Active galaxy A galaxy that radiates large amounts of energy quite differently in character than that of a normal galaxy.
Adaptation The response to a changing environment of an organism’s structure or function in a way that improves its ability to survive and reproduce; any property of an organism that adds to its fitness.
Agriculturist The main job description of our ancestors beginning some 10,000 years ago, namely those who mastered farming.
Amino acid An organic molecule containing carboxyl (COOH) and amino (NH2) groups, of which twenty different types (plus two rare ones) form the building blocks of the proteins that direct the metabolism in all life-forms on Earth.
Angular momentum The tendency of an object to keep spinning or moving in a circle; rotary inertia.
Anthropic principle The idea that the Universe is the way it is because we (intelligent beings) are here to observe it; the Universe is made for us.
Anthropocentrism The idea that events can be viewed and interpreted in terms of human activities and values.
Anthropomorphism The attribution of human characteristics to other organisms or objects.
Anthropology The study of humanity, including its origin, evolution, development, culture, race, customs, and beliefs.
Antimatter A form of matter having an opposite charge than is normally the case; for example, a positively charged positron is the antimatter opposite of a negatively charged electron.
Archaeology The study of old artifacts that ancient humans left behind.
Arrow of time In thermodynamics: the irreversible and inexorable increase in entropy for all natural events. In cosmology: the regular and apparent increase in complexity throughout the history of the Universe.
Artifact An object made by humans that has been preserved and can be studied to learn about a particular time period.
Asteroid A small, rocky object revolving around the Sun, sometimes called a minor planet or planetoid.
Asteroid belt A region of space between Mars and Jupiter where the great majority of asteroids are found.
Astrobiology The study of the origin, evolution, and distribution of past and present life in the Universe; also known as bioastronomy or exobiology.
Astronomical unit The average distance between the Earth and the Sun, about eight light-minutes.
Astronomy The study of material events in the Universe beyond Earth’s atmosphere.
Astrophysics The study of interactions between matter and radiation in space.
Atmosphere Those gases surrounding the surface of a planet, moon, or star.
Atom A submicroscopic component of matter, composed of positively charged protons and neutral neutrons in the nucleus, surrounded by negatively charged electrons.
Atom period A time in the early Universe when elementary particles began to cluster, thus fashioning the first atoms.
ATP An acronym for adenosine triphosphate, an organic molecule that acts as energy currency in life-forms; the central conveyor of phosphate-bond energy in a cell’s metabolism.
Australopithecus The designation given to those prehuman creatures having a mixture of apelike and humanlike qualities, and who lived several million years ago.
Autotroph Any organism capable of self-nourishment by feeding on inorganic matter and external energy, such as plants with the help of sunlight.
Axon The main extrusion of a neuron that acts as a transmitter of information.
Barred-spiral galaxy A type of spiral galaxy having a linear extension or “bar” made of stars and interstellar matter passing through its center.
Baryons Matter made mainly of protons and neutrons; “normal” matter (as opposed to “dark” matter) composing stars, planets, and life-forms.
Big Bang A popular term describing the sudden, expansive start of the Universe.
Binary-star system A pair of stars in orbit about their common center of mass and held together by their mutual gravitational attraction.
Biochemistry The study of chemical processes in living organisms.
Biological evolution The changes experienced by life-forms, from generation to generation, throughout the history of life on Earth.
Biology The study of life in all its forms and phenomena.
Biosphere That part of Earth’s crust, water, and atmosphere capable of sustaining life.
Bipedal An adjective meaning “having two feet,” “an ability to walk on two legs,” or both.
Black hole A region containing a huge amount of mass compacted into an extremely small volume, thus making its pull of gravity so strong that not even light can escape—hence its name.
Brain That part of the central nervous system enclosed in the cranium of humans and other vertebrates, made of a soft, convoluted mass of gray and white matter acting to control and coordinate mental and physical actions.
Carbonaceous chondrite A meteorite having embedded pebble-sized granules that contain significant quantities of organic (carbon-rich) matter.
Catalyst A facilitator or accelerator of a chemical reaction without itself being consumed or changed in the process.
Cell A minimal, usually microscopic, structural unit made of chemicals that can be considered alive.
Central dogma In biology: the assertion that information in biological systems passes unilaterally from nucleic acids to proteins, but not conversely. In physics: the assertion that energy is conserved in all systems, in all environments, and at all times in the Universe.
Chance A happening without known cause; fortuitous, accidental, contingent, unpredictable.
Change To make different the form, nature, and content of something; the transformation of one system into another that is different in at least one respect.
Chaos In the old sense, unconstrained randomness, disorder; in the new sense, the behavior of a deterministic system under conditions that allow for the possibility of multiple outcomes.
Chemical compound A tightly knit cluster of elements, also sometimes called a molecule.
Chemical evolution The prebiological changes that transformed simple atoms and molecules into the more complex chemicals needed for the origin of life.
Chemistry The study of the properties, compositions, and structures of substances and elements and the ways they interact with one another.
Chemosynthesis The production of organic matter by microorganisms that use chemical energy stored in certain inorganic substances, such as hydrogen sulfide.
Chromosome A threadlike molecule in the nucleus of a cell, consisting of mostly DNA and containing the bulk of the cell’s hereditary genes; in humans, twenty-three types vary in length from 50 million to 250 million nucleotide base pairs.
Civilization An advanced stage of development in the arts and sciences accompanied by corresponding social, political, and cultural complexity.
Closed system A system able to exchange energy, but not matter, with its surrounding environment.
Closed Universe A model Universe that stops expanding at some time in the future, after which it contracts to a point much like that from which it began.
Collisional process An event involving a collision of objects; for example, the excitation of a hydrogen atom when hit by another hydrogen atom.
Collision model The idea, now out of favor, that planets form from hot streaming debris torn from a star during a near-collision or close encounter with another passing star.
Comet A small ball of rock and ice from which extends a long wispy tail of gas and dust while nearing the Sun.
Complexity A state of intricacy, complication, variety, or involvement, as in the interconnected parts of a structure—a quality of having many interacting, different components; operationally, a measure of the information needed to describe a system’s structure and function or of the rate of energy flowing through a system of given mass.
Condensation model The idea, broadly accepted today, that planets originate from gravitationally contracting and chemically condensing eddies as a natural by-product of the formation of a star.
Consciousness That property of human nature generally, or of the brain specifically, that grants us self-awareness and a sense of wonder.
Conservation of mass and energy A basic principle of science stipulating that the sum of all mass and energy in a closed system remains constant during any event.
Constellation A geometric pattern of bright stars that appear grouped in the sky, named after gods, heroes, animals, and mythological beings by ancient astronomers.
Continental drift The movement of Earth’s crust over geological times caused by the plate tectonics within Earth’s mantle.
Convection The transfer of heat via circulation, resulting from the upwelling of warm matter and the concurrent downward flow of cool matter to take its place.
Core The central region of a planet, star, or galaxy.
Cosmic abundances A standard listing of the relative numbers of the various elements, determined by studies of the spectral lines in astronomical objects and averaged for many stars in our cosmic neighborhood.
Cosmic background radiation A weak, nearly isotropic electromagnetic (mostly microwave) signal permeating all of space, thought to be a remnant of the big bang.
Cosmic evolution A grand synthesis of all the many varied changes in the assembly and composition of radiation, matter, and life throughout the history of the Universe.
Cosmic-ray particle A charged, subatomic particle of matter (not radiation) that races throughout interstellar space and that regularly strikes Earth’s atmosphere.
Cosmological principle The idea (really an assumption) in modern cosmology that the Universe is homogeneous (uniform at every point) and isotropic (uniform in every direction) on scales larger than galaxy superclusters.
Cosmology The study of the structure, evolution, and destiny of the Universe.
Cosmos A complete, orderly, harmonious system; from the Greek, kosmos, meaning an orderly whole.
Critical universal density The density of matter above which the Universe is closed (and will collapse back on itself) and below (or equal to) which the Universe is open (and will expand forevermore).
Cultural evolution The changes in the ways, means, actions, and ideas of society, including their transmission from one generation to another.
Culture The totality of activities, artifacts, values, and behavior acquired by members of society through learning.
Cyclic Universe A model Universe that continuously oscillates between expansion and contraction.
Cytoplasm The contents of a cell around its nucleus.
Dark dust cloud A region of interstellar space containing a rich concentration of gas and dust in an irregular but well-defined cloud that obscures the light from stars beyond it.
Dark matter Unseen mass in galaxies and galaxy clusters whose existence is only inferred indirectly but that has not been confirmed directly by any observations.
Darwinism The idea that life-forms evolve by descent with modification among individuals within populations, by means of natural selection of those best adapted to survive environmental changes.
Decoupling An event in the early Universe when atoms first formed, after which photons moved freely in space, causing matter and radiation to behave differently.
Dehydration condensation The linking of two or more amino acids by means of removing water.
Dendrite One of a network of extrusions of a neuron that acts as a receiver of information.
Density A measure of compactness, namely the quantity of something in a unit of volume.
Determinism The idea that all events have specific, definite causes and obey precise, natural laws, making their outcomes completely predictable; from any particular initial state, one and only one sequence of future states is possible.
Development Any process of change, usually of growth or elaboration, between a system’s origin and its maturity.
Differentiation The separation of heavy matter from light matter, thus causing a variation in density and composition.
Disorder An irregularity in arrangement or behavior; a synonym for entropy; an absence of order.
DNA An acronym for deoxyribonucleic acid, a self-replicating, double-helical molecule resident chiefly in biological nuclei, mainly responsible for storing hereditary information needed for the building of proteins.
Doppler effect The apparent change in the wavelength (or frequency) of a wave, caused by line-of-sight motion of the source or of the observer (or both).
Dwarf star Any star comparable to or smaller in size than the Sun.
Earth Humankind’s home planet in space, third out from the Sun.
Earthquake A sudden dislocation of rocky matter near Earth’s surface.
Ecology The study of the interrelatedness among all systems and between those systems and their environment; most common in biology where the systems comprise all living things.
Ecosystem A community of systems and their shared environment, regarded as a unit, all interacting so as to perpetuate the grouping more or less indefinitely; most common in biology, where the systems are plants, animals, and other organisms and the environments are often seafloor, forest, and grassland areas.
Electromagnetic force The force that binds charges of opposite electrical charge and repels charges of identical electrical charge.
Electromagnetism The phenomenon of electricity and magnetism studied together.
Electromagnetic spectrum The entire range of all the various kinds of radiation; light (or the visible spectrum) is just one small segment of this much broader spectrum.
Electron A negatively charged elementary particle that resides outside (but is bound to) the nucleus of an atom.
Element A substance comprising one and only one distinct kind of atom; one impossible to separate into simpler substances by chemical means.
Elementary particle A basic building block of atoms.
Elliptical galaxy A galaxy having a spherical or elliptical shape, some more than others, and composed mostly of old stars and little interstellar matter.
Emergence The appearance of entirely new system properties at higher levels of complexity not preexisting among, nor predictable from knowledge of, lower-level components; the process of a system “becoming” from its environment at certain critical stages in its development or evolution.
Energy The ability to do work or to produce change; an abstract concept invented by nineteenth-century physicists to quantify many different phenomena in Nature.
Energy density A measure of compactness of energy; an amount of energy per given volume.
Energy rate density The amount of energy (available to do work) flowing through a system per unit time and per unit mass.
Entropy A measure of randomness, or disorder, of a system, reaching a maximum state of inert uniformity at thermodynamic equilibrium; a lack of information about a system’s organization.
Environment Any part of the Universe not included in a system; a combination of all things, conditions, and influences surrounding a system.
Enzyme Any of numerous complex proteins that catalyze specific biochemical reactions.
Equilibrium A state wherein a system’s gradients are negligible, its probability maximized, and its free energy minimized; one that constantly reacquires any and all of its possible configurations randomly, thus from which it exhibits no tendency to depart.
Escape velocity The minimum speed needed for an object to escape the gravitational pull of a massive object.
Euclidean geometry The terrestrially familiar geometry of “flat space” that all of us learn in high school.
Eukaryote A life-form whose cells have well-developed biological nuclei; all organisms above the level of prokaryotes, including protists, fungi, plants, and animals.
Event Any occurrence in spacetime defined by its location and date; a happening.
Event horizon A region within which no event can ever be seen, heard, or known by anyone outside; also termed the “surface” of a black hole.
Evolution Any process of growth and change with time, including an accumulation of historical information; in its broadest sense, both developmental and generational change.
Extraterrestrial An adjective meaning “beyond the Earth.”
Fauna The species of animals living in a given geographical area at a given time.
Fermentation The extraction of energy via the capture and chemical breakdown of small molecules.
First law of thermodynamics A principle stipulating that in any real process, energy is conserved, that is, never created or destroyed but allowably changed from one form to another.
First-generation star A star made of primordial matter and thus having no heavy elements.
Fission A nuclear process that releases energy when heavyweight nuclei break down into lightweight nuclei.
Flora The species of plants living in a given geographical area at a given time.
Flow The movement of an entity from one place to another; to issue or proceed from a source.
Force An agent of change in or on any system.
Form The structure, pattern, organization, or essential makeup of anything.
Fossil The hardened remains of a dead organism whose skeletal outlines or bony features are preserved in ancient rocks.
Fragmentation Developing inhomogeneities in the gas density of a cloud that eventually break down into smaller clumps of matter within the cloud.
Function The ability of a system’s components, beyond its mere structure, to execute an internal action, role, or job assignment, such as breathing, running, writing, or reproducing.
Fusion A nuclear process that releases energy when lightweight nuclei combine to form heavyweight nuclei.
Galactic center The hub or core of the Milky Way, some 30,000 light-years from the Sun.
Galactic cluster A loose collection of tens to hundreds of relatively young stars spread over several light-years, sometimes termed an open cluster.
Galactic evolution The changes experienced by galaxies, either intrinsically because of localized changes among myriad stars or environmentally because of merges, acquisitions, and close-encounters among neighboring galaxies.
Galactic halo A nearly spherical region that surrounds the Milky Way Galaxy and extends some 50,000 light-years from the galactic center.
Galactic plane A relatively thin disk or plane in which most of the Milky Way’s stars and interstellar matter now reside.
Galaxy An open, coherent, spacetime structure maintained far from thermodynamic equilibrium by a flow of energy through it—a colossal system of billions of stars and much loose gas held together by gravity.
Galaxy cluster A group of galaxies held together by their mutual gravitational attraction.
Galaxy period A time in the relatively early history of the Universe when the bulk of most galaxies formed.
Galaxy supercluster A truly huge cluster of galaxy clusters, often stretching over a hundred million light-years or more.
Gaseous nebula A region of ionized gas (plasma) surrounding one or more young, hot stars (sometimes termed an emission nebula).
Gene A segment of any DNA molecule containing information for the construction of proteins, hence responsible for directing inheritance from generation to generation; in humans, approximately 30,000 such genes vary in size from 3000 to 2.4 million nucleotide bases.
Gene pool The spread or distribution of the variations or traits among a given population of a species; all the genes in a given population.
Genetic code An encyclopedic blueprint of the physical and chemical properties of all of an organism’s cells and all of its functions.
Genetics The study of heredity and the biological processes by which inherited characteristics are passed from one generation to the next.
Genome The sum of all genes carried by a single organism; humans, for example, have nearly 30,000 genes (comprising about 3 billion nucleotide bases), bacteria, a few hundred genes (including about a million bases).
Genotype The sum of all genes of an individual.
Geocentric An adjective meaning “centered on the Earth.”
Geography The study of positions, shapes, sizes, and numerous other qualities of Earth’s continents.
Geology The study of the physical history of Earth, especially the rocks of which it is composed and the events it has undergone.
Geometry The study of the size, shape, and scale of things.
Giant star Any star much larger in size than the Sun.
Globular cluster A tight-knit collection of many thousands, sometimes even millions, of old stars spread throughout the halos of galaxies.
Grand-unified theory An idea that three forces—the electromagnetic force, the strong nuclear force, and the weak force—are different manifestations of one and the same force.
Gravitational force The force that holds matter together on a large scale, such as stars within galaxies, atoms within stars, and people on Earth.
Gravitational instability A condition whereby an object’s (inward-pulling) gravitational potential energy exceeds its (outward-pushing) thermal energy, thus causing the object to infall.
Gravity An attractive force that any massive object exerts on all other massive objects.
Gravity wave The gravitational analog of an electromagnetic wave whereby gravitational radiation is emitted at the speed of light from any mass that undergoes rapid acceleration.
Greenhouse effect The trapping of radiation by an atmosphere (or greenhouse), thus causing greater heating than would normally be the case.
Habitable zone A three-dimensional region of “comfortable” temperatures that surrounds every star.
Hadron period A very early time in the history of the Universe when heavy, strongly interacting, elementary particles, such as protons and neutrons, were the most abundant type of matter.
Heat The amount of energy transferred to or from a substance; the thermodynamic state of an object by virtue of the random motions of the particles within it.
Heliocentric An adjective meaning “centered on the Sun.”
Heredity The transmission of genetic traits from parents to offspring, thus ensuring the preservation of certain characteristics among future generations of a species.
Heterotroph Any organism requiring organic matter for food, such as primitive cells that survived by absorbing acids and bases floating on primordial seas or most animals today.
Hierarchical clustering The idea that large objects are built from small objects, including, for example, galaxies having originated partly by collecting already-made star clusters.
Holism The idea that a whole entity, as a basic component of reality, has an existence greater than the mere sum of its parts.
Hominid Both our erect-walking human ancestors (present and extinct Homo) and their predecessor near-relatives (bipedal austrolopithecines) arising after the split from the gorilla-chimp lineage roughly six million years ago.
Homo erectus The species designation given to all human creatures who lived from roughly 200,000 to 1 million years ago; literally, the Latin means “erect man.”
Homo sapiens The species designation given to all human creatures who lived during about the past 200,000 years, including ourselves; literally, the Latin means “wise man.”
Hubble’s constant The proportionality factor between the distance of a galaxy and the velocity with which it recedes; currently, its best estimate is 20 kilometers/second/million light-years.
Hubble’s law An empirical finding linking the distance of a galaxy and the velocity with which it recedes.
Humankind The human race on Earth, considered collectively.
Hunter-gatherer The main job description of our ancestors during most of the past few million years, namely those who survived by hunting and gathering food.
Hydrosphere The liquid part of a planet’s surface, including any lakes, streams, oceans, and rivers.
Ice age A period of cool, dry climate that intermittently plagues planets, causing, in the case of Earth, a long-term buildup of glacial ice far from the poles.
Inflation A period of extremely rapid expansion of the Universe very shortly after the beginning of all things.
Information The number of bits needed to specify a message or structure; the difference between the maximum possible and actual entropies of any given system.
Intelligence The capacity to comprehend relationships and address multiple tasks simultaneously; a biological adaptation for complex behavior, probably synonymous with language.
Intergalactic space Regions outside galaxies and especially galaxy clusters where matter has never been conclusively found.
Interplanetary matter Debris in the great spaces between the planets of the Solar System.
Interplanetary space Regions among the planets, moons, and related objects of the Solar System.
Interstellar matter Sparse gas and dust in the vast domains among the stars.
Interstellar space Dark regions among the stars of any galaxy.
Invertebrate An organism without a backbone.
Invisible radiation Those kinds of radiation to which the human eye is not sensitive, including radio, infrared, and ultraviolet waves, as well as X rays and gamma rays.
Ion An atom with one or more electrons removed (or added), giving it a positive (or negative) charge.
Ionosphere A radio-reflecting region high in Earth’s atmosphere in which solar radiation has removed an electron from some of the atoms.
Irregular galaxy A strangely shaped galaxy, often rich in interstellar matter but apparently not a member of any of the major classes of spiral or elliptical galaxies.
Isotope An atom having more or fewer neutrons than normal.
Jovian planets The four, big, gassy planets in the outer parts of the Solar System: Jupiter, Saturn, Uranus, and Neptune.
Kelvin scale An international temperature scale, equal to the Celsius (or Centigrade) scale plus 273 degrees.
Kepler’s laws Three principles, discovered empirically by a seventeenth-century German astronomer, that describe the motions of the planets in their orbits about the Sun.
Kinetic energy The energy of an object or system due to its mass and motion; the ability to do work actively via motion.
Kin selection In individuals related by common descent, such as siblings, altruistic selection for the shared parts of their genotypes.
Lamarckism The idea that an organism is a result of environmental influences rather than genetic inheritance; traits can be acquired through habit, use, or disuse during a single lifetime and then passed on intact to the next generation.
Lepton period A very early time in the history of the Universe when the lightweight, weakly interacting, elementary particles, such as electrons and neutrinos, were the most abundant type of matter.
Life An open, coherent, spacetime structure kept far from thermodynamic equilibrium by a flow of energy through it—a carbon-based system operating in a water-based medium, with higher forms metabolizing oxygen.
Life Era An advanced period in the history of the Universe when technological life-forms manipulate their genes and their environments more than conversely.
Light The kind of radiation to which the human eye is sensitive.
Light-year The distance traveled by light in a full year; equal to some 10 trillion kilometers.
Linear momentum The tendency of an object to keep moving in a straight line.
Lithosphere The solid part of a planet’s surface, including any continents and seafloor.
Local Group The specific name given to the galaxy cluster that includes the Milky Way Galaxy as a member.
Luminosity The rate of electromagnetic energy released from any object, sometimes called the absolute brightness.
Magnetism An attractive or repulsive influence that a magnet exerts on another magnet (or on a charged particle).
Magnetosphere A region of space, usually high above a planet’s atmosphere, where charged particles are magnetically deflected or trapped.
Mantle The interior of a planet, namely that matter below the crust yet above the core.
Mass A measure of the total amount of physical substance, or “stuff,” contained within an object.
Mass extinction The destruction of a large part of the biosphere of Earth by means of climatic, geologic, cosmic, or other environmental events.
Materialism The idea that there is nothing other than matter, energy, and their various arrangements, motions, and changes in the Universe.
Matter Anything that has mass and occupies space.
Matter Era A mature period in the history of the Universe when the density of energy contained within matter exceeds the density of energy contained within radiation.
Matter-antimatter annihilation A highly efficient process in which equal amounts of matter and antimatter collide and destroy each other, thus producing a burst of energy.
Meiosis The division of a cell nucleus, whereby the chromosomes do not divide but are equally shared by the two new (sex) cells.
Meme In analogy with gene, a cultural replicator—an idea, behavior, style, or usage that spreads from person to person within a culture.
Metabolism The sum of all chemical reactions that energetically support a living organism, starting from energy sources that are either chemical (environmental nutrients) or physical (sunlight).
Meteor A heated, glowing object streaking through Earth’s atmosphere but not yet having hit the surface.
Meteorite A meteoroid that manages to survive passage through an atmosphere to collide ultimately with the surface of a planet or moon.
Meteoroid On average, a meter-sized boulder that has probably escaped from the asteroid belt and thus roams the Solar System.
Microbe A unicellular microorganism, or bacterium, distinguishable from plant or animal.
Milky Way Humankind’s home galaxy, comprising some hundred billion stars of which the Sun is one and so named because its stars resemble a milky band running across the dark night sky.
Mitochondria An organelle of a eukaryotic cell that transforms nutrients into energy, generally by oxidation; thought to be an ancient bacterium, originally captured by symbiosis as a parasite by an infected cell.
Mitosis The division of a cell nucleus into two equal parts, in which all the chromosomes divide equally, yielding two cells identical to the first.
Modern synthesis A conceptual unity in contemporary biology, based on Darwinian evolution, including natural selection, adaptation, diversity, and Mendelian genetics; also termed neo-Darwinism.
Molecular clock The regularity in the change of a gene or a whole genotype over geological time.
Molecular cloud A relatively dense, cold region of interstellar matter where molecules are abundant.
Molecule A bound cluster of two or more atoms held together by electromagnetic forces.
Multicell A group of cells that collaborate with other cells.
Mutation A microscopic change in the DNA base sequence of any gene, transmissible by replication.
Natural selection In general: a normative process whereby environmental resistance tends to eliminate nonrandomly those members of a group of systems least well adapted to cope and thus, in effect, choose or “select” those best suited for survival. In biology: the Darwinian process whereby a population’s life-forms having advantageous traits are able to adapt to a changing environment, thereby surviving, reproducing, and passing on to their descendants those favorable traits which then accumulate in the population over time.
Nature The Universe, including all its natural phenomena.
Nebular model The idea that the Solar System originated in a contracting, swirling cloud of gas that left behind a concentric series of rings from which the planets formed.
Necessity The inevitable “force” of circumstances.
Neo-Darwinism A combination of traditional Darwinian evolution and Mendelian genetics; also termed “the modern synthesis.”
Neuron A biological, or nerve, cell in a brain.
Neutrino A neutral, weakly interacting elementary particle having almost no mass.
Neutron A neutral elementary particle having slightly more mass than a proton and that resides in the nucleus of most atoms.
Neutron star An extremely compact ball of neutrons having the mass of a star but a size smaller than a planet.
Non-equilibrium A state characterized by nonnegligible gradients and a regular energy flow, allowing for further change, growth, and evolution.
Non-thermal radiation Radiation released by virtue of a fast-moving charged particle (such as an electron) interacting with a magnetic force field; heat has no role in this process.
Normal galaxy A galaxy that radiates energy much as expected from a large accumulation of stars.
Nova An unstable star that rapidly brightens while expelling a small fraction of its matter after which it slowly fades back to normal.
Nuclear force The force that binds atomic nuclei.
Nuclear transformation Changes in atomic nuclei owing to the reaction of one nucleus with another nucleus.
Nucleic acid A class of long-chain, organic molecules, made by grouping many nucleotides with sugars and often inhabiting the biological nuclei of cells.
Nucleotide base An organic molecule, of which five different types constitute the building blocks of all nucleic acids within genes that transmit hereditary characteristics from one generation of life-forms to the next.
Nucleus In physics: the positively charged core of an atom where nearly all of its mass resides and that comprises protons and (except for in hydrogen) neutrons, around which electrons orbit. In biology: the inner, central part of a eukaryotic cell, containing the genetic recipe (DNA) for making similar cells.
Oceanography The study of the ocean’s motion, history, and physical and chemical behavior.
Ontogeny The developmental history of an individual system; in biology, the developmental life cycle of an individual organism.
Open system A system able to exchange both energy and matter with its surrounding environment.
Open Universe A model Universe that expands forever.
Order A regularity in arrangement or behavior; a restriction on the number of possible states; an absence of disorder.
Organism Anything that lives—plant, animal, or microbe—or has ever been living.
Organization Relations existing among the components of a system for it to be a member of a specific class.
Origin A coming into being; a process whereby a given state precedes all other such states in time.
Ozone layer A layer in Earth’s atmosphere rich in triatomic oxygen (O3) molecules, which block high-frequency radiation.
Paleoanthropology The study of prehistoric humanity.
Paleomagnetism The study of ancient magnetism.
Paleontology The study of the fossilized remains of dead organisms.
Panspermia An idea stipulating that germs are everywhere in the Universe and that primitive life on Earth originated when some of these germs came to our planet from outer space.
Particulate evolution The changes among elementary particles, including photons, in the early Universe.
Periodic table of the elements A systematic listing, according to increasing mass, of all the known kinds of atoms.
Phenotype The sum of all traits of an individual (anatomical, physiological, behavioral) resulting from the interaction of its genes with the environment.
Photon A packet of pure quantum energy; the massless, chargeless carrier of electromagnetic radiation and mediator of the electromagnetic force.
Photosphere The visible portion of the Sun or any star, although this “surface” is made exclusively of hot, opaque gas.
Photosynthesis The production of organic matter by (usually) green plants that use sunlight to make glucose from carbon dioxide and water, the byproduct being oxygen.
Phylogeny The evolutionary history of a group of systems; in biology, the evolution of ancestral relations among species, often illustrated by a “tree of life” branching diagram where organisms are connected by the number of mutations separating them.
Physics The study of matter, energy, space, and time.
Planet An open, coherent, spacetime structure maintained far from thermodynamic equilibrium by a flow of energy through it—a rocky and/or gaseous system, more massive than an asteroid yet less massive than the star about which it orbits.
Planetary evolution The changes in the physical or chemical properties of planets during the course of their histories.
Planetary nebula A twofold object comprising an old yet hot white-dwarf star surrounded by a thin, ionized, spherical shell of expanding gas.
Planetesimal An asteroid-sized blob of matter that gradually collided with others in the formative stages of the Solar System, thus fabricating the planets.
Plasma A state of matter wherein all atoms are ionized; a mixture of free electrons and free atomic nuclei, often called the “fourth state of matter,” after solids, liquids, and gases.
Platonism The idea that the changing, shifting world of physical phenomena masks a deeper reality—an underlying set of eternal ideas and unchanging forms, and it is these alone that grant true knowledge.
Positron A positively charged antiparticle of the electron.
Potential energy The energy of an object or system due to its mass and position; the ability to do work passively stored.
Power The rate at which work is done; an amount of energy transferred per unit time.
Primate The order of mammals that includes monkeys, apes, and humans.
Primordial nucleosynthesis Element building that occurred in the early Universe when the nuclei of primordial matter collided and fused with one another.
Principle of equivalence The idea that the pull of gravity on an object and the acceleration of that object (i.e., its gravitational and inertial forces) can be viewed as conceptually equivalent.
Probability The likelihood of an event, expressed by the ratio of the number of actual occurrences to that of possible occurrences.
Process The change of a quantity over time; the act of proceeding.
Prokaryote A life-form whose single cell lacks a well-developed biological nucleus, such as various types of bacterial microorganisms.
Protein A class of long, folded organic molecules, made of typically hundreds of amino acids and inhabiting the cytoplasm of cells; a major structural component as well as a functional enzyme in both plants and animals.
Proteinoid microsphere A microscopic proteinlike cluster rich in amino acids, artificially produced in the laboratory.
Proteome The sum of all proteins in a cell.
Protist A general name for any of the vast variety of unicellular eukaryotes, bigger and more complex than any prokaryote.
Protogalaxy A forerunner of a present-day galaxy, also sometimes termed a “baby galaxy.”
Proton A positively charged elementary particle that resides in the nucleus of every atom.
Proton-proton cycle A series of nuclear events whereby hydrogen nuclei (protons) are converted into helium nuclei, releasing energy in the process.
Protoplanet A forerunner or progenitor of a genuine planet.
Protostar An embryonic condensation of interstellar matter perched at the dawn of star birth.
Pulsar A compact, celestial object that emits rapid and periodic pulses of radiation and that is thought to be a rotating neutron star.
Punctuated equilibrium The idea that life’s species remain essentially unchanged for long periods of time, after which they change rapidly in response to sudden, drastic changes in the environment.
Quantum physics A branch of physics dealing with submicroscopic parts of a system, including its inherent uncertainty.
Quark A fractionally charged, basic building block of protons, neutrons, and many other elementary particles.
Quasar An acronym for quasi-stellar source; a high-red-shift object whose image resembles a star but whose energy budget seems comparable to or larger than that of a normal galaxy.
Radiation In physics: a form of energy that travels at the velocity of light, of which light itself is a special kind. In biology: divergence of members of a single evolutionary line into different niches.
Radiation Era An early period in the history of the Universe when the density of energy contained within radiation exceeded the density of energy contained within matter.
Radioactivity The spontaneous decay of certain rare, unstable, heavyweight nuclei into more stable lightweight nuclei, a natural by-product of which is the release of energy.
Red-giant star An old, bright star, much larger in size and cooler than the Sun.
Red shift The Doppler lengthening of the wavelength of radiation (or the shifting of spectral lines toward longer wavelengths) caused by some net motion of recession.
Reductionism The idea asserting that all natural phenomena can be understood only by reducing them to their smallest component parts.
Relativistic physics A branch of physics dealing with matter moving at high speeds (special theory) or in strong gravitational fields (general theory); the worldview according to Einstein.
Relativity A theory of physics that describes the dynamical behavior of matter and energy under peculiar circumstances, especially at very high velocities and very high densities.
Reproduction The natural process among life-forms by which new individuals are generated; a copy, duplicate.
Respiration A chemical process whereby cells use oxygen to release energy; a technical term for “breathing.”
Revolution The orbital motion of one object about another.
RNA An acronym for ribonucleic acid, a single-stranded organic helix found chiefly in the cytoplasm of cells, often instrumental in protein synthesis.
Rotation The spin of an object about its own axis.
Scientific method An investigative technique used by all natural scientists throughout the world. In general, some data or ideas are first gathered, then a theory is proposed to explain them, and finally an experiment is devised to test the theory.
Second law of thermodynamics A principle stipulating that in any real process, the entropy of the Universe increases, that is, irreversibly tends toward greater disorder; energy naturally flows from hotter to colder systems, and not the reverse.
Secondary atmosphere Gases that a planet exhales from its interior after having lost its primary or primordial atmosphere.
Second-generation star A star having some heavy elements and that is thus made of matter that has been previously processed through other stars.
Selection A process of Nature that causes some systems having certain properties, which are not the norm for a population of systems, to preferentially adapt to their environment and thus to enhance their state; those things that work well survive, and those that don’t, don’t.
Shock wave A rapidly rushing shell of gas that tends to push aside and sometimes implode matter in its wake.
Simplicity A state free of complexity or of the possibility of confusion.
Singularity In physics: a superhot, superdense state of matter, where the known laws of physics are likely to break down. In mathematics: a point where a mathematical function ceases to be defined, usually because it becomes infinite.
Sociobiology The systematic study of the biological basis of social behavior in humans as well as in other life-forms.
Solar core The region immediately surrounding the center of the Sun where nuclear reactions release vast quantities of energy.
Solar System Humankind’s home planetary system, comprising nine planets, dozens of moons, and countless smaller asteroids and meteoroids, all orbiting the Sun.
Space An indefinitely great three-dimensional expanse in which all material objects are located and all events occur.
Spacetime A synthesis of the three dimensions of space and of a fourth dimension, time; a hallmark of relativity theory.
Speciation The change of a single species into two or more new species; also termed “disruptive selection.”
Species Any organism—plant, animal, or microbe—of a single kind; a fundamental biological classification denoting a group of individuals not only structurally similar but also able to mate among themselves and produce fertile offspring.
Spectroscopy An observational technique designed to disperse radiation into its component wavelengths in order to study in fine detail the way that matter emits or absorbs radiation.
Spiral arm Part of a pinwheel structure of young stars and interstellar clouds usually winding out from a galaxy’s center.
Spiral galaxy A galaxy having a pinwheel shape, some more than others, and composed of a mixture of old and young stars as well as loose interstellar matter.
Spontaneous generation The theory, now refuted, that life-forms have suddenly emerged fully developed from peculiar arrangements of nonliving matter.
Standard model In physics: an acknowledged description of microscopic phenomena, bolstered by accelerator experiments and the quantum field theory of particles and forces. In cosmology: an acknowledged description of macroscopic phenomena, bolstered by observations of galaxy recession, background radiation, and elemental abundances.
Star An open, coherent, spacetime structure maintained far from thermodynamic equilibrium by a flow of energy through it—a glowing ball of gas held together by its own gravity and powered by nuclear fusion at its center.
State The status or condition of a system as specified by certain dynamical variables.
Statistical fluctuation Continual change from one course or condition to another; an irregularity or instability.
Statistical physics A branch of physics dealing with vast numbers of particles and their probable states, enabling some averaging of properties within macroscopic systems.
Stellar evolution The changes experienced by stars as they originate, mature, and terminate.
Stellar nucleosynthesis Element building that occurs in stars when nuclei collide and fuse with one another.
Stellar period A time in the history of the Universe (including now) when the stars form.
Structure The arrangement of the basic components of a system, including form but not function.
Sun Humankind’s parent star, a resident of the Milky Way.
Supernova An explosive death of a massive star whose glowing debris produce for a few weeks a great brightening comparable to a whole galaxy.
Supernova remnant The remains of a supernova, namely glowing debris scattered over a light-year or more.
Symbiosis The living together, in a mutually beneficial union that aids survival or evolution, of two organisms of different species.
Symmetry The ordered repetition of identical parts of a structure or state.
Synapse A microscopic gap separating an axon of one neuron from a dendrite of another neuron.
System A finite assemblage of interdependent things in the Universe, separated from its surrounding environment by topological and organizational boundaries; any entity of interest, usually one having interconnected components acting as a unitary whole.
Tectonics The study of crustal displacements and deformations of large continental plates on a planet’s surface; for Earth, popularly termed “continental drift.”
Temperature A measure of the heat of an object, by virtue of the random motions of the particles within it.
Terrestrial An adjective meaning “of the Earth.”
Terrestrial planets The four small, rocky planets in the inner parts of the Solar System: Mercury, Venus, Earth, and Mars.
Thermal energy The energy of an object or system due to its heat, a measure of which is temperature.
Thermal radiation Radiation released by virtue of an object’s heat; namely, by charged particles interacting with other charged particles.
Thermodynamics The study of the macroscopic changes in the energy of a system, for which temperature is a central property and meaning literally “movement of heat.”
Time The fourth dimension that distinguishes past, present, and future; a quantity easily measured yet hard to define.
Turbulence The disordered, irregular motion of matter, so complex as to defy description except in a statistical manner.
Unicell A single cell that does not collaborate with other cells.
Universe The totality of all known or supposed objects and phenomena, formerly existing, now present, or to come, taken as a whole.
Van Allen belts Zones of intense radiation surrounding Earth’s midsection, caused by charged particles trapped in Earth’s magnetic force field.
Velocity of light The fastest speed that any object can move, approximately 300,000 kilometers per second.
Vertebrate An organism having a backbone, including fishes, amphibians, reptiles, and mammals.
Virus The smallest and simplest entity that sometimes appears to be alive.
Visible spectrum The narrow range of wavelengths in the electromagnetic spectrum to which the human eye is sensitive; namely, light.
Volcano The site of hot lava upwelling from below the crust of a planet or moon.
Wavelength The distance between successive crests of a wave.
Weak force The force that governs the change of one kind of elementary particle into another.
White-dwarf star An old, dim star, much smaller in size and hotter than the Sun.