Glossary term: Vacuum

Description: The air that surrounds us contains about 10 billion billion molecules per cubic centimeter. An ideal vacuum would be a region that contains no molecules whatsoever. In practice, we talk about a vacuum if a region contains considerably fewer particles than usual. A "high vacuum" has only one millionth of the usual number of particles per cubic centimeter, and an "ultra-high vacuum" has less than a billion particles per cubic centimeter. Most regions of outer space are even emptier than that. On average, the density of ordinary matter particles in our Universe amounts to less than one per square meter. What astronomers consider comparatively dense, such as a giant molecular gas cloud, is still a better vacuum than any we can produce here on Earth.

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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".