G-type star
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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 continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hert ...
(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 masses and an effective temperature between about 5,300 and 6,000 K. Like other main-sequence stars, a G-type main-sequence star is converting the element
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
to
helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
in its core by means of
nuclear fusion Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles ( neutrons or protons). The difference in mass between the reactants and products is manife ...
, but can also fuse
helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
when hydrogen runs out. The
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, the star in the center of the
Solar System The Solar System Capitalization 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 "Solar ...
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 tons of hydrogen into helium in a process known as the
proton–proton chain The proton–proton chain, also commonly referred to as the chain, is one of two known sets of nuclear fusion reactions by which stars convert hydrogen to helium. It dominates in stars with masses less than or equal to that of the Sun, where ...
(4 hydrogens form 1 helium),
converting Converting companies are companies that specialize in modifying or combining raw materials such as polyesters, adhesives, silicone, adhesive tapes, foams, plastics, felts, rubbers, liners and metals, as well as other materials, to create new pro ...
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 part ...
to
energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...
. Besides the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, other well-known examples of G-type main-sequence stars include Alpha Centauri,
Tau Ceti Tau Ceti, Latinized from τ Ceti, is a single star in the constellation Cetus that is spectrally similar to the Sun, although it has only about 78% of the Sun's mass. At a distance of just under from the Solar System, it is a rela ...
,
Capella Capella is the brightest star in the northern constellation of Auriga. It has the Bayer designation α Aurigae, which is Latinised to Alpha Aurigae and abbreviated Alpha Aur or α Aur. Capella is the sixth-brightest star in ...
and 51 Pegasi. 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 the less massive and luminous G-type main-sequence stars lower down.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 harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
's atmosphere due to atmospheric
Rayleigh scattering Rayleigh scattering ( ), named after the 19th-century British physicist Lord Rayleigh (John William Strutt), is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of th ...
, 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 is a star with substantially larger radius and luminosity than a main-sequence (or ''dwarf'') star of the same surface temperature.Giant star, entry in ''Astronomy Encyclopedia'', ed. Patrick Moore, New York: Oxford University Press ...
s or bigger, stars similar to the Sun still outshine 90% of the stars in the
Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
(which are largely much dimmer
orange dwarf A K-type main-sequence star, also referred to as a K-type dwarf or an orange dwarf, is a main-sequence (hydrogen-burning) star of spectral type K and luminosity class V. These stars are intermediate in size between red M-type main-sequence stars ...
s, red dwarfs, and
white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes ...
s which are much more common, the latter being stellar remnants). A G-type main-sequence star 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 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 Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
, and the star passes on to the horizontal branch, 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 (PN, plural PNe) 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 unrelate ...
, 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 A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes ...
, which cools slowly from its high initial temperature as the nebula fades.


Spectral standard stars

The revised Yerkes Atlas system (Johnson & Morgan 1953)Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas
H.L. Johnson & W.W. Morgan, 1953, Astrophysical Journal, 117, 313 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 In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar He ...
dwarf stars, i.e. those standard stars that have remained unchanged over years, are beta CVn (G0V), the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
(G2V), Kappa1 Ceti (G5V),
61 Ursae Majoris 61 Ursae Majoris, abbreviated 61 UMa, is a single star in the northern circumpolar constellation of Ursa Major. It has a yellow-orange hue and is dimly visible to the naked eye with an apparent visual magnitude of 5.35. The distance ...
(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 (G6V). There are not yet any generally agreed upon G7V and G9V standards.


Planets

Besides the Sun and its planets, some of the nearest G-type stars known to have planets include
61 Virginis 61 Virginis (abbreviated 61 Vir) is the Flamsteed designation of a G-type main-sequence star (G7V) slightly less massive than the Sun (which has a hotter G2V spectral type), located about 27.9 light-years away in the constellation of Vir ...
,
HD 102365 HR 4523 (66 G. Centauri) is a binary star system that is located in the northeastern part of the Centaurus constellation, at a distance of about from the Solar System. The larger member of the system is a G-type star that is smaller than ...
, HD 147513, 47 Ursae Majoris,
Mu Arae Mu Arae (μ Arae, abbreviated Mu Ara, μ Ara), often designated HD 160691, officially named Cervantes , is a main sequence G-type star approximately 50 light-years away from the Sun in the constellation of Ara. The star has a planetary ...
, and
Tau Ceti Tau Ceti, Latinized from τ Ceti, is a single star in the constellation Cetus that is spectrally similar to the Sun, although it has only about 78% of the Sun's mass. At a distance of just under from the Solar System, it is a rela ...
.


See also


References


External links

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