The majority of stars in the galaxy, including our Sun, Sirius and Alpha Centauri A and B are all main sequence stars. The Sun"s relative longevity and also stability have provided the problems necessary for life to evolve here on Planet. Our understanding of the procedures associated and also features of this crucial team of stars has actually advanced in parallel via our expertise of nuclear physics.

Properties of Key Sequence StarsNucleosynthesis and Fusion Reactions

Properties of Key Sequence Stars

Key sequence stars are qualified by the resource of their power. They are all undergoing fusion of hydrogen into helium within their cores. The rate at which they execute this and also the amount of fuel easily accessible counts upon the mass of the star. Mass is the crucial factor in determining the lifeexpectations of a main sequence star, its size and its luminosity. Stars on the main sequence additionally show up to be untransforming for long periods of time. Any version of such stars need to be able to account for their stcapacity.

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Hydrostatic Equilibrium

The straightforward design of any kind of main sequence star is of a dense gas/liquid in a state of hydrostatic equilibrium. The inward acting pressure, gravity, is well balanced by exterior acting pressures of gas push and the radiation press. Acomponent from the exceptionally warm but tenuous corona , the push and temperature of stars basically boosts as you method the core.

Key sequence stars basically have actually a fixed size that is a role of their mass. The more enormous the star, the higher its gravitational pull inwards. This consequently compresses the gas more. As you try and compush a gas it exerts a gas pressure earlier, it resists the compression. In stars this gas push alone is not adequate to withstand also the gravitational collapse. Once the core temperature has actually got to around 10 million K, fusion of hydrogen occurs, releasing power. This energy exerts an outwards radiation pressure as a result of the activity of the photons on the exceptionally dense matter in the core. The radiation press combined with the gas push balances the inward pull of gravity avoiding even more collapse.

Stellar Mass

As was evident from the evolutionary Hayashi tracks on the previous page, a star"s place on the primary sequence its actually a role of its mass. This is an incredibly helpful connection, referred to as the mass-luminosity relation. If we understand where on the primary sequence a star is we can infer its mass. In general the more enormous a star is, the additionally up the major sequence it is discovered and also the more luminous it is. Mathematically this relation is expressed by:

wright here n is around 4 for Sun-favor stars, 3 for the even more huge stars and 2.5 for dim red main sequence stars. (*Keep in mind this formula is not compelled for HSC exams). A 0.1 solar mass star has only around one-thousandth the luminosity of the Sun whereas a 10-solar mass star is has actually a luminosity 10,000 × that of our Sun.



Limits on the upper mass of stars is thneed to be somewhere between 150 and also 200 solar masses based on theoretical modeling. Such stars are exceptionally rare and also short-lived.

The greater the mass of a major sequence star, the greater its effective temperature. This, linked with the bigger radius of better mass major sequence stars accounts for their much greater luminosity. Remember, LT4 and also LR2 so also a little increase in efficient temperature will certainly significantly increase luminosity.

Main-Sequence Lifespan

The major sequence is the phase wright here a star spends the majority of of its existence. Relative to other stages in a star"s "life" it is extremely long; our Sun took around 20 million years to form but will certainly spfinish about 10 billion years (1 × 1010 years) as a major sequence star before evolving into a red large. What determines the main sequence lifeexpectancy of a star?

Main sequence stars differ in mass. You might imagine that an extra huge star has even more fuel accessible so deserve to spfinish more time on the major sequence fmaking use of hydrogen to helium. You would certainly be wrong - the opposite is true. More enormous stars have a more powerful gravitational pressure acting inwards so their core gets hotter. The better temperatures mean that the nuclear reactions occur at a much greater price in huge stars. They for this reason use up their fuel much faster than reduced mass stars. This is analogous to the instance through many kind of chemical reactions, the higher the temperature the quicker the reaction price.

Lifespans for major sequence stars have actually a vast array. Whilst our Sun will certainly spend 10 billion years on the major sequence, a high-mass, ten solar-mass (10MSun) star will certainly only last 20 million years (2.0× 107 years) on the major sequence. A star via a only half the mass of Sun deserve to spfinish 80 billion years on the main sequence. This is a lot longer than the age of the Universe which implies that all the low-mass stars that have developed are still on the major sequence - they have not had actually time to evolve off it.

Mass/MSunLuminosity/LSunEffective Temperature (K)Radius/RSunMain sequence lifeexpectations (yrs)


Although tbelow are 92 normally emerging aspects and a few hundred isotopes, the composition of stars is remarkably equivalent and straightforward. Stars are created practically totally of hydrogen and also helium. A star such as our Sun is around 73% hydrogen by mass and also 25% helium. If determined by variety of nuclei then it is 92% hydrogen and 7.8% helium. The continuing to be 2% by mass or 0.2% by number is all the heavier facets. Historically astronomers termed these facets through atomic numbers greater than two as metals. These incorporate facets such as carbon and also oxygen. The usage of "metals" is not to be confused with the even more common chemical meaning of the term.

Metallicity is a measure of the abundance of aspects heavier than helium in a star and also is expressed as the fraction of steels by mass. It have the right to be figured out or at leastern inferred from spectroscopic and also photometric monitorings. In basic stars through higher metallicities are inferred to be younger than those with extremely low values. This is because of the fact that facets heavier than helium are made inside stars by nucleosynthesis and also released right into interstellar space by mass-loss events such as supernova explosions in the late stperiods of stellar advancement. Early generations of stars

Stars uncovered in the spiral arms of galaxies, consisting of our Sun, are mainly younger and also have actually high metallicities. They are referred to as Population I stars. Population II stars are older, red stars through reduced metallicities and are frequently located in globular clusters in galactic halos, in elliptical galaxies and near the galactic centre of spiral galaxies.

Nucleosynthesis and also Fusion Reactions

Nucleosynthesis sindicate describes the production of nuclei heavier than hydrogen. This occurs in primary sequence stars with 2 primary processes, the proton-proton chain and the CNO cycle (carbon, nitrogen, oxygen). Primordial nucleosynthesis emerged very early in the background of the Universe, leading to some helium and tiny traces of lithium and deuterium, the heavy isotope of hydrogen. Fusion procedures in post-main sequence stars are responsible for many of the heavier nuclei. Other mechanisms such as neutron capture likewise take place in the last steras of huge stars. Both questioned in later peras.

Main sequence stars fuse hydrogen right into helium within their cores. This is periodically dubbed "hydrogen burning" yet you should be cautious through this term. "Burning" implies a combustion reaction with oxygen yet the procedure within stellar cores is a nuclear reaction, not a chemical one.

The nuclear fusion in the cores of primary sequence stars entails positive hydrogen nuclei, ionised hydrogen atoms or protons, to slam together, releasing energy in the procedure. At each phase of the reaction, the linked mass of the commodities is much less than the full mass of the reactants. This mass distinction is what accounts for the energy released according to Einstein"s well known equation: E = m c2 where E is the power, m the mass and also c the speed of light in a vacuum. This is much better expressed as:

In conditions such as those on Planet, if we attempt to bring two proloads (hydrogen nuclei) together the electrostatic interactivity often tends to reason them to repel. This coulombic repulsion have to be overcome if the proloads are to fusage. The actual procedure whereby 2 protons deserve to fusage requires a quantum mechanical result recognized as tunneling and in exercise requires the protons to have exceptionally high kinetic energies. This suggests that they have to be traveling very quick, that is have exceptionally high temperatures. Nuclear fusion only starts in the cores of stars as soon as the density in the core is great and the temperature reaches about 10 million K.

Tright here are 2 primary procedures by which hydrogen fusion takes area in major sequence stars - the proton-proton chain and also the CNO (for carbon, nitrogen, oxygen) cycle.

Proton-Proton (pp) Chain

The primary procedure responsible for the energy developed in a lot of major sequence stars is the proton-proton (pp) chain. It is the leading procedure in our Sun and also all stars of less than 1.5 solar masses. The net result of the process is that four hydrogen nuclei, prolots, undergo a sequence of fusion reactions to create a helium-4 nucleus. The sequence displayed below is the a lot of prevalent form of this chain and also is likewise dubbed the ppI chain. It accounts for 85% of the fusion energy released in the Sun.


The neutrinos are neutral and have exceptionally low rest masses. They basically do not interact via normal issue and so travel straight out from the core and also escape from the star at almost the speed of light. About 2% of the energy released in the pp chain is lugged by these neutrinos.

Positrons are the antiparticle of electrons. Although the pp chain entails the fusion of hydrogen nuclei, the cores of stars still contain electrons that have been ionised or ripped off from their hydrogen or helium nuclei. When a positron collides with an electron, an antimatter-issue occasion occurs in which each annihilates the various other, releasing yet even more high-energy gamma pholoads.

Two various other develops of the pp chain deserve to occur in stars and contribute around 15% of the energy manufacturing in the Sun. In the ppII chain, a He-3 nucleus created through the first steras of the ppI chain undergoes fusion via a He-4 nucleus, producing Be-7 and also releasing a gamma photon. The Be-7 nucleus then collides with a positron, releasing a neutrino and also creating Li-7. This consequently fuses via a proton, separating to release 2 He-4 nuclei. A rarer occasion is the ppIII chain whereby a Be-7 nucleus produced as above fprovides with a proton to form B-8 and also release a gamma photon. B-8 is unsteady, undergoing beta positive degeneration into Be-8, releasing a positron and a neutrino. Be-8 is likewise unsecure and splits right into 2 He-4 nuclei. This process just contributes 0.02% of the Sun"s power. These forms are summarised as:

CNO Cycle

Stars through a mass of about 1.5 solar masses or more produce many of their energy by a different develop of hydrogen fusion, the CNO cycle. CNO stands for carbon, nitrogen and also oxygen as nuclei of these elements are connected in the process. As its name means, this process is cyclical. It requires a proton to fusage through a C-12 nuclei to begin the cycle. The resultant N-13 nucleus is unsteady and also undergoes beta positive degeneration to C-13. This then fprovides via another proton to from N-14 which consequently fprovides with a proton to provide O-15. Being unstable this undergoes beta positive degeneration to create N-15. When this fuses via a proton, the resultant nucleus instantly splits to develop a He-4 nucleus and also a C-12 nucleus. This carbon nucleus is then able to initiate one more cycle. Carbon-12 thus acts prefer a nuclear catalyst, it is essential for the procedure to continue yet eventually is not used up by it.


Why does the CNO cycle dominate in higher-mass stars? The answer has to carry out via temperature. The first phase of the pp chain requires two proloads fmaking use of together whereas in the CNO cycle, a proton has to fuse through a carbon-12 nucleus. As carbon has six protons the coulombic repulsion is higher for the first action of the CNO cycle than in the pp chain. The nuclei hence require higher kinetic energy to overcome the stronger repulsion. This suggests they need to have actually a greater temperature to initiate a CNO fusion. Higher-mass stars have a stronger gravitational pull in their cores which leads to greater core temperatures.

The CNO cycle becomes the chief resource of power in stars of 1.5 solar masses or higher. Core temperatures in these stars are 18 million K or better. As the Sun"s core temperature is about 16 million K, the CNO cycle accounts for just a minute fractivity of the complete energy released. The loved one power developed by each procedure is shown on the plot listed below.


Calculating the Sun"s Key Sequence Lifespan

As we have actually currently watched, the Sun has a major sequence lifespan of around 10 billion (1 × 1010) years. How execute astronomers calculate such a value? A initially order approximation for this value is surprisingly simple to derive.

You will certainly recall that the mass of a helium-4 nucleus is slightly less than the amount of the 4 separate prolots required to form it. In nuclear physics, the masses dealt with are so little that the atomic mass unit or amu is supplied instead of the kilogram where 1 amu = 1.66 × 10-27 kg. A proton has a mass of 1.0078 amu so four prolots add up to 4.0312 amu. A helium-4 nucleus has actually a mass of 4.0026 which means that the mass defect, the distinction between the 2 full masses, is 0.0286 amu or just 0.7%. From equation 6.2:

E = Δm c2 so substituting in worths givesE = 0.0286(1.66 × 10-27)(3 × 108)2 ∴ E = 4.3 × 10-12 J

The production of each helium nucleus releases just a small amount of energy, 10-12 J which does not seem a lot. We recognize though measurement that the Sun"s luminosity is 3.90 × 1026 J.s-1. To produce this amount of energy, substantial numbers of helium, (3.90 × 1026)/(4.3 × 10-12) = 9 × 1037, should be formed eextremely second. Each second, 600 million tons of hydrogen fusage to form 596 million lots of helium. This means 4 million loads of matter is damaged and also converted into power each second.

The high temperature needed for hydrogen fusion is just found in the core region of the Sun. This comprises only around 10% of its complete mass. The power potentially available from this mass of hydrogen is roughly:

Ecomplete = (mass defect per He nucleus produced) × c2 × (mass of H in core) ∴ Ecomplete = 0.0071(9 × 1016)(0.1 × 2.0 × 1030) = 1.28 × 1044 J

Given that the Sun"s power output is currently 3.90 × 1026 J. s-1 and assuming that it will certainly be around continuous for its main sequence lifespan, then the Sun has actually sufficient core hydrogen for around 10 billion years. As it is presently about about 5 billion years old this implies it is half method via its primary sequence life.

Energy Transport in a Star

We have actually currently viewed just how energy is produced in a star such as the Sun. How, though, does this energy escape from the star? Two processes, radiation and also convection, play a critical function.

The Sun"s interior comprises three primary regions. The core, only 25% of the Sun"s diameter, a radiative zone extfinishing from the core to 70% of the diameter and the external area wbelow convection procedures conquer.

High-energy gamma pholots created in the core carry out not escape quickly from it. The high temperature plasma in the core is about ten times denser than a thick metal on Planet. A photon have the right to only take a trip a centimeter or so on average in the core before connecting via and also scattering from an electron or positive ion. Each of these interactions alters both the power and take a trip direction of the photon. The direction a photon travels after an interactivity is random so occasionally it is reflected ago into the core. Nonetheless over many kind of succeeding interactions the net impact is that the photon slowly renders its method out from the core. The route it takes is referred to as a random walk. Photons shed power to the electrons and also ions via each interactivity developing a variety of photon energies. This procedure is recognized as thermalisation and also outcomes in the characteristic blackbody spectrum that forms the continuum background spectrum of stars.

Interactions between ions and electrons additionally produce many kind of additional pholots of assorted energies. These additionally add to the blackbody spectrum.

The electrons and also nuclei formed in fusion reactions additionally lug kinetic energy that they can imcomponent to other pposts with interactions, increasing the thermal power of the plasma. Neutrinos developed by the various fusion and also decay reactions take a trip out from the core at virtually the rate of light. They are efficiently unimpeded by the dense matter in the core of main sequence stars. They lug away about 2% of the total energy.

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The outer 30% of the Sun is at reduced temperature and also thickness than the inner parts. Here, convection curleas are responsible for transferring power to the surconfront. Deep cells, 30,000 km across are responsible for supergranulation. The cells just below the photospbelow are just 1,000 kilometres throughout and are responsible for the granulation watched on the surconfront of the Sun as in the picture below.