Description: Hydrogen is the most abundant and lightest element in the Universe; it has the smallest nuclear charge as it has only one proton. When hydrogen atoms are excited, for instance by radiation from a nearby hot star, they emit light in characteristic narrow regions of the spectrum. These hydrogen emission lines can be used to detect atomic hydrogen: in particular, the hydrogen alpha line, with its rich red color which makes hydrogen clouds show up in splendid red in astronomical images, and the 21 centimeter (cm) hydrogen line which can be used to map out large clouds of gas using radio telescopes.

Related Terms:

• Proton

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".