Asia is the world's largest continent, encompassing an area of 17,139,000 square miles (44,390,000 square kilometers), almost 30 percent of the world's land area. Because Asia covers such an enormous area and contains so many countries and islands, its exact borders remain unclear.


Asteroids are relatively small, rocky chunks of matter that shine like small stars, but orbit the Sun like planets. Most asteroids, or minor planets as they are sometimes called, are made of carbon-rich rock, while others (those farthest from the Sun) contain iron, nickel, and a few other elements.


Asthma is a reversible lung disease that affects approximately four million people in the United States. It is sometimes referred to as a disease of twitchy or reactive lungs, which means that the airways of the lungs are extremely sensitive to irritants such as pollen, animal dander (hair, feathers, or skin), dust, or tobacco smoke.


Astrophysics uses the already understood theories of physics (the study of matter and energy) to describe astronomical (universal) phenomena or events. Astrophysicists try to understand the processes that cause our universe and everything in it to behave the way it does.

Atmosphere, Composition and Structure

Earth's atmosphere is composed of about 78 percent nitrogen, 21 percent oxygen, and a 1-percent mixture of minor gases dominated by argon. The atmosphere can be divided into vertical layers.

Atmosphere Observation

Atmosphere observation refers to all equipment and techniques used to study properties of the atmosphere, including temperature, pressure, air movements, and chemical composition. Basic instruments for measuring the atmosphere, such as the barometer, were developed during the seventeenth and eighteenth centuries.

Atmospheric Circulation

Atmospheric circulation is the movement of air at all levels of the atmosphere over all parts of the planet. The driving force behind atmospheric circulation is solar energy, which heats the atmosphere with different intensities at the equator, the middle latitudes, and the poles.

Atmospheric Optical Effects

Rainbows, mirages, auroras, the twinkling of stars, and even the blue color of the sky are all considered atmospheric optical effects. These visual events in the sky occur when light bounces off or is bent by solid particles, liquids droplets, and other materials present in the atmosphere.

Atmospheric Pressure

Earth's atmosphere consists of gases that surround the surface of the planet. Like any gas, which is made up of molecules that are constantly in motion, the atmosphere exerts a force or pressure on everything within it.


An atom is the smallest particle of a element that has all the properties of that element. Imagine that you decide to cut a chunk of aluminum metal into half, over and over again.

Atomic Mass

The atomic mass of an atom is the mass of that atom compared to some standard, such as the mass of a particular type of carbon atom. The terms atomic mass and atomic weight are often used interchangeably, although, strictly speaking, they do not mean the same thing.

Atomic Theory

An atomic theory is a model developed to explain the properties and behaviors of atoms. As with any scientific theory, an atomic theory is based on scientific evidence available at any given time and serves to suggest future lines of research about atoms.

Attention-Deficit Hyperactivity Disorder (ADHD)

Attention-deficit hyperactivity disorder (ADHD) is a condition that is characterized by a person's inability to focus attention. The condition is present at birth and is usually evident by early childhood, although some persons are not diagnosed until adulthood.


Of the seven continents, Australia is the flattest, smallest, and, except for Antarctica, the most arid (dry). Including the southeastern island of Tasmania, the island continent encompasses 2,967,877 square miles (7,686,810 square kilometers).


Automation is the use of computers and robots to automatically control and operate machines or systems to perform work normally done by humans. Although ideas for automating tasks have been in existence since the time of the ancient Greeks, the development of automation came during the Industrial Revolution of the early eighteenth century.


No invention in modern times has had as much of an impact on human life as the invention of the automobile. It has become an important influence on the history, economy, and social life of much of the world.


Bacteria are very small organisms, usually consisting of one cell, that lack chlorophyll (a green pigment found in plants that allows for the production of food). Except for viruses, they are the smallest living things on Earth.


Ballistics is the study of projectile motion. A projectile is an object that has been launched, shot, hurled, thrown, or projected by any other means and that then travels on its own along a ballistic path.


A balloon is a type of aircraft consisting of a thin envelope filled with a gas less dense than the surrounding air. The envelope can be made of rubber, plastic, treated paper or cloth, or other material through which gases cannot seep.


A barometer is an instrument for measuring atmospheric pressure. Two kinds of barometers are in common use, a mercury barometer and an aneroid barometer.


A battery is a device for converting chemical energy into electrical energy. Batteries can consist of a single voltaic cell or a series of voltaic cells joined to each other.


Behavior is the way that all organisms or living things respond to stimuli in their environment. Stimuli include chemicals, heat, light, touch, and gravity.

Big Bang Theory

The big bang is the foremost model that scientists use to describe the creation of the universe. This theory proposes that the universe was created in a violent event approximately 12 to 15 billion years ago.

Binary Star

A binary star, often called a double star, is a star system in which two stars linked by their mutual gravity orbit around a central point of mass. Binary stars are quite common.


Biochemistry is the science dealing with the chemical nature of the bodily processes that occur in all living things. It is the study of how plants, animals, and microbes function at the level of molecules.


The term biodegradable is used to describe substances that are capable of being broken down, or decomposed, by the action of bacteria, fungi, and other microorganisms. Temperature and sunlight may also play a role in the decomposition of biodegradable substances.


The term biodiversity refers to the wide range of organisms—plants and animals—that exist within any given geographical region. That region may consist of a plot of land no more than a few square meters or yards, a whole continent, or the entire planet.


Bioenergy or "biomass energy" is any type of fuel or power that is made from living matter or biomass. Biomass is a scientific term for organic or living matter that is available on a renewable basis, such as plants.