Main sequence

main-sequencemain sequence dwarfmain-sequence starmain sequence stardwarfevolutionary trackzero age main sequencemain sequence dwarfsmain-sequence starsmain sequence stars
In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness.wikipedia
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Nuclear fusion

fusionhydrogen fusionfusion reaction
After condensation and ignition of a star, it generates thermal energy in its dense core region through nuclear fusion of hydrogen into helium.
Fusion is the process that powers active or "main sequence" stars, or other high magnitude stars.

Dwarf star

dwarfDSdwarfs
Stars on this band are known as main-sequence stars or dwarf stars.
Most main sequence stars are dwarf stars.

Sun

solarSolThe Sun
These are the most numerous true stars in the universe, and include the Earth's Sun.
The Sun is a G2V star, with G2 indicating its surface temperature of approximately 5,778 K (5,505 °C, 9,941 °F), and V that it, like most stars, is a main-sequence star.

Stellar evolution

evolvedevolvingevolution
After the hydrogen fuel at the core has been consumed, the star evolves away from the main sequence on the HR diagram, into a supergiant, red giant, or directly to a white dwarf.
Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star.

White dwarf

white dwarfswhite dwarf starcentral star
After the hydrogen fuel at the core has been consumed, the star evolves away from the main sequence on the HR diagram, into a supergiant, red giant, or directly to a white dwarf.
, § 1. After the hydrogen-fusing period of a main-sequence star of low or medium mass ends, such a star will expand to a red giant during which it fuses helium to carbon and oxygen in its core by the triple-alpha process.

Stellar classification

spectral typeK-typeG-type
Annie Jump Cannon and Edward C. Pickering at Harvard College Observatory developed a method of categorization that became known as the Harvard Classification Scheme, published in the Harvard Annals in 1901.
Luminosity class 0 or Ia+ is used for hypergiants, class I for supergiants, class II for bright giants, class III for regular giants, class IV for sub-giants, class V for main-sequence stars, class sd (or VI) for sub-dwarfs, and class D (or VII) for white dwarfs.

Hertzsprung–Russell diagram

color-magnitude diagramHR diagramcolor magnitude diagram
These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell.
Most of the stars occupy the region in the diagram along the line called the main sequence.

Red giant

red giant starred giantsred giant stars
After the hydrogen fuel at the core has been consumed, the star evolves away from the main sequence on the HR diagram, into a supergiant, red giant, or directly to a white dwarf.
Red giants are evolved from main-sequence stars with masses in the range from about to around.

Pre-main-sequence star

pre-main-sequencepre–main sequence starpre-main sequence star
(The mass distribution of newly formed stars is described empirically by the initial mass function.) During the initial collapse, this pre-main-sequence star generates energy through gravitational contraction.
A pre-main-sequence star (also known as a PMS star and PMS object) is a star in the stage when it has not yet reached the main sequence.

Red dwarf

redred dwarf starsred dwarfs
For the cooler stars, dwarfs such as red dwarfs, orange dwarfs, and yellow dwarfs are indeed much smaller and dimmer than other stars of those colors.
A red dwarf (or M dwarf) is a small and cool star on the main sequence, of M spectral type.

K-type main-sequence star

orange dwarfKK-type star
For the cooler stars, dwarfs such as red dwarfs, orange dwarfs, and yellow dwarfs are indeed much smaller and dimmer than other stars of those colors.
A K-type main-sequence star (K V), also referred to as an K 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 ("red dwarfs") and yellow G-type main-sequence stars.

Astronomy

astronomicalastronomerastronomers
In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness.
A sufficiently dense, and hot, core region will trigger nuclear fusion, thus creating a main-sequence star.

G-type main-sequence star

yellow dwarfGG-type
For the cooler stars, dwarfs such as red dwarfs, orange dwarfs, and yellow dwarfs are indeed much smaller and dimmer than other stars of those colors.
A G-type main-sequence star (Spectral type: G-V), often (and imprecisely) called a yellow dwarf, or G dwarf star, is a main-sequence star (luminosity class V) of spectral type G. Such a star has about 0.84 to 1.15 solar masses and surface temperature of between 5,300 and 6,000 K.

Protostar

protostellarprotostarsbirth of new stars
When a protostar is formed from the collapse of a giant molecular cloud of gas and dust in the local interstellar medium, the initial composition is homogeneous throughout, consisting of about 70% hydrogen, 28% helium and trace amounts of other elements, by mass. The initial mass of the star depends on the local conditions within the cloud.
It ends when the infalling gas is depleted, leaving a pre-main-sequence star, which contracts to later become a main sequence star at the onset of Hydrogen fusion.

Initial mass function

Mass functiondistribution of stellar massesIMF
(The mass distribution of newly formed stars is described empirically by the initial mass function.) During the initial collapse, this pre-main-sequence star generates energy through gravitational contraction.
The IMF is often given as a probability distribution function (PDF) for the mass at which a star enters the main sequence (begins hydrogen fusion).

Solar mass

mass of the SunSun's masssolar masses
Stars below about 1.5 times the mass of the Sun primarily fuse hydrogen atoms together in a series of stages to form helium, a sequence called the proton–proton chain.
The original mass of the Sun at the time it reached the main sequence remains uncertain.

Luminosity

luminousbolometric luminosityluminosities
He published the first plots of color versus luminosity for these stars.
The vast majority of stars are found along the main sequence with blue Class O stars found at the top left of the chart while red Class M stars fall to the bottom right.

Mass–luminosity relation

mass-luminosity relationmass-luminosity ratiomass-luminosity relationship
First is the Stefan–Boltzmann law, which relates the luminosity L, the radius R and the surface temperature T eff . Second is the mass–luminosity relation, which relates the luminosity L and the mass M.
The value a = 3.5 is commonly used for main-sequence stars.

Beta Pictoris

β Picβ Pictoristhat more famous system
style="text-align: left;"|Beta Pictoris
The Beta Pictoris system is very young, only 20 to 26 million years old, although it is already in the main sequence stage of its evolution.

Beta Comae Berenices

β Comβ Comae Berenices
style="text-align: left;"|Beta Comae Berenices
Beta Comae Berenices (β Comae Berenices, β Com) is a main sequence dwarf star in the northern constellation of Coma Berenices.

Alpha Coronae Borealis

α CrBAlpheccaAlpha Coronae Borealis A
style="text-align: left;"|Alpha Coronae Borealis A
It is located about 75 light years from the Sun and contains two main sequence stars, one class A and one class G.

Brown dwarf

brown dwarfsbrown dwarvesPlanetar
Below this threshold are sub-stellar objects that can not sustain hydrogen fusion, known as brown dwarfs.
Unlike the stars in the main sequence, brown dwarfs are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1 H) to helium in their cores.

Sirius

SothisDog StarSirius B
Intermediate-mass stars such as Sirius may transport energy primarily by radiation, with a small core convection region.
The binary system consists of a main-sequence star of spectral type A0 or A1, termed Sirius A, and a faint white dwarf companion of spectral type DA2, designated Sirius B. The distance between the two varies between 8.2 and 31.5 astronomical units as they orbit every 50 years.

CNO cycle

CNOcarbon-nitrogen-oxygen (CNO) cyclecarbon-nitrogen-oxygen cycle
Above this mass, in the upper main sequence, the nuclear fusion process mainly uses atoms of carbon, nitrogen and oxygen as intermediaries in the CNO cycle that produces helium from hydrogen atoms.
Red giant stars are observed to have lower carbon-12/carbon-13 and carbon-12/nitrogen-14 ratios than do main sequence stars, which is considered to be convincing evidence for the operation of the CNO cycle.

Pi Andromedae

π Andromedaeπ AndPi Andromedae A
style="text-align: left;"|Pi Andromedae A
The pair is classified as a blue-white B-type main sequence dwarf, with an apparent magnitude of +4.34.