Main sequence

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Image:H-R diagram.png The main sequence of the Hertzsprung-Russell diagram is the curve where the majority of stars are located in this diagram. Stars located on this band are known as main-sequence stars or dwarf stars. The coolest dwarfs are the red dwarfs.

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.

At closer inspection, one notices that the main sequence is not exactly a line but instead somewhat fuzzy. There are many reasons for this fuzziness, the most important one still being observational uncertainties which mainly affect the distance of the star in question but range all the way to unresolved binary stars.

But 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. 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 has the spectral classification of G2 V. After the hydrogen supply in the core is exhausted, it will expand to become a red giant.

1 Main sequence lifetime and luminosity
2 Main sequence data
3 In the arts
4 Reference
5 External links

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Main sequence lifetime and luminosity

The physics of stellar lifetime and luminosity have been known for some time, though there is still calibration to be done for stars at the bottom and top of the H-R diagram. Stellar mass, lifetime, and luminosity are all tightly coupled on the main sequence, so all must be discussed at once.

A star's luminosity increases exponentially with its mass; the formula typically given for the mass-luminosity relationship is approximately <math>L=M^4</math>, where <math>L</math> is luminosity and <math>M</math> is mass in solar masses.(Massey and Meyer, 3104) However, this is a generalization, as the M-L relationship changes with stellar mass.(Massey and Meyer, 3104) Note the following approximations with varying mass, from the same source:

<math>L=M^{1.6}</math> for M ≈ 100 solar masses

<math>L=M^{3.1}</math> for M ≈ 10 solar masses

<math>L=M^{4.7}</math> for M ≈ 1 solar mass

<math>L=M^{2.7}</math> for M ≈ 0.1 solar mass

This dramatic change in luminosity is an indicator of the sensitivity of the fusion reactions at a star's core; this fusion rate is far more important to stellar evolution than mass. For most stars that are not red dwarfs, roughly the same proportion of mass is available for fusion in the core: 10%.(Massey and Meyer, 3104) Thus, a star with 10 times more hydrogen in its core than the Sun will exhaust its supply much more rapidly, consuming it over a thousand times faster. From this relation one can extract a rough formula for main sequence lifetime, from the same source:

τ_ms ≈ <math>\frac{M}{L}</math>

Another common method of calculating main sequence lifetime is to take the result of the above calculation and multiplying the answer by the Sun's main sequence lifetime of 10 billion years. Applying this method to the 10 solar-mass star mentioned earlier gives an estimated lifetime of 80 million years.

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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. (Note: the following data are not in accord with the external link, and the luminosity per unit surface does not follow the law of being proportional to T⁴.)

Stellar Class	Radius	Mass	Luminosity	Temperature
Stellar Class	R/R_☉	M/M_☉	L/L_☉	K
O2	16	158		54,000
O5	14	58		46,000
B0	5.7	16	16,000	29,000
B5	3.7	5.4	750	15,200
A0	2.3	2.6	63	9,600
A5	1.8	1.9	24	8,700
F0	1.5	1.6	9.0	7,200
F5	1.2	1.35	4.0	6,400
G0	1.05	1.08	1.45	6,000
G2	1.0	1.0	1.0	5,900
G5	0.98	0.95	0.70	5,500
K0	0.89	0.83	0.36	5,150
K5	0.75	0.62	0.18	4,450
M0	0.64	0.47	0.075	3,850
M5	0.36	0.25		3,200

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In the arts

Main sequence is also the name of a composition of music by composer Vangelis on his 1976 album, Albedo 0.39 and is in reference to the astronomical term.

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Reference

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 1561592684

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