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Glossary term: G-type Star

Also known as G star or G-star

Description: A star with spectral type "G". Astronomers identify G-type stars by the presence of strong absorption lines from ionized calcium; more generally, absorption lines from metals are stronger in G-type stars than in hotter stars (such as F-type stars) and weaker than in cooler stars (such as K-type stars). G-type stars have typical (effective) temperatures between around 5200 kelvins (K) and 6000 K. Compared to other stars, they appear yellow to human eyes, unless interstellar or atmospheric reddening is important. G-type stars that are main sequence stars, that is, that are burning hydrogen to helium in their core region with nuclear fusion, are called yellow dwarfs. The Sun is an example of a G-type star that is a yellow dwarf.

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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".

Related Diagrams


A quite ragged line peaking about 470 nm then declining at longer wavelengths with a few deeper dips.

Spectrum of a G-type star

Caption: The spectrum of the G-type star UCAC4 700-069569. The colour of the line between 400 nm and 700 nm roughly corresponds to the colour the human eye would see light of that wavelength. Below 400 nm and above 700 nm, where the human eye can see little to no light, the lines are coloured blue and red respectively. The black lines show spectral absorption lines caused by atoms and ions of different elements in the star’s atmosphere. These atoms and ions absorb at specific wavelengths, causing sharp, dark lines in the spectra. How strong these lines are depends on the temperature of the star’s atmosphere. Two stars made from the same mix of elements could have spectra with vastly different sets of lines in their spectra if they have different temperatures in their atmospheres. In G-type stars lines from hydrogen atoms are weaker than in F-type stars and lines from ionised calcium stronger. Lines from metal atoms such as atoms of iron, sodium and calcium also begin to become prominent.
Credit: IAU OAE/SDSS/Niall Deacon

License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons

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