appears stellar. Most quasars have very large redshifts.
quasar absorption lines
When the light from a distant quasar intercepts an intervening galaxy, an absorption line is created in the quasar spectrum at the redshift of the galaxy. Such lines are called quasar absorption lines.


r-process reactions
Rapid reactions, probably occurring inside supernovae, in which heavy elements are formed as atomic nuclei capture neutrons. (See also s-process reactions.)
radial velocity
The velocity component along the line of sight toward or away from an observer. Recession is positive; approach is negative.
(1) Any electromagnetic waves or atomic particles that transmit energy across space; (2) one of three modes of heat (energy) transmission through stars or planets from warm regions to cool regions.
radiation pressure
An outward pressure on small particles exerted by electromagnetic radiation in a direction away from the light source.
radio galaxy
A galaxy that emits unusually large amounts of radio radiation.
radio source
A point or small portion of the sky giving stronger radio emission than the sky in its vicinity.
radio telescope
A device that gathers and concentrates radio waves.
radioactive atom
Any atom whose nucleus spontaneously disintegrates.
radioactive dating
A technique for determining the age of a material by measuring the amount of a radioactive isotope and its decay product.
The spontaneous decay of a massive atomic nucleus, releasing particles or radiation.
random error
Error that can be reduced when separate observations are combined. For example, four times the data will reduce the error by a factor of two.
random process
A process in which the timing of an individual event is unpredictable, even though the rate of events may be well-determined. Examples are impacts from space and decays of radioactive atoms.
red giant
A post-main-sequence star whose surface layers have expanded to many solar radii and have relatively low temperatures.
A Doppler shift of spectral features toward longer wavelengths, indicating recession of the source.
redshift of galaxies
The shift toward longer wavelengths in light of distant galaxies, due to their recession from the solar system. It increases with galaxies' distances.
A type of telescope using a mirror as the light collector.
reflex motion
The wobble, or periodic motion, of a star due to the influence of a much smaller planet or companion.
A type of telescope using a lens as the light collector.
A powdery soil layer on the Moon and some other bodies caused by meteorite bombardment.
The smallest angle that can be discerned with an optical system; for example, the eye can resolve about 2 minutes of arc.
A close, or simple-number, relationship between periodicities in two phenomena. For example, if one body has half the orbital period of another, they are said to be in orbital resonance.
retrograde motion
Revolution or rotation from east to west contrary to the usual motion in the solar system.
retrograde rotation
Spinning on an axis from east to west, or clockwise as seen from the North Pole (opposite to the spin direction of Earth).
The orbital motion of a planet around the Sun or a moon around a planet.
right ascension
Longitude lines projected onto the celestial sphere. (Abbreviation: R.A.)
ring systems
Systems of orbiting particles seen in the equatorial planes of each of the four gas giant planets. The rings are transparent, and the individual particles follow orbits governed by Kepler's laws.
riverbeds on Mars
Evidence of erosion on Mars in the form of dry riverbeds, the best evidence that Mars was once wetter than it is today.
RNA (ribonucleic acid)
The molecule that assembles proteins from DNA instructions.
Roche's limit
The distance from a large body within which tidal forces would disrupt a satellite.
Rocke surface
An imaginary surface in three dimensions that contains the gas in a binary star system. Also called a Roche lobe.
The spin of a planet or a moon on its axis.
rotation curve
Orbital velocity as a function of distance from the center of a galaxy. The flat rotation curves observed for many spiral galaxies provide good evidence for dark matter.
RR Lyrae star
A type of variable star similar to the Cepheids that has been found associated with Population II and not Population I.
Russell-Vogt theorem
The theorem stating that the equilibrium structure of a star is determined by its mass and chemical composition.


Sagittarius A*
The powerful radio source located at the center of the Milky Way Galaxy.
Any small body orbiting a larger body.
The sixth planet out from the Sun, famous for its prominent rings.
Saturn's atmosphere
The thick, cloudy gases around Saturn, composed mostly of hydrogen.
Saturn's ring system
A system of innumerable icy particles orbiting Saturn.
Saturn's satellite system
A family of at least 18 moons orbiting Saturn, ranging from 20 km diameter up to a size slightly exceeding that of the planet Mercury.
Schwarzschild radius
The radius corresponding to the event horizon of a black hole, proportional to the mass of the black hole.
scientific method
The method of learning about nature from making observations, formulating hypotheses, and constructing observational or experimental tests to see if the hypotheses are accurate.
scientific notation
A convenient way of recording and manipulating very large and very small numbers. Also called exponential notation, or powers of ten.
search for extraterrestrial intelligence (SETI)
A scientific field devoted to calculating the probability of intelligent life in the universe and designing the optimum strategies for making contact.
seasonal changes on Mars
Changes in shape and darkness of the dusky patches on Mars from summer to winter and year to year. Once thought to indicate Martian vegetation, the changes are now known to result from blowing dust deposits.
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.
sedimentary rock
Rock formed from sediments.
seismic waves
Waves passing through the interior or surface layers of a planet due to a seismic disturbance, such as an earthquake or large meteorite impact.
Study of vibrational waves passing through planets, revealing internal structure.
selection effect
Any effect that systematically biases observations or statistics away from a correct understanding.
The search for extraterrestrial intelligence.
Seyfert galaxy
A type of galaxy with a bright, bluish nucleus, possibly marking a transition between ordinary galaxies and quasars.
shepherd satellites
Satellites that move near planetary rings and act to confine the ring particles onto certain orbits.
SI metric system
The internationally standardized scientific system of units, in which length is given as meters, mass as kilograms, and time as seconds.
significant figures
The number of digits known for certain in a quantity.
small-angle equation
The equation giving the relation between the distance D of an object, its diameter d, and its angular size a - alpha (expressed in seconds of arc): a/206,265 = d/D
solar composition
A gas composed of 76 percent hydrogen and 22 percent helium, with much smaller trace amounts of all other elements.
solar core
The Sun's central region of high-pressure gases, where nuclear energy is produced.
solar cycle
22-year cycle of solar activity.
solar eclipse
Partial or total blocking of the Sun's light by an astronomical body (in most usages, by the Moon).
solar flare
A brilliant outbreak in the Sun's outer atmosphere, usually associated with active groups of sunspots.
solar nebula
The cloud of gas around the Sun during the formation of the solar system.
solar seismology
The study of natural vibrations and oscillations in the Sun as a way to probe the structure of the solar interior.
solar system
The Sun and all bodies orbiting around it.
solar wind
An outrush of gas past the Earth and beyond the outer planets. Near the Earth, the solar wind travels at velocities near 600 km/s, sometimes reaching 1000 km/s.
Material with a rigid structure resulting from the forces between atoms. The atoms in a solid can be arranged in an irregular (or amorphous) structure or in a regular (or crystalline) structure.
The date when the Sun reaches maximum distance from the celestial equator (occurs twice annually).
soot line
A distance of about 2.7 AU in the middle of the asteroid belt, separating stony and metal-rich asteroids closer in from black carbon-rich asteroids farther out.
space astronomy
Astronomy that uses the unique advantages of the space environment, including sensitivity to infrared and high-energy electromagnetic waves, and freedom from the blurring effects of the Earth's atmosphere. The term usually applies to observations made with telescopes in orbit around Earth.
space curvature
The curvature of three-dimensional space; one of the consequences of general relativity. Matter tells space how to curve, and space tells matter how to move.
space-time diagram
A plot of time as the y coordinate and space as the x coordinate, with the three spatial dimensions collapsed to one dimension. It reflects the fact that events must be specified in time and space.
special theory of relativity
Einstein's theory that deals with relative motions, and takes as its starting point the fact that the speed of light is a universal constant.
spectral class
A class to which a star belongs because of its spectrum, which in turn is determined by its temperature. The spectral classes are O, B, A, F, G, K, and M, from hottest to coolest.
An instrument for tracing the intensity of a spectrum at different wavelengths; the result is a graph.
Study of spectra, especially as revealing the properties of the light source.
Light from an object arranged in order of wavelength; specifically, the colors of visible light, arranged in this order.
speed of light
Designated as c, the speed of light is about 300,000 km/s and is constant as perceived by all observers.
sphere of gravitational influence
The region in which the gravitational influence of a body is the dominant influence on a passing small body's motions.
spiral arm
In spiral galaxies, one of the arms lying at an angle to the Sun-center line. The arms contain open clusters, O and B stars, and nebulae.
spiral galaxy
A disk-shaped galaxy with a spiral pattern, typically containing 1010 to 1012 solar masses of stars, dust, and gas.
standard candle
An idealized astronomical source (star or galaxy) with well-understood physics and a well-determined luminosity. The known luminosity and the apparent brightness can be combined to give the distance.
standard error
A measure of the uncertainty in a single measurement, based on a scatter of multiple measurements of the same quantity. Also called standard deviation.
A mass of material, usually wholly gaseous, massive enough to initiate (or to have once initiated) nuclear reactions in its central region.
Stefan-Boltzmann law
A law giving the total energy E radiated from a surface of area A and temperature T per second: E= sigmaT4 A. Sigma (sigma), the Stefan-Boltzmann constant, equals 5.67*10-8W/m2*K4 in SI units.
stellar associations
Loose groups of 10 to 100 young stars found in the plane of the Milky Way and other galaxies.
stellar evolution
Evolution of every star from one form to another forced by changes in composition as nuclear reactions proceed.
stellar populations
Large collections of stars that share properties of age, motions, or chemical composition.
A prehistoric English ruin with built-in astronomical alignments.
stony-iron meteorite
Stony meteorite that probably comes from deep within the parent body, where melted stony and metallic material coexisted.
strong nuclear force
The force of nature that binds quarks into neutrons and protons in the atomic nucleus.
Breakup of a contracting cloud into smaller condensations.
The process by which frozen solids in a comet turn directly into gases under the heating influence of the Sun.
A magnetic disturbance on the Sun's surface that is cooler than the surrounding area.
supercluster of galaxies
Cluster of clusters of galaxies.
supergiant star
An extremely luminous star in the uppermost part of the H-R diagram.
supermassive black hole
The hypothesized power source of a quasar or active galaxy, formed by the gradual accretion of material in the center of a galaxy.
A very energetic stellar explosion expending about 1042 to 1044 joules and blowing off most of the star's mass, leaving a dense core.
The idea that diverse physical phenomena have a simple underlying basis; one of the basic assumptions of modern science.
synchrotron radiation
Radiation emitted when electrons move at nearly the speed of light in a magnetic field.
system of knowledge
Ways of dealing with information, such as superstition, appeals to authority, and the advocacy system. The scientific method is the only system that has been successful in leading to an understanding of the natural world.
systematic error
Error that cannot be reduced by simply increasing the number of observations. In astronomy, systematic errors are usually caused by an incomplete understanding of the physics of an astronomical object.


T Tauri star
A type of variable star, often shedding mass, believed to be still forming and contracting onto the main sequence.
Geological features that result from stresses and pressures in the crust of a planet. Tectonic forces can lead to earthquakes and motion of the crust.
An instrument for collecting electromagnetic radiation and producing magnified images of distant objects.
A measure of the average energy of a molecule of a material.
temperature (color)
The temperature of a star as estimated from the intensity of the stellar radiation at two or more colors or wavelengths.
temperature (Kelvin)
Absolute temperature measured in Celsius degrees, with the zero point at absolute zero.
terrestrial planet
(1) Mercury, Venus, Earth, or Mars; (2) a planet composed primarily of rocky material.
A body of hypotheses, often with mathematical backing and having passed some observational tests; often implying more validity than the term hypothesis.
theory of cosmological redshifts
The theory that galaxies' redshifts are all due to recessional motion, increase with distance, and thus give an indicator of distance.
theory of galaxy formation
The (speculative) theory that describes how galaxies form from the gravitational collapse of enormous clouds of gas in the early universe.
theory of noncosmological redshifts
The theory that at least some galaxies' redshifts are not Doppler shifts due to recession, but are due to some other cause.
theory of star formation
The theory that describes how stars form by the gravitational collapse of interstellar clouds of dust and gas.
thermal equilibrium
A balance between the input and outflow of heat in a system.
thermal radiation
Electromagnetic radiation emitted by a body and associated with an object's temperature; it grows greater and bluer in color as the temperature increases.
third quarter
The phase of the Moon when it is three-fourths of the way around its orbit from new moon; the third quarter moon is seen in the dawn sky with a straight terminator and half the disk illuminated.
tidal force
The variation in one body's gravitational force from place to place across another body, e.g., the variation of the Moon's gravity across Earth.
tidal interaction
Close encounter between galaxies that leads to observable effects on the morphology.
A bulge raised in a body by the gravitational force of a nearby body.
A measure of the flow of events.
Titius-Bode law
A nonphysical formula that gives the approximate distances of the planets from the Sun in AU.
total solar eclipse
(1) An eclipse in which the light source is totally obscured from a specified observer; (2) an eclipse in which a body is entirely immersed in another's shadow. (See also eclipse.)
transformation of energy
The process where one form of energy turns into another form. The natural world is full of examples of transforming energy. Mass is also a form of energy.
(1) Passage of a planet across the Sun's disk; (2) any passage of a body with a small angular size across the face of a body with a large angular size.
Trapezium cluster
A small cluster of young massive stars located in the Orion Nebula; the hot stars ionize the gas around them.
triple-alpha process
A nuclear reaction in which helium is transformed into carbon in red giant stars.
Trojan asteroids
Asteroids caught near the Lagrangian points in Jupiter's orbit, 60° ahead of and 60° behind the planet.
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.
turnoff point
The point on the H-R diagram of a cluster at which the main sequence appears to terminate at the high-luminosity end.
Type I supernova
One possible explosive death of a star. A white dwarf in a binary-star 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.


Region of the electromagnetic spectrum just beyond the visible range, corrosponding to wavelengths slightly shorter than blue light.
uncertainty principle
Heisenberg uncertainty principle. It is fundamentally impossible to make simultaneous measurements of a particle's position and velocity with infinite accuracy.
The idea that the four forces of nature are just different manifestations of one basic superforce; the properties of the superforce can be realized only at phenomenally high temperatures or energies.
A well-defined entity for representing a measurable quantity.
universal law of gravitation
Newton's expression for gravity, which states that the gravity force between two objects is equal to the product of their masses divided by the square of the distance between them, multiplied by the gravitational constant.
universality of physical laws
The assumption, borne out by some evidence, that the physical laws understood locally apply throughout the universe and perhaps to the universe as a whole.
Everything that exists. Astronomers distinguish between the observable universe, the region from which light has had time to reach us in the age of the universe, and the physical universe, which may be much larger.
The seventh planet outward from the Sun.
Uranus' rotation axis
Notable for its almost right-angle tilt (obliquity) of 97° to Uranus' orbital plane.


Van Allen belts
Doughnut-shaped zones around the Earth (or another planet with a strong magnetic field) that trap energetic ions from the Sun.
variable star
A star that varies in brightness.
velocity dispersion
The range of velocities in a self-contained dynamical system, like a globular cluster or a galaxy.
The planet most similar in size to Earth and second closest to the Sun.
Virgo cluster
The nearest large cluster of galaxies containing thousands of galaxies; it appears to lie in the direction of the constellation of Virgo and makes up a large part of the Local Supercluster.
virtual pair
A particle and antiparticle that exist for such a brief interval that they cannot be observed.
visible spectrum
The small range of the electromagnetic spectrum that human eyes percieve as light. The visible spectrum ranges from about 4000-7000 angstroms, corrosponding to blue through red light.
visual binary
A binary in which both components can be seen.
Low-density regions in the large-scale distribution of galaxies.
Eruption of molten materials at the surface of a planet or satellite.
A measure of the total space occupied by a body.


wave-particle duality
The concept that particles can show wave-like properties and that radiation can show particle-like properties.
(1) The length of the wavelike characteristic of electromagnetic radiation; (2) in any wave, the distance from one maximum to the next.
wavelike properties of light
Characteristics of light, such as frequency and diffraction, that mimic properties of waves.
weak nuclear force
The force of nature that converts neutrons into protons and is responsible for radioactive decay.
A measure of the force due to gravitational attraction.
white dwarf star
A planet-sized star of roughly solar mass and very high density (108 to 1011 kg/m3) produced as a terminal state after nuclear fuels have been consumed.
Wien's law
A formula giving the wavelength W at which the maximum amount of radiation comes from a body of temperature T. The formula is W = 0.00290/T.


X-ray burster
A binary system composed of a neutron star and a main-sequence star, in which the thermonuclear detonation of a layer of helium on the neutron star leads to an intense burst of X rays.


The point directly overhead.
zero curvature
The curvature of a surface or space in which parallel lines remain parallel and the sum of the angles of a triangle is exactly 180 degrees.
zero-age main sequence star
A newly formed star that has just arrived on the main sequence.
A band around the sky about 18 degrees wide, centered on the ecliptic, in which the planets move.
zodiacal light
A glow, barely visible to the eye, caused by dust particles spread along the ecliptic plane.