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Star class · YDW

G-type main-sequence star

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A G-type main-sequence star is a star of spectral class G that is fusing hydrogen into helium in its core. The Sun is one: a G2V star with a surface temperature of 5,772 K 1. G dwarfs have masses of about 0.9 to 1.1 times the Sun's and surface temperatures of about 5,300 to 6,000 K 2. They are often called yellow dwarfs, but seen from space they look white with only a faint warm tint 3. About one star in twenty near the Sun is a G dwarf 4, and because the Sun is one, they are the reference against which every other star, and the search for other Earths, is measured.

Characteristics

The G sequence

TypeTemperature (K)Mass (Sun = 1)Radius (Sun = 1)Luminosity (Sun = 1)B-VM_V
G0V5,9301.061.101.350.604.48
G2V5,7701.001.011.020.654.80
G5V5,6600.980.980.890.684.98
G9V5,3800.900.850.550.785.55

Values are the mean main-sequence sequence compiled by Eric Mamajek, building on Pecaut and Mamajek (2013) 2 5. A G dwarf's spectrum is dominated by the strong lines of ionised calcium and by the lines of many metals; the hydrogen lines that dominate hotter stars are much weaker.

The Sun as a G dwarf

The International Astronomical Union's nominal solar values, adopted in 2015 as conversion constants, are a radius of 695,700 km, a luminosity of 3.828 × 10²⁶ watts, an effective temperature of 5,772 K and a total solar irradiance at Earth of 1,361 W/m² 1. The oldest solids in the Solar System, calcium- and aluminium-rich inclusions in meteorites, date to 4,567.3 million years ago, which fixes the Sun's age 6. Inside, energy travels outward as radiation through the inner 71% of the Sun's radius and by convection above that; helioseismology puts the base of the convection zone at 0.713 solar radii 7. The convective layer drives the granulation visible on the surface and, with the Sun's rotation, the magnetic field that produces sunspots, flares and the 11-year activity cycle.

The Sun's full disc in visible light, bright at the centre and darker toward the edge, with a few small dark sunspot groups
Figure 1Observation: the Sun in visible light from NASA's Solar Dynamics Observatory; the disc darkens toward its edge because we see cooler, higher layers there. Credit: NASA/SDO/HMI/Goddard Space Flight Center.
PD-NASA

What colour is the Sun?

The Sun's light peaks in the green, but it is spread across all visible wavelengths, and to the eye it adds up to white. Converting model spectra of G2V stars into the colour the eye would see gives a white with the faintest yellow tint 3. The Sun looks yellow or orange from the ground only because the atmosphere scatters away blue light, most strongly when the Sun is low.

Evolution

A G dwarf brightens slowly as helium accumulates in its core. The Sun began its main-sequence life at about 70% of its present luminosity and is now brightening at about 1% every 110 million years 8. Models agree on the broad course of what follows and differ in the details 8 9:

StageSun's age (billion years)Luminosity (Sun today = 1)Radius (Sun today = 1)
Arrival on the main sequence00.700.89
Today4.581.001.00
End of the main sequence10.01.841.37
Tip of the red giant branch12.172,730256 (1.2 AU)
Helium burning begins12.175411
Tip of the asymptotic giant branch12.30about 2,100 to 4,200149 to 179

(Values from Schröder and Smith 2008.) In that model the Sun loses a third of its mass as a red giant, its planets' orbits widen as a result, and Earth still does not escape: it would need to orbit at about 1.15 AU or more today to survive 8. An earlier model, with less mass loss, has the red giant Sun reach 170 solar radii, engulf Mercury and spare Venus and Earth 9. Either way the Sun ends as a white dwarf of about 0.54 solar masses. See Red giant for this phase.

Earlier G types, slightly heavier than the Sun, run through the same sequence faster; later ones, lighter, more slowly.

Planets around G dwarfs

The habitable zone of a G dwarf lies roughly where Earth orbits the Sun: from about 0.95 to 1.68 AU for the Sun itself, using the conservative limits of Kopparapu et al. (2014), shifting inward to about 0.73 to 1.30 AU for a G9 dwarf 10. The first planet found around a Sun-like star orbited a G dwarf, 51 Pegasi 11. Kepler-452 b, announced in 2015 as a planet 1.6 times Earth's radius in the habitable zone of a G2 star 12, is a caution about such claims: a later reanalysis found it must still be considered a candidate 13. The nearest G dwarf, Alpha Centauri A, has a candidate giant planet in its habitable zone imaged by JWST, which awaits confirmation 14.

Notable examples

StarTypeDistanceNotes
The SunG2V1 AU1 solar mass, 4.57 billion years old 1 6
Alpha Centauri AG2V1.33 pc (4.34 ly)1.079 solar masses, 1.22 solar radii; bound to the K dwarf Alpha Centauri B 15
Tau CetiG8V3.65 pc (11.9 ly)Four planet candidates from radial velocities 16
18 ScorpiiG2Va14.1 pcA solar twin: 5,823 K and 1.04 solar masses, about 1.6 billion years younger than the Sun 17 18
51 PegasiG2IV15.5 pcHost of the first planet found around a Sun-like star 11
Two bright stars with diffraction spikes close together against a dark sky
Figure 2Observation: Alpha Centauri A (left), the nearest G dwarf, and its K-type companion Alpha Centauri B, imaged by Hubble. Credit: ESA/Hubble & NASA.
CC-BY-4.0

See also

References

  1. 1Prša, A. et al. (2016). Nominal Values for Selected Solar and Planetary Quantities: IAU 2015 Resolution B3. The Astronomical Journal 152, 41. doi:10.3847/0004-6256/152/2/41
  2. 2Mamajek, E. E.. A Modern Mean Dwarf Stellar Color and Effective Temperature Sequence (version 2022.04.16). www.pas.rochester.edu/~emamajek/EEM_dwarf_UBVIJHK_colors_Teff.txt
  3. 3Harre, J. V. and Heller, R. (2021). Digital color codes of stars. Astronomische Nachrichten 342, 578-587. doi:10.1002/asna.202113868
  4. 4Reylé, C. et al. (2021). The 10 parsec sample in the Gaia era. Astronomy & Astrophysics 650, A201. doi:10.1051/0004-6361/202140985
  5. 5Pecaut, M. J. and Mamajek, E. E. (2013). Intrinsic Colors, Temperatures, and Bolometric Corrections of Pre-main-sequence Stars. The Astrophysical Journal Supplement Series 208, 9. doi:10.1088/0067-0049/208/1/9
  6. 6Connelly, J. N. et al. (2012). The Absolute Chronology and Thermal Processing of Solids in the Solar Protoplanetary Disk. Science 338, 651-655. doi:10.1126/science.1226919
  7. 7Basu, S. and Antia, H. M. (1997). Seismic measurement of the depth of the solar convection zone. Monthly Notices of the Royal Astronomical Society 287, 189-198. doi:10.1093/mnras/287.1.189
  8. 8Schröder, K. P. and Connon Smith, R. (2008). Distant future of the Sun and Earth revisited. Monthly Notices of the Royal Astronomical Society 386, 155-163. doi:10.1111/j.1365-2966.2008.13022.x
  9. 9Sackmann, I. J., Boothroyd, A. I. and Kraemer, K. E. (1993). Our Sun. III. Present and Future. The Astrophysical Journal 418, 457. doi:10.1086/173407
  10. 10Kopparapu, R. K. et al. (2014). Habitable Zones around Main-sequence Stars: Dependence on Planetary Mass. The Astrophysical Journal Letters 787, L29. doi:10.1088/2041-8205/787/2/L29
  11. 11Mayor, M. and Queloz, D. (1995). A Jupiter-mass companion to a solar-type star. Nature 378, 355-359. doi:10.1038/378355a0
  12. 12Jenkins, J. M. et al. (2015). Discovery and Validation of Kepler-452b: A 1.6 Earth-radius Super Earth Exoplanet in the Habitable Zone of a G2 Star. The Astronomical Journal 150, 56. doi:10.1088/0004-6256/150/2/56
  13. 13Mullally, F. et al. (2018). Kepler's Earth-like Planets Should Not Be Confirmed without Independent Detection: The Case of Kepler-452b. The Astronomical Journal 155, 210. doi:10.3847/1538-3881/aabae3
  14. 14Beichman, C. et al. (2025). Worlds Next Door: A Candidate Giant Planet Imaged in the Habitable Zone of alpha Centauri A. I. Observations, Orbital and Physical Properties, and Exozodi Upper Limits. The Astrophysical Journal Letters 989, L22. doi:10.3847/2041-8213/adf53f
  15. 15Akeson, R. et al. (2021). Precision Millimeter Astrometry of the alpha Centauri AB System. The Astronomical Journal 162, 14. doi:10.3847/1538-3881/abfaff
  16. 16Feng, F. et al. (2017). Color Difference Makes a Difference: Four Planet Candidates around tau Ceti. The Astronomical Journal 154, 135. doi:10.3847/1538-3881/aa83b4
  17. 17Meléndez, J. et al. (2014). 18 Sco: A Solar Twin Rich in Refractory and Neutron-capture Elements. Implications for Chemical Tagging. The Astrophysical Journal 791, 14. doi:10.1088/0004-637X/791/1/14
  18. 18Porto de Mello, G. F. and da Silva, L. (1997). HR 6060: The Closest Ever Solar Twin?. The Astrophysical Journal 482, L89-L92. doi:10.1086/310693