A galaxy is a large collection of stars, glowing nebulae (clouds), gas, and dust bound together by gravity. Many scientists now believe that a black hole, the remains of a massive star, lies at the center of many galaxies.

Game Theory

Game theory is a branch of mathematics concerned with the analysis of conflict situations. The term conflict situation refers to a condition involving two or more people or groups of people trying to achieve some goal.

Gamma Ray

Gamma rays are high-energy subatomic particles formed either by the decay of radioactive elements or by nuclear reactions. The wavelength of a gamma ray is very short—less than the radius of an atom—and the energy they carry can measure millions of electron volts.

Gamma-ray Burst

Gamma-ray bursts are unexplained intense flashes of light that occur several times a day in distant galaxies. The bursts give off more light than anything else in the universe and then quickly fade away.

Gases, Liquefaction of

Liquefaction of gases is the process by which a gas is converted to a liquid. For example, oxygen normally occurs as a gas.

Gases, Properties of

Gases are a state of matter characterized by two properties: their lack of definite volume and their lack of definite shape. This definition suggests, in the first place, that a given mass of gas can occupy any volume whatsoever.


A generator is a machine that converts mechanical energy into electrical energy. Generators can be subdivided into two major categories depending on whether the electric current produced is alternating current (AC) or direct current (DC).

Genetic Disorders

Genetic disorders are conditions that have some origin in an individual's genetic make-up. Many of these disorders are inherited and are governed by the same genetic rules that determine dimples and red hair.

Genetic Engineering

Genetic engineering is any process by which genetic material (the building blocks of heredity) is changed in such a way as to make possible the production of new substances or new functions. As an example, biologists have now learned how to transplant the gene that produces light in a firefly into tobacco plants.


Genetics is the branch of biology concerned with the science of heredity. The term heredity refers to the way in which specific characteristics are transmitted from one generation to the next.

Geologic Map

Geologic maps display the arrangement of geologic features of a particular area. These features can include such things as types of rocks, faults, minerals, and groundwater.

Geologic Time

Geologic time describes the immense span of time—billions of years—revealed in the complex rock surface of Earth. Geologists have devised a geologic time scale that divides Earth's history into units of time.


Geology is the scientific study of Earth. Geologists study the planet—its formation, its internal structure, its materials, its chemical and physical processes, and its history.


The term geometry is derived from the Greek word geometria, meaning "to measure the Earth." In its most basic sense, then, geometry was a branch of mathematics originally developed and used to measure common features of Earth. Most people today know what those features are: lines, circles, angles, triangles, squares, trapezoids, spheres, cones, cylinders, and the like.


Gerontology is a branch of sociology that studies aging and the problems—psychological, economic, and social—that arise in old age. Gerontology includes the field of geriatrics, the medical study of the biological process of aging and the treatment of illnesses of old age.


Glaciers are flowing masses of ice, created by years of snowfall and cold temperatures. Approximately one-tenth of Earth is covered by glaciers, including Antarctica and parts of Greenland, Iceland, Canada, Russia, and Alaska.


Glass is a hard, brittle substance that is usually transparent or translucent. It is made by melting together sand (silicon dioxide), soda (sodium carbonate), limestone (calcium carbonate), and other ingredients.

Global Climate

Global climate is the term used to describe the sum total of all weather patterns formed over all parts of the planet. In contrast to the term weather, which applies to relatively short-term phenomena such as storms, global climate is used to describe patterns that extend over much longer periods of time, at least a few decades.

Graphs and Graphing

A graph is a pictorial representation of a set of data. These data can be of two distinct types: continuous or discontinuous.

Gravity and Gravitation

Gravity is the force of attraction between any two objects in the universe. That force depends on two factors: the mass of each object and the distance between them.

Greenhouse Effect

The greenhouse effect is a natural phenomenon that is responsible for the relatively high temperature maintained on Earth's surface and in its atmosphere. The name comes from the process by which greenhouses are thought to collect and hold heat.


Gynecology is the specialized field of medicine dealing with the health of a woman's genital system.


A gyroscope is an instrument consisting of a frame supporting a disk or wheel that spins rapidly about an axis. Technically, a gyroscope is any body that spins on a movable axis, including a child's toy top and the planet Earth.


Half-life is a measurement of the time it takes for one-half of a radioactive substance to decay (in this sense, decay does not mean to rot, but to diminish in quantity).


Hallucinogens are natural and human-made substances that often cause people to believe they see random colors, patterns, events, and objects that do not exist. The hallucinatory experiences can either be very pleasant or very disturbing.


The halogens are the five chemical elements that make up Group 17 on the periodic table: fluorine, chlorine, bromine, iodine, and astatine. The term halogen comes from Greek terms meaning "to produce sea salt." The halogens are all chemically active.

Hand Tools

The earliest hand tools date back to the Old Stone Age (of the Paleolithic period), the earliest period of human development, which started roughly two million years ago. These early hand tools included sticks and rocks picked up and used to pound, dig, or throw.


In humans, the heart is a pulsating organ that pumps blood throughout the body. On average, the heart weighs about 10.5 ounces (300 grams).