# G-type main-sequence star
A **G-type main-sequence star** (spectral type G, luminosity class V), often called a **yellow dwarf** or **G dwarf**, is a star of spectral class G that is fusing hydrogen into helium in its core while lying on the main sequence of the [Hertzsprung–Russell diagram](https://www.edgepedia.org/hertzsprung-russell-diagram). The Sun is the best-known example and serves as the defining standard of the subtype G2V.[1](https://content-calpoly-edu.s3.amazonaws.com/evolution/1/images/3universe/stardust/starWikis/G-type_main-sequence_star%20-%20wiki.pdf) • [2](https://www.livescience.com/space/the-sun/is-the-sun-really-a-dwarf-star)
| Key fact | Detail |
|---|---|
| Definition | A main-sequence star (luminosity class V) of spectral type G, converting hydrogen to helium in its core by nuclear fusion[1](https://content-calpoly-edu.s3.amazonaws.com/evolution/1/images/3universe/stardust/starWikis/G-type_main-sequence_star%20-%20wiki.pdf) |
| Temperature range | About 5,125 to 5,725 degrees Celsius (9,260 to 10,340 degrees Fahrenheit) for G-type stars[2](https://www.livescience.com/space/the-sun/is-the-sun-really-a-dwarf-star) |
| The Sun | Classified G2V, with an effective temperature of 5800 K and a luminosity of 3.8 × 1026 W[3](https://openstax.org/books/astronomy-2e/pages/15-1-the-structure-and-composition-of-the-sun) |
| Anchor standards | beta CVn (G0V), the Sun (G2V), Kappa1 Ceti (G5V), and 61 Ursae Majoris (G8V)[1](https://content-calpoly-edu.s3.amazonaws.com/evolution/1/images/3universe/stardust/starWikis/G-type_main-sequence_star%20-%20wiki.pdf) |
| Population share | About 8% of the main-sequence stellar population, compared with about 12% for K dwarfs and 3% for F dwarfs[4](https://ntrs.nasa.gov/api/citations/20205001152/downloads/STRIVES%2020-18.pdf) |
| Lifetime | About 10 billion years for the Sun, which is already roughly halfway through its main-sequence life[5](https://science.nasa.gov/missions/hubble/goldilocks-stars-are-best-places-to-look-for-life/) |
| Interior structure | A radiative core beneath a convective envelope; the fully convective regime begins below about 0.35 solar masses[6](https://springerlink.fh-diploma.de/article/10.1007/s00159-020-00130-3) |
## Classification within the MK system
The two-dimensional classification of a G dwarf, written for example as G2 V, assigns a temperature type (G, with subclasses 0 through 9) and a luminosity class (V for main-sequence dwarfs). The system was founded by W.W. Morgan and P.C. Keenan with the 1943 publication of *An Atlas of Stellar Spectra* (Morgan, Keenan & Kellman), and it classifies a star by matching its spectrum against a defined set of standard stars, using only directly observable spectral features.[7](https://ned.ipac.caltech.edu/level5/Gray/Gray1.html) Morgan assigned types such as G2 V purely by visual inspection of high signal-to-noise spectrograms, deliberately separating spectral type and luminosity class from any absolute-magnitude calibration, so later physical measurements could refine the calibration without changing the classification itself.[8](https://www.nationalacademies.org/read/5859/chapter/17)
The revised Yerkes Atlas system published by Johnson and Morgan in 1953 listed 11 G-type dwarf spectral standard stars, though not all have remained in use. The anchor points of the MK system among G-type dwarfs, the standards that have stayed unchanged over the years, are beta CVn (G0V), the Sun (G2V), [Kappa1 Ceti](https://www.edgepedia.org/kappa1-ceti) (G5V), and [61 Ursae Majoris](https://www.edgepedia.org/61-ursae-majoris) (G8V).[1](https://content-calpoly-edu.s3.amazonaws.com/evolution/1/images/3universe/stardust/starWikis/G-type_main-sequence_star%20-%20wiki.pdf) The Sun itself is awkward to observe directly with stellar spectrographs, so the normal practice is to observe the sunlight reflected from an airless solar-system body.[9](https://assets.press.princeton.edu/chapters/s8963.pdf)
Spectrally, a G star is identified by lines of ionized iron and other metals; a star with hydrogen lines of intermediate strength is a B star if it also shows helium lines, but a G star if it shows these metal lines instead.[10](https://openstax.org/books/astronomy/pages/17-3-the-spectra-of-stars-and-brown-dwarfs)
## Physical properties and energy generation
The Sun illustrates the class. Its effective temperature is 5800 K, its luminosity is 3.8 × 1026 W, and its photospheric diameter is 109.3 times Earth's diameter; about 73% of its mass is hydrogen and 25% helium.[3](https://openstax.org/books/astronomy-2e/pages/15-1-the-structure-and-composition-of-the-sun) The G band of the classification corresponds to surface temperatures of roughly 5,125 to 5,725 degrees Celsius.[2](https://www.livescience.com/space/the-sun/is-the-sun-really-a-dwarf-star)
Like other main-sequence stars, G dwarfs convert hydrogen to helium in their cores.[1](https://content-calpoly-edu.s3.amazonaws.com/evolution/1/images/3universe/stardust/starWikis/G-type_main-sequence_star%20-%20wiki.pdf) In the Sun the dominant mechanism is the proton-proton chain, which accounts for 99% of solar hydrogen burning; the carbon-nitrogen-oxygen cycles contribute the remainder and dominate only in hotter, more massive main-sequence stars.[11](https://www1.psfc.mit.edu/research/hedp/Papers/Acharya_RMP-2025.pdf) Energy generated in the core travels outward first by radiation through the [radiative zone](https://www.edgepedia.org/radiative-zone) and then by convection through the outer envelope, a structure shared by solar-like stars with masses above about 0.35 solar masses; below that mass, main-sequence stars become fully convective.[6](https://springerlink.fh-diploma.de/article/10.1007/s00159-020-00130-3) The outer [convection zone](https://www.edgepedia.org/convection-zone) drives surface magnetic activity, and its turbulent motions also stochastically excite the solar-like oscillations that asteroseismology uses to probe stellar interiors.[12](https://doi.org/10.1146/annurev-astro-082812-140938)
## Lifetimes and population
G dwarfs are moderately numerous but comparatively short-lived members of the main sequence. G dwarfs make up about 8% of the main-sequence stellar population, against about 12% for K dwarfs, and 3% for F dwarfs.[4](https://ntrs.nasa.gov/api/citations/20205001152/downloads/STRIVES%2020-18.pdf) The Sun, now about halfway through its lifetime, will last about 10 billion years on the main sequence before swelling into a red giant; by comparison, K dwarfs live from 15 billion to 45 billion years.[5](https://science.nasa.gov/missions/hubble/goldilocks-stars-are-best-places-to-look-for-life/) Because G and F dwarf lifetimes are comparable to the age of the Galaxy, the G dwarfs ever born are assumed to still exist, which makes their metallicity distribution a record of the Galaxy's chemical history.[13](https://ned.ipac.caltech.edu/level5/March03/McWilliam/McWilliam4.html)
## The G-dwarf problem
The metallicity distribution of local G dwarfs exposed a long-standing puzzle in galactic chemical evolution. In a closed-box "Simple model," in which the solar neighborhood evolved from primordial gas with no gas flowing in or out, the cumulative metallicity distribution predicts many more low-metallicity G dwarfs than are observed; van den Bergh first noted in 1962 that the [Milky Way](https://www.edgepedia.org/milky-way) contains far fewer such stars than the model's steep initial rise requires.[14](https://academic.oup.com/mnrasl/article/502/1/L95/6124634) The deficit extends to the longer-lived K and M dwarfs, so it cannot be explained by old metal-poor stars having already evolved off the main sequence.[15](https://galaxiesbook.org/chapters/II-05.-Chemical-Evolution-in-Galaxies_2-Classical-models-of-galactic-chemical-evolution.html) The most accepted resolution is that the solar vicinity did not evolve as a closed box but experienced gas flows, in particular gas infall, which suppresses the predicted population of metal-poor stars.[16](https://link.springer.com/article/10.1007/s00159-021-00133-8)
## G dwarfs as planet hosts
G dwarfs have been central to exoplanet surveys. Kepler's input catalog mirrored its science aim of establishing how often Earth-like planets orbit stars resembling the Sun: of the 150,000 stars sampled, 90,000 were G-type dwarfs or close to the main sequence.[17](https://www.sciencedirect.com/topics/physics-and-astronomy/g-dwarf-stars) An optimistic habitable-zone definition yields about 0.114 Earth-size exoplanets per star in the habitable zones of main-sequence G stars among confirmed exoplanets.[18](https://www.sciencedirect.com/science/article/pii/S003206331400172X)
In habitability discussions, G stars occupy a middle ground. NASA describes K dwarfs, which are cooler, less luminous, and three times more numerous than Sun-like stars, as the "Goldilocks stars," intermediate between the rarer, more luminous but shorter-lived G stars and the more numerous but more violent M dwarfs.[5](https://science.nasa.gov/missions/hubble/goldilocks-stars-are-best-places-to-look-for-life/) A K2V star is only about a third as luminous as a G2V star and a K6V star about a tenth as luminous, which improves the planet-star contrast for direct imaging of their planets.[4](https://ntrs.nasa.gov/api/citations/20205001152/downloads/STRIVES%2020-18.pdf) The Sun's comparatively rapid evolution sets a clock on planetary habitability: about a billion years from now Earth will orbit inside the inner edge of the Sun's habitable zone, which moves outward as the Sun grows warmer and brighter.[5](https://science.nasa.gov/missions/hubble/goldilocks-stars-are-best-places-to-look-for-life/)
## References
1. [G-type main-sequence star (archived reference text), Cal Poly](https://content-calpoly-edu.s3.amazonaws.com/evolution/1/images/3universe/stardust/starWikis/G-type_main-sequence_star%20-%20wiki.pdf)
2. [Is the sun really a dwarf star? Live Science](https://www.livescience.com/space/the-sun/is-the-sun-really-a-dwarf-star)
3. [15.1 The Structure and Composition of the Sun, Astronomy 2e, OpenStax](https://openstax.org/books/astronomy-2e/pages/15-1-the-structure-and-composition-of-the-sun)
4. [The K Dwarf Advantage for Biosignatures on Directly Imaged Exoplanets, NASA NTRS](https://ntrs.nasa.gov/api/citations/20205001152/downloads/STRIVES%2020-18.pdf)
5. [Goldilocks Stars Are Best Places to Look for Life, NASA Science](https://science.nasa.gov/missions/hubble/goldilocks-stars-are-best-places-to-look-for-life/)
6. [Magnetic fields of M dwarfs, The Astronomy and Astrophysics Review](https://springerlink.fh-diploma.de/article/10.1007/s00159-020-00130-3)
7. [A Digital Spectral Classification Atlas, R. O. Gray](https://ned.ipac.caltech.edu/level5/Gray/Gray1.html)
8. [William Wilson Morgan, Biographical Memoirs Volume 72, National Academies Press](https://www.nationalacademies.org/read/5859/chapter/17)
9. [The Sun as a star (chapter), Princeton University Press](https://assets.press.princeton.edu/chapters/s8963.pdf)
10. [17.3 The Spectra of Stars (and Brown Dwarfs), Astronomy, OpenStax](https://openstax.org/books/astronomy/pages/17-3-the-spectra-of-stars-and-brown-dwarfs)
11. [Solar fusion III: New data and theory for hydrogen-burning stars, Reviews of Modern Physics](https://www1.psfc.mit.edu/research/hedp/Papers/Acharya_RMP-2025.pdf)
12. [Asteroseismology of Solar-Type and Red-Giant Stars, Annual Review of Astronomy and Astrophysics](https://doi.org/10.1146/annurev-astro-082812-140938)
13. [Abundance Ratios and Galactic Chemical Evolution, Andrew McWilliam](https://ned.ipac.caltech.edu/level5/March03/McWilliam/McWilliam4.html)
14. [SDSS-IV MaNGA: the 'G-dwarf problem' revisited, MNRAS Letters](https://academic.oup.com/mnrasl/article/502/1/L95/6124634)
15. [Classical models of galactic chemical evolution, galaxiesbook.org](https://galaxiesbook.org/chapters/II-05.-Chemical-Evolution-in-Galaxies_2-Classical-models-of-galactic-chemical-evolution.html)
16. [Modelling the chemical evolution of the Milky Way, The Astronomy and Astrophysics Review](https://link.springer.com/article/10.1007/s00159-021-00133-8)
17. [G Dwarf Stars, ScienceDirect Topics](https://www.sciencedirect.com/topics/physics-and-astronomy/g-dwarf-stars)
18. [Relative stellar occurrence of exoplanets in habitable zones of the main sequence F, G, K stars, Planetary and Space Science](https://www.sciencedirect.com/science/article/pii/S003206331400172X)
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*Topic: Encyclopedia › Physical world and mathematics › Astronomy › Stars and galaxies › Stellar astrophysics, structure, evolution, and variables › Stellar classification and star types › G-type main-sequence stars*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license