Information about Main Sequence

The main sequence of the Hertzsprung-Russell diagram is the curve along which the majority of stars are located. Stars on this band are known as main-sequence stars or dwarf stars.

This line is so pronounced because both the spectral type and the luminosity depend only on a star's mass (to zeroth order) as long as it is fusing hydrogen—and that is what almost all stars spend most of their "active" life doing. These main-sequence (and therefore "normal") stars are called dwarf stars not because they are unusually small. They simply have smaller radii and are less luminous than the other main type of stars, the giant stars. "White dwarfs" are a different kind of star which are indeed small.

The main sequence does not follow a completely even curve; this is primarily because of the observational uncertainties which mainly affect the distance of the star in question but range all the way to unresolved binary stars.

However, even perfect observations would lead to a fuzzy main sequence, because mass is not a star's only parameter. Chemical composition and—related—its evolutionary status also move a star slightly on the main sequence, as do close companions, rotation, or magnetic fields, to name just a few factors. Actually, there are very metal-poor stars (subdwarfs) that lie just below the main sequence although they are fusing hydrogen, thus marking the lower edge of the main sequence's fuzziness due to chemical composition.

Astronomers will sometimes refer to the "zero age main sequence", or ZAMS. This is a line calculated by computer models of where a star will be when it begins hydrogen fusion; its brightness and surface temperature typically increase from this point with age. Stars usually enter and leave the main sequence from about when they are born or when they are starting to die, respectively.

Our Sun is a main-sequence star—it has been one for about 4.5 billion years and will continue to be one for another 4.5 billion years. It is a dwarf star with a spectral classification of G2 V. After the hydrogen supply in the core is exhausted, it will expand to become a red giant.

The total main sequence lifetime of a star can be estimated from its mass relative to the Sun's as follows:^[1]

where is the mass of the Sun, is the mass of the star and is the star's estimated main sequence lifetime in years. The lightest stars, of less than a tenth of solar mass, may last over a trillion years. However, this estimate poorly matches the lifetime of the heaviest stars, which last at least a few million years.

Main sequence data

The table below shows typical values for stars along the main sequence. The values of luminosity (L), radius (R), and mass (M) are relative to the Sun. The actual values for a star may vary by as much as 20-30%. The coloration of the stellar class column gives an approximate representation of the star's photographic color. A popular mnemonic for memorizing the sequence is "Oh Be A Fine Girl/Guy, Kiss Me".

Stellar Class	Radius	Mass	Luminosity	Temperature	Example
	R/R☉	M/M☉	L/L☉	K
O2	16	158	2,000,000	54,000	Sanduleak −71° 51
O5	14	58	800,000	46,000	Sanduleak −66° 41
B0	5.7	16	16,000	29,000	Phi1 Orionis
B5	3.7	5.4	750	15,200	Pi Andromedae A
A0	2.3	2.6	63	9,600	Vega
A5	1.8	1.9	24	8,700	Beta Pictoris
F0	1.5	1.6	9.0	7,200	Gamma Virginis
F5	1.2	1.35	4.0	6,400	Eta Arietis
G0	1.05	1.08	1.45	6,000	Beta Comae Berenices
G2	1.0	1.0	1.0	5,700	Sun
G5	0.98	0.95	0.70	5,500	Alpha Mensae
K0	0.89	0.83	0.36	5,150	70 Ophiuchi A
K5	0.75	0.62	0.18	4,450	61 Cygni A
M0	0.64	0.47	0.075	3,850	Gliese 185
M5	0.36	0.25	0.013	3,200	EZ Aquarii A
M8	0.15	0.10	0.0008	2,500	Van Biesbroeck's star
M9.5	0.10	0.08	0.0001	1,900	LP 647-013

See also

• Instability

References

1. ^ Richmond, Michael. Stellar evolution on the main sequence (English). Retrieved on 2006-08-24.

• Massey, Philip and Michael R. Meyer. "Stellar Masses." The Encyclopedia of Astronomy and Astrophysics. Ed. Paul Murdin. London: Institute of Physics Publishing Ltd and Nature Publishing Group, 2001. 3103-09. ISBN 1-56159-268-4

External links

• Table of Features of the "Life Zones" of Main Sequence Stars
• A java based applet for stellar evolution.

The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a Colour-Magnitude diagram, or CMD) shows the relationship between absolute magnitude, luminosity, classification, and effective temperature of
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In mathematics, the concept of a curve tries to capture the intuitive idea of a geometrical one-dimensional and continuous object. A simple example is the circle.
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STAR is an acronym for:

Organizations:

• Society for Telescopy, Astronomy, and Radio, a non-profit astronomy club in New Jersey
• Special Tasks and Rescue or Special Tactics and Response, synonyms for SWAT

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In astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequently refined in terms of other characteristics.
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Luminosity has different meanings in several different fields of science.

In photometry and color imaging

Main article: luminance

In photometry, luminosity
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Orders of approximation have been used not only in science, engineering, and other quantitative disciplines to make approximations with various degrees of precision but also more generally, and more loosely, to indicate relative precision outside these disciplines in the form of
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nuclear fusion is the process by which multiple atomic particles join together to form a heavier nucleus. It is accompanied by the release or absorption of energy. Iron and nickel nuclei have the largest binding energies per nucleon of all nuclei and therefore are the most stable.
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1, −1
(amphoteric oxide)
Electronegativity 2.20 (Pauling scale) More

Atomic radius 25 pm
Atomic radius (calc.) 53 pm
Covalent radius 37 pm
Van der Waals radius 120 pm
Miscellaneous

Thermal conductivity (300 K) 180.
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A giant star is a star with substantially larger radius and luminosity than a main sequence star of the same surface temperature.^[1] Typically, it will have a radius of between 10 and 100 solar radii and luminosity between 10 and 1,000 times the Sun's.
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white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. As white dwarfs have mass comparable to the Sun's and their volume is comparable to the Earth's, they are very dense.
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binary star is a stellar system consisting of two stars orbiting around their center of mass. For each star, the other is its companion star. Recent research suggests that a large percentage of stars are part of systems with at least two stars.
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metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium. (This terminology is used differently to the usual meaning of the word 'metal', since on the grandest of scales the universe is overwhelmingly composed of hydrogen
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In astronomy, stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from hundreds of thousands to billions of years.
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binary star is a stellar system consisting of two stars orbiting around their center of mass. For each star, the other is its companion star. Recent research suggests that a large percentage of stars are part of systems with at least two stars.
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Stellar rotation is the angular motion of a star about its axis. The rate of rotation can be measured from the spectrum of the star, or by timing the movements of active features on the surface.

The rotation of a star produces an equatorial bulge due to centrifugal force.
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stellar magnetic field is a magnetic field generated by the motion of conductive plasma inside a main sequence (hydrogen-burning) star. This motion is created through convection, which is a form of energy transport involving the physical movement of material.
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metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium. (This terminology is used differently to the usual meaning of the word 'metal', since on the grandest of scales the universe is overwhelmingly composed of hydrogen
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A subdwarf star, sometimes denoted by "sd", is luminosity class VI under the Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral type.
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The Sun

Observation data
Mean distance
from Earth 1.49610¹¹ m
(8.31 min at light speed)
Visual brightness (V) −26.74^{m [1]}
Absolute magnitude 4.
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A red giant is a luminous giant star of low or intermediate mass that is in a later phase of its evolution, with nuclear fusion going on in a shell outside the core but not in the core itself.
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The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a Colour-Magnitude diagram, or CMD) shows the relationship between absolute magnitude, luminosity, classification, and effective temperature of
..... Click the link for more information.

In astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequently refined in terms of other characteristics.
..... Click the link for more information.

Brown dwarfs are sub-stellar objects with a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores, as do stars on the main sequence, but which have fully convective surfaces and interiors, with no chemical differentiation by depth.
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white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. As white dwarfs have mass comparable to the Sun's and their volume is comparable to the Earth's, they are very dense.
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red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type. They constitute the vast majority of stars and have a mass of less than one-half that of the Sun (down to about 0.
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A subdwarf star, sometimes denoted by "sd", is luminosity class VI under the Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral type.
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Subgiant star is a class of stars that are brighter than normal main sequence (dwarf) stars of the same spectral class, but not as bright as true giant stars. They are believed to be stars that are ceasing or already ceased fusing hydrogen in their cores.
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A giant star is a star with substantially larger radius and luminosity than a main sequence star of the same surface temperature.^[1] Typically, it will have a radius of between 10 and 100 solar radii and luminosity between 10 and 1,000 times the Sun's.
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The luminosity class II in the Yerkes spectral classification is given to bright giants. These are stars which straddle the boundary between giants and supergiants, and the classification is in general given to giant stars with exceptionally high luminosity, but
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Supergiants are among the most massive stars. In the Hertzsprung-Russell diagram they occupy the top region of the diagram. In the Yerkes spectral classification supergiants are class Ia (most luminous supergiants) or Ib (less luminous supergiants).
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