Description: In physics, gas is a state of matter where molecules or atoms are loosely bound to each other so as to allow for constant, chaotic motion, with atoms and molecules going every which way at different speeds. The air we breathe is a mixture of gases such as molecular nitrogen and oxygen. The mean energy of motion is a measure of the temperature of the gas. The main effect responsible for a gas's pressure (exerted, for instance, upon the walls of a container) is gas particles hitting those walls and recoiling. In astronomy, one may encounter gases as the constituents of interstellar gas clouds, giant molecular clouds, intergalactic gas, or the gas of a planetary atmosphere. In a slight misuse of language, astronomers also refer to plasma (where the atoms in question are ionized) as gas – for example, when they call stars "balls of gas," or refer to gas swirling in an accretion disk.

Related Terms:

• Accretion Disk

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".