TheSun, a typical example of a G-type main-sequence star
AG-type main-sequence star (spectral type: G-V), also often, and imprecisely, called ayellow dwarf, orG star, is amain-sequence star (luminosity class V) ofspectral type G. Such a star has about 0.9 to 1.1solar masses and aneffective temperature between about 5,300 and 6,000 K (5,000 and 5,700 °C; 9,100 and 10,000 °F). Like other main-sequence stars, a G-type main-sequence star converts theelementhydrogen tohelium in its core by means ofnuclear fusion. TheSun, the star in the center of theSolar System 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 milliontons of hydrogen into helium in a process known as theproton–proton chain (4 hydrogens form 1 helium),converting about 4 million tons ofmatter toenergy.[1][2] Besides theSun, other well-known examples of G-type main-sequence stars includeAlpha Centauri,Tau Ceti, and51 Pegasi.[3][4][5][6]
The termyellow 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.[7] The Sun is in fact white, but it can often appear yellow, orange or red throughEarth's atmosphere due to atmosphericRayleigh scattering, especially at sunrise and sunset.[8][9][10] In addition, although the term "dwarf" is used to contrast G-type main-sequence stars withgiant stars or bigger, stars similar to the Sun still outshine 90% of the stars in theMilky Way (which are largely much dimmerorange dwarfs,red dwarfs, andwhite dwarfs which are much more common, the latter beingstellar remnants).[11]
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 thesubgiant branch and ultimately expanding into many times its previous size at the tip of thered giant phase, about 1 billion years after leaving the main sequence. After this, the star's degenerate helium core abruptly ignites in ahelium flash fusinghelium, and the star passes on to thehorizontal branch, and then to theasymptotic 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 aplanetary nebula, radiating as it absorbs energetic photons from the photosphere. Eventually, the core begins to fade as nuclear reactions cease, and becomes a dense, compactwhite dwarf, which cools slowly from its high initial temperature as the nebula fades.[12][13]
The revised Yerkes Atlas system (Johnson & Morgan 1953)[16] listed 11 G-typedwarf spectral standard stars; however, not all of these still exactly conform to this designation.
The "anchor points" of theMK spectral classification system among the G-typemain-sequence dwarf stars, i.e. those standard stars that have remained unchanged over years, areChara (G0V), theSun (G2V),Kappa1 Ceti (G5V),61 Ursae Majoris (G8V).[17] Other primary MK standard stars include HD 115043 (G1V) and16 Cygni B (G3V).[18] The choices of G4 and G6 dwarf standards have changed slightly over the years among expert classifiers, but often-used examples include70 Virginis (G4V) and82 Eridani (G6V). There are not yet any generally agreed upon G7V and G9V standards.
G-type main sequence stars can provide habitability for life to develop, such as the Sun with life on Earth.[19] 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.[20]
