G-type main-sequence star

Template:Short description Template:Infobox astronomical formation A G-type main-sequence starTemplate:Efn is a main-sequence star of spectral type G. The spectral luminosity class is typically Template:Rn. Such a star has about 0.9 to 1.1 solar masses and an effective temperature between about Template:Cvt. Like other main-sequence stars, a G-type main-sequence star converts the element hydrogen to helium in its core by means of nuclear fusion.

The Sun is an example of a G-type main-sequence star. Each second, the Sun fuses approximately 600 million tons of hydrogen into helium in a process known as the proton–proton chain (4 hydrogens form 1 helium), converting about 4 million tons of matter to energy.<ref>"Why Does The Sun Shine?" Template:Webarchive, lecture, Barbara Ryden, Astronomy 162, Ohio State University, accessed on line June 19, 2007.</ref><ref>"Sun" Template:Webarchive, entry at ARICNS, accessed June 19, 2007.</ref> Besides the Sun, other well-known examples of G-type main-sequence stars include Alpha Centauri, Tau Ceti, and 51 Pegasi.<ref>"Alpha Centauri A" Template:Webarchive, SIMBAD query result. Accessed on line December 4, 2007.</ref><ref>"Tau Ceti" Template:Webarchive, SIMBAD query result. Accessed on line December 4, 2007.</ref><ref>"51 Pegasi" Template:Webarchive, SIMBAD query result. Accessed December 4, 2007.</ref><ref>Template:Cite web</ref>

Description

The term yellow dwarf is a misnomer, because G-type stars actually range in color from white, for more luminous types like the Sun, to only very slightly yellowish for less massive and luminous G-type main-sequence stars.<ref name="vend">What Color Are the Stars? Template:Webarchive, Mitchell N. Charity's webpage, accessed November 25, 2007</ref> The Sun is in fact white, but it can often appear yellow, orange or red through Earth's atmosphere due to atmospheric Rayleigh scattering, especially at sunrise and sunset.<ref>Template:Cite web</ref><ref>Template:Cite web</ref><ref>Template:Cite journal</ref> In addition, although the term "dwarf" is used to contrast G-type main-sequence stars with giant stars or bigger, stars similar to the Sun still outshine 90% of the stars in the Milky Way (which are largely much dimmer orange dwarfs, red dwarfs, and white dwarfs which are much more common, the latter being stellar remnants).<ref>Template:Cite web</ref>

A G-type main-sequence star with the mass of the Sun will fuse hydrogen for approximately 10 billion years, until the hydrogen element is exhausted at the center of the star. When this happens, the star rapidly expands, cooling and darkening as it passes through the subgiant branch and ultimately expanding into many times its previous size at the tip of the red giant phase, about 1 billion years after leaving the main sequence. After this, the star's degenerate helium core abruptly ignites in a helium flash fusing helium, and the star passes on to the horizontal branch, and then to the asymptotic giant branch. As helium starts running out, it expands even further and pulses violently, with the star's gravity insufficient to hold its outer envelope. This results in significant mass loss and shedding. The ejected material remains as a planetary nebula, radiating as it absorbs energetic photons from the photosphere. Eventually, the core begins to fade as nuclear reactions cease, and becomes a dense, compact white dwarf, which cools slowly from its high initial temperature as the nebula fades.<ref>Template:Cite journal</ref><ref>Template:Cite web</ref>

Spectral standard stars

Properties of typical G-type main-sequence stars<ref>Template:Cite journal</ref><ref>Template:Cite web</ref>

Spectral type	Mass (Template:Solar mass)	Radius (Template:Solar radius)	Luminosity (Template:Solar luminosity)	Effective temperature (K)	Color index Template:Nowrap
G0V	1.06	1.100	1.35	style="background-color:#Template:Color temperature"|5,930	0.60
G1V	1.03	1.060	1.20	style="background-color:#Template:Color temperature"|5,860	0.62
G2V	1.00	1.012	1.02	style="background-color:#Template:Color temperature"|5,770	0.65
G3V	0.99	1.002	0.98	style="background-color:#Template:Color temperature"|5,720	0.66
G4V	0.985	0.991	0.91	style="background-color:#Template:Color temperature"|5,680	0.67
G5V	0.98	0.977	0.89	style="background-color:#Template:Color temperature"|5,660	0.68
G6V	0.97	0.949	0.79	style="background-color:#Template:Color temperature"|5,600	0.70
G7V	0.95	0.927	0.74	style="background-color:#Template:Color temperature"|5,550	0.71
G8V	0.94	0.914	0.68	style="background-color:#Template:Color temperature"|5,480	0.73
G9V	0.90	0.853	0.55	style="background-color:#Template:Color temperature"|5,380	0.78

The revised Yerkes Atlas system (Johnson & Morgan 1953)<ref>Template:Cite journal</ref> listed 11 G-type dwarf spectral standard stars; however, not all of these still exactly conform to this designation.

The "anchor points" of the MK spectral classification system among the G-type main-sequence dwarf stars, i.e. those standard stars that have remained unchanged over years, are Chara (G0V), the Sun (G2V), Kappa1 Ceti (G5V), 61 Ursae Majoris (G8V).<ref name="Garrison">MK ANCHOR POINTS Template:Webarchive, Robert F. Garrison</ref> Other primary MK standard stars include HD 115043 (G1V) and 16 Cygni B (G3V).<ref name="Keenan89">Template:Cite journal</ref> The choices of G4 and G6 dwarf standards have changed slightly over the years among expert classifiers, but often-used examples include 70 Virginis (G4V) and 82 Eridani (G6V). There are not yet any generally agreed upon G7V and G9V standards.

Habitability

Template:Main G-type main sequence stars can provide habitability for life to develop, such as the Sun with life on Earth.<ref name="astrobiology">Template:Cite web</ref> They also live long enough to give life enough time to develop, between 7.9 and 13 billion years. Our Sun's lifetime is about 10 billion years.<ref>Template:Cite web</ref>

Planets

Besides the Sun and its planets, some of the nearest G-type stars known to have planets include 61 Virginis, HD 102365, HD 147513, 47 Ursae Majoris (Chalawan), and Mu Arae (Cervantes).

A famous example of a G-type star with a planetary system was Tau Ceti, which was once known to host eight planets. As of July 2025, all of these planets have been disconfirmed as a 2025 study using ESPRESSO data failed to unambiguously detect any planets.<ref name=Figueira2025>Template:Cite journal</ref>

See also

• G-dwarf problem
• Hertzsprung–Russell diagram
• Solar analog
• Star count, survey of stars
• Yellow hypergiant