Glossary term: Density

Description: Density is a physical characteristic of a substance or object that expresses the relationship of its volume to its mass. The higher the density, the greater the mass per unit volume. The average density of an object is its total mass divided by its total volume. Its SI unit is kilograms per cubic meter (kg/m3).

Densities in the region of the Milky Way around the Sun can range from about 10-20 kg/m3 for interstellar gas to more than 1017 kg/m3 for the interiors of neutron stars.

Everyday densities on Earth fall between those extremes, with iron at about 7800 kg/m3, water at around 1000 kg/m3 and the air that surrounds us at sea-level at a bit more than 1 kg/m3.

The Universe includes not just the stars, planets, and gas in galaxies, but also the relatively empty space between galaxies and between clusters of galaxies. This leads to an average density of matter in the Universe of the order of 10-27 kg/m3.

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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".