A G-type main-sequence star (spectral type: G-V), also often, and imprecisely, called a yellow dwarf, or G star, is a

main-sequence star In astronomy, the main sequence is a classification of stars which appear on plots of stellar color versus brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or dwarf stars, and positions of star ...

(luminosity class V) of

spectral type In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

G. Such a star has about 0.9 to 1.1

solar mass The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxie ...

es and an

effective temperature The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...

between about . Like other main-sequence stars, a G-type main-sequence star converts the element

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...

in its core by means of

nuclear fusion Nuclear fusion is a nuclear reaction, reaction in which two or more atomic nuclei combine to form a larger nuclei, nuclei/neutrons, neutron by-products. The difference in mass between the reactants and products is manifested as either the rele ...

. The

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

, the star in the center of the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

to which the Earth is gravitationally bound, is an example of a G-type main-sequence star (G2V type). Each second, the Sun fuses approximately 600 million

ton Ton is any of several units of measure of mass, volume or force. It has a long history and has acquired several meanings and uses. As a unit of mass, ''ton'' can mean: * the '' long ton'', which is * the ''tonne'', also called the ''metric ...

s 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 In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic pa ...

energy Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...

. Besides the

, other well-known examples of G-type main-sequence stars include

Alpha Centauri Alpha Centauri (, α Cen, or Alpha Cen) is a star system in the southern constellation of Centaurus (constellation), Centaurus. It consists of three stars: Rigil Kentaurus (), Toliman (), and Proxima Centauri (). Proxima Centauri ...

, Tau Ceti, and

51 Pegasi 51 Pegasi (abbreviated 51 Peg), formally named Helvetios , is a Sun-like star located from Earth in the constellation of Pegasus. It was the first main-sequence star found to have an exoplanet (designated 51 Pegasi b, officially named ...

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.What Color Are the Stars?
, Mitchell N. Charity's webpage, accessed November 25, 2007 The Sun is in fact white, but it can often appear yellow, orange or red through

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

's atmosphere due to atmospheric

Rayleigh scattering Rayleigh scattering ( ) is the scattering or deflection of light, or other electromagnetic radiation, by particles with a size much smaller than the wavelength of the radiation. For light frequencies well below the resonance frequency of the scat ...

, especially at sunrise and sunset. In addition, although the term "dwarf" is used to contrast G-type main-sequence stars with

giant star A giant star has a substantially larger radius and luminosity than a main-sequence (or ''dwarf'') star of the same surface temperature. They lie above the main sequence (luminosity class V in the Yerkes spectral classification) on the Hertzsp ...

s or bigger, stars similar to the Sun still outshine 90% of the stars in the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

(which are largely much dimmer orange dwarfs,

red dwarf A red dwarf is the smallest kind of star on the main sequence. Red dwarfs are by far the most common type of fusing star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs are ...

s, and

white dwarf A white dwarf is a Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place i ...

s which are much more common, the latter being

stellar remnant In astronomy, the term compact object (or compact star) refers collectively to white dwarfs, neutron stars, and black holes. It could also include exotic stars if such hypothetical, dense bodies are confirmed to exist. All compact objects have a ...

s). 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 A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution ...

branch and ultimately expanding into many times its previous size at the tip of the

red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The stellar atmosphere, outer atmosphere is inflated and tenuous, making the radius large and the surface t ...

phase, about 1 billion years after leaving the main sequence. After this, the star's degenerate helium core abruptly ignites in a

helium flash A helium flash is a very brief thermal runaway nuclear fusion of large quantities of helium into carbon through the triple-alpha process in the core of low-mass stars (between 0.5-0.44 solar masses () and 2.0 ) during their red giant phase. The Su ...

fusing

, and the star passes on to the

horizontal branch The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red-giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha proc ...

, and then to the asymptotic giant branch. Expanding even further as helium starts running out as it pulses violently, the star's gravity is not sufficient to hold its outer envelope, resulting in significant mass loss and shedding. The ejected material remains as a

planetary nebula A planetary nebula is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The term "planetary nebula" is a misnomer because they are unrelated to planets. The ...

, radiating as it absorbs energetic photons from the photosphere. Eventually, the core begins to fade as nuclear reactions cease, and becomes a dense, compact

, which cools slowly from its high initial temperature as the nebula fades.

Spectral standard stars

The revised Yerkes Atlas system (Johnson & Morgan 1953) listed 11 G-type

dwarf Dwarf, dwarfs or dwarves may refer to: Common uses *Dwarf (folklore), a supernatural being from Germanic folklore * Dwarf, a human or animal with dwarfism Arts, entertainment, and media Fictional entities * Dwarf (''Dungeons & Dragons''), a sh ...

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 In astronomy, the main sequence is a classification of stars which appear on plots of stellar color versus brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or dwarf stars, and positions of star ...

dwarf stars, i.e. those standard stars that have remained unchanged over years, are Chara (G0V), the

(G2V), Kappa1 Ceti (G5V), 61 Ursae Majoris (G8V).MK ANCHOR POINTS
, Robert F. Garrison Other primary MK standard stars include HD 115043 (G1V) and 16 Cygni B (G3V).The Perkins Catalog of Revised MK Types for the Cooler Stars
, P.C. Keenan & R.C McNeil, "Astrophysical Journal Supplement Series" 71 (October 1989), pp. 245–266. 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 82 G. Eridani (HD 20794, Gliese 139, e Eridani) is a star away from Earth in the constellation Eridanus (constellation), Eridanus. It is a main sequence, main-sequence star with a stellar classification of G6 V, and it hosts a sys ...

(G6V). There are not yet any generally agreed upon G7V and G9V standards.

Habitability

G-type main sequence stars can provide habitability for life to develop, such as the Sun with life on Earth. 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.

Planets

Besides the Sun and its

planets A planet is a large, rounded astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets by the most restrictive definition of the te ...

, some of the nearest G-type stars known to have planets include 61 Virginis, HD 102365, HD 147513, Chalawan,

Cervantes Miguel de Cervantes Saavedra ( ; ; 29 September 1547 (assumed) – 22 April 1616 NS) was a Spanish writer widely regarded as the greatest writer in the Spanish language and one of the world's pre-eminent novelists. He is best known for his no ...

and Tau Ceti.

References

External links