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 (luminosity class V) of
spectral type G. Such a star has about 0.9 to 1.1
solar mass
The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...
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 5,300 and 6,000
K. Like other main-sequence stars, a G-type main-sequence star is converting the
element hydrogen to
helium in its core by means of
nuclear fusion, but can also fuse
helium when hydrogen runs out. The
Sun, the star in the center of the
Solar 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 million
ton
Ton is the name of any one of several units of measure. It has a long history and has acquired several meanings and uses.
Mainly it describes units of weight. Confusion can arise because ''ton'' can mean
* the long ton, which is 2,240 pounds
...
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 to
energy. Besides the
Sun, other well-known examples of G-type main-sequence stars include
Alpha Centauri
Alpha Centauri ( Latinized from α Centauri and often abbreviated Alpha Cen or α Cen) is a triple star system in the constellation of Centaurus. It consists of 3 stars: Alpha Centauri A (officially Rigil Kentaurus), Alpha Centaur ...
,
Tau Ceti,
Capella 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 ...
.
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?](_blank)
, 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's atmosphere due to atmospheric
Rayleigh scattering, especially at sunrise and sunset. 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 dwarf
''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave since 2009, gaining a cult following. T ...
s, and
white dwarfs which are much more common, the latter being
stellar remnant
In astronomy, the term compact star (or compact object) refers collectively to white dwarfs, neutron stars, and black holes. It would grow to include exotic stars if such hypothetical, dense bodies are confirmed to exist. All compact objects ha ...
s).
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 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 outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...
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 0.8 solar masses () and 2.0 ) during their red giant phase (the Sun is ...
fusing
helium, 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 process) ...
, 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, 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.
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
Dwarf or dwarves may refer to:
Common uses
*Dwarf (folklore), a being from Germanic mythology and folklore
* Dwarf, a person or animal with dwarfism
Arts, entertainment, and media Fictional entities
* Dwarf (''Dungeons & Dragons''), a humanoid ...
spectral standard stars; however, not all of these still exactly conform to this designation.
The "anchor points" of the MK spectral classification
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 ...
system among the G-type main-sequence dwarf stars, i.e. those standard stars that have remained unchanged over years, are beta CVn
Beta Canum Venaticorum (β Canum Venaticorum, abbreviated Beta CVn, β CVn), also named Chara , is a G-type main-sequence star in the northern constellation of Canes Venatici. At an apparent visual magnitude of 4.25, it is the second-brigh ...
(G0V), the Sun (G2V), Kappa1 Ceti
Kappa1 Ceti, Latinized from κ1 Ceti, is a variable yellow dwarf star approximately 30 light-years away in the equatorial constellation of Cetus.
The star was discovered to have a rapid rotation, roughly once every nine days. Thou ...
(G5V), 61 Ursae Majoris (G8V).[MK ANCHOR POINTS](_blank)
, Robert F. Garrison Other primary MK standard stars include HD 115043 (G1V) and 16 Cygni
16 Cygni or 16 Cyg is the Flamsteed designation of a triple star system approximately 69 light-years away from Earth in the constellation of Cygnus. It consists of two Sun-like yellow dwarf stars, 16 Cygni A and 16 Cygni B, together wit ...
B (G3V).[The Perkins Catalog of Revised MK Types for the Cooler Stars](_blank)
, 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
70 Virginis is a binary star located 58 light years from the Sun in the equatorial constellation of Virgo, near the northern constellation border with Coma Berenices. ''70 Virginis'' is its Flamsteed designation. The star is visible ...
(G4V) and 82 Eridani
82 G. Eridani (HD 20794, HR 1008, e Eridani) is a star about 20 light years away from Earth in the constellation Eridanus. It is a main-sequence star with a stellar classification of G6 V, and it hosts a system of at least thre ...
(G6V). There are not yet any generally agreed upon G7V and G9V standards.
Planets
Besides the Sun and its planets
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a young ...
, some of the nearest G-type stars known to have planets include 61 Virginis, HD 102365, HD 147513
HD 147513 (62 G. Scorpii) is a star in the southern constellation of Scorpius. It was first catalogued by Italian astronomer Piazzi in his star catalogue as "XVI 55". With an apparent magnitude of 5.38, according to the Bortle scale it is ...
, 47 Ursae Majoris
47 Ursae Majoris (abbreviated 47 UMa), formally named Chalawan , is a yellow dwarf star approximately 46 light-years from Earth in the constellation of Ursa Major. , three extrasolar planets (designated 47 Ursae Majoris b, c and d; t ...
, Mu Arae, and Tau Ceti.
See also
References
External links
{{Portal bar, Astronomy, Stars, Outer space, Solar System
Star types