The majority of stars in the galaxy, including our Sun, Sirius and also Alpha Centauri A and also B space all key sequence stars. The Sun"s loved one longevity and also stability have detailed the conditions necessary for life to evolve below on Earth. Our knowledge of the procedures involved and also characteristics of this crucial group the stars has evolved in parallel with our knowledge of nuclear physics.

Properties of key Sequence StarsNucleosynthesis and combination Reactions

Properties of main Sequence Stars

Main sequence stars are qualified by the resource of your energy. They are all undergoing combination of hydrogen into helium within your cores. The price at i m sorry they do this and also the lot of fuel obtainable depends upon the mass of the star. Massive is the vital factor in identify the lifespan of a key sequence star, its size and its luminosity. Stars ~ above the main sequence likewise appear to be unchanging for lengthy periods of time. Any kind of model of together stars must have the ability to account for their stability.

You are watching: The main sequence has a limit at the lower end because

Hydrostatic Equilibrium

The an easy model of any type of main sequence star is of a thick gas/fluid in a state the hydrostatic equilibrium. The inward exhilaration force, gravity, is well balanced by outside acting pressures of gas pressure and the radiation pressure. Apart from the incredibly hot but tenuous corona , the pressure and temperature the stars basically rises as you technique the core.

Main sequence stars basically have a fixed size that is a duty of their mass. The an ext massive the star, the greater its gravitational traction inwards. This in turn compresses the gas more. Together you try and compress a gas it exerts a gas press back, it resists the compression. In stars this gas press alone is not enough to stand up to the gravitational collapse. As soon as the core temperature has reached about 10 million K, combination of hydrogen occurs, publication energy. This power exerts one outwards radiation pressure due to the activity of the photons on the extremely dense matter in the core. The radiation pressure combined with the gas push balances the inward traction of gravity preventing further collapse.

Stellar Mass

As was evident from the evolution Hayashi monitor on the vault page, a star"s position on the main sequence its actually a role of that mass. This is one incredibly valuable relationship, referred to as the mass-luminosity relation. If we know where ~ above the main sequence a star is we have the right to infer that mass. In basic the an ext massive a star is, the further up the main sequence the is found and the an ext luminous that is. Mathematically this relation is express by:

where n is about 4 because that Sun-like stars, 3 for the more massive stars and also 2.5 for dim red key sequence stars. (*Note this formula is not compelled for HSC exams). A 0.1 solar mass star has only about one-thousandth the luminosity the the sunlight whereas a 10-solar fixed star is has a luminosity 10,000 × the of our Sun.



Limits top top the top mass the stars is believed to it is in somewhere between 150 and 200 solar masses based on theoretical modeling. Such stars are incredibly rare and short-lived.

The higher the fixed of a key sequence star, the higher its effective temperature. This, combined with the larger radius of higher mass key sequence stars accounts for their much higher luminosity. Remember, LT4 and LR2 so also a little increase in effective temperature will substantially increase luminosity.

Main-Sequence Lifespan

The main sequence is the phase where a star spends most of the existence. Family member to other stages in a star"s "life" that is extremely long; our sunlight took around 20 million years to form but will spend around 10 billion years (1 × 1010 years) as a key sequence star before evolving right into a red giant. What identify the main sequence lifespan of a star?

Main succession stars vary in mass. You may imagine the a much more massive star has more fuel accessible so have the right to spend an ext time on the main sequence fusing hydrogen come helium. You would certainly be wrong - the opposite is true. Much more massive stars have actually a stronger gravitational force acting inwards for this reason their core gets hotter. The higher temperatures typical that the nuclear reactions happen at a much higher rate in huge stars. They for this reason use up your fuel much quicker than lower mass stars. This is analogous to the situation with many chemical reactions, the greater the temperature the much faster the reaction rate.

Lifespans for main sequence stars have a vast range. Whilst our sunlight will spend 10 billion years on the key sequence, a high-mass, ten solar-mass (10MSun) star will only last 20 million year (2.0× 107 years) on the main sequence. A star through a only half the fixed of Sun can spend 80 billion year on the main sequence. This is much much longer than the age of the universe which way that every the low-mass stars the have developed are tho on the main sequence - they have not had actually time come evolve off it.

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


Although there are 92 naturally developing elements and also a couple of hundred isotopes, the ingredient of stars is remarkably comparable and simple. Stars space composed nearly entirely of hydrogen and also helium. A star such together our sun is around 73% hydrogen by mass and also 25% helium. If figured out by number of nuclei then it is 92% hydrogen and also 7.8% helium. The continuing to be 2% by massive or 0.2% by number is all the more heavier elements. In the history astronomers termed these elements with atomic numbers greater than 2 as metals. These include aspects such as carbon and also oxygen. The usage of "metals" is no to be confused with the much more common chemical meaning of the term.

Metallicity is a measure of the diversity of aspects heavier 보다 helium in a star and is expressed as the portion of steels by mass. It can be identified or at the very least inferred indigenous spectroscopic and also photometric observations. In general stars with higher metallicities are inferred to it is in younger 보다 those with an extremely low values. This is as result of the reality that facets heavier 보다 helium are made within stars by nucleosynthesis and released right into interstellar space by mass-loss occasions such as supernova explosions in the late stages of stellar evolution. At an early stage generations of stars

Stars found in the spiral arms of galaxies, consisting of our Sun, are normally younger and have high metallicities. They are described as population I stars. Population II stars are older, red stars with reduced metallicities and also are commonly located in globular swarm in galactic halos, in elliptical galaxies and near the galactic center of spiral galaxies.

Nucleosynthesis and combination Reactions

Nucleosynthesis merely refers to the manufacturing of nuclei heavier than hydrogen. This occurs in key sequence stars with two key processes, the proton-proton chain and the CNO cycle (carbon, nitrogen, oxygen). Primordial nucleosynthesis occurred an extremely early in the history of the Universe, bring about some helium and small traces the lithium and also deuterium, the heavy isotope the hydrogen. Combination processes in post-main succession stars are responsible for numerous of the heavier nuclei. Various other mechanisms such as neutron capture likewise occur in the last stages of enormous stars. Both discussed in later on pages.

Main succession stars fuse hydrogen right into helium within their cores. This is sometimes called "hydrogen burning" but you need to be mindful with this term. "Burning" suggests a burning reaction v oxygen however the process within stellar cores is a nuclear reaction, no a chemistry one.

The nuclear fusion in the cores of main sequence stars entails positive hydrogen nuclei, ionised hydrogen atom or protons, come slam together, releasing energy in the process. At each phase of the reaction, the merged mass that the products is much less than the full mass of the reactants. This mass distinction is what accounts for the power released follow to Einstein"s well known equation: E = m c2 whereby E is the energy, m the mass and c the rate of light in a vacuum. This is much better expressed as:

In conditions such together those ~ above Earth, if we try to bring two protons (hydrogen nuclei) with each other the electrostatic interaction tends to cause them come repel. This coulombic repulsion have to be get over if the protons space to fuse. The actual process whereby 2 protons deserve to fuse entails a quantum mechanical impact known as tunneling and also in exercise requires the protons to have incredibly high kinetic energies. This method that they must be traveling really fast, that is have very high temperatures. Nuclear blend only start in the cores the stars as soon as the density in the main point is good and the temperature reaches about 10 million K.

There space two key processes whereby hydrogen fusion takes location in key sequence stars - the proton-proton chain and also the CNO (for carbon, nitrogen, oxygen) cycle.

Proton-Proton (pp) Chain

The main procedure responsible because that the energy produced in most key sequence stars is the proton-proton (pp) chain. It is the dominant process in our Sun and all stars of much less than 1.5 solar masses. The net impact of the procedure is that 4 hydrogen nuclei, protons, experience a succession of blend reactions to produce a helium-4 nucleus. The sequence shown listed below is the many common type of this chain and also is additionally called the ppI chain. It accounts because that 85% that the fusion energy released in the Sun.


The neutrinos are neutral and have incredibly low rest masses. They basically do not connect with typical matter and also so travel directly out from the core and also escape from the star at practically the speed of light. About 2% of the energy released in the pp chain is carried by this neutrinos.

Positrons are the antiparticle of electrons. Although the pp chain entails the combination of hydrogen nuclei, the cores of stars quiet contain electrons that have been ionised or ripped off from your hydrogen or helium nuclei. When a positron collides v an electron, one antimatter-matter event occurs in which each annihilates the other, publication yet more high-energy gamma photons.

Two other develops of the pp chain can occur in stars and also contribute around 15% of the energy production in the Sun. In the ppII chain, a He-3 nucleus created via the an initial stages of the ppI chain undergoes combination with a He-4 nucleus, developing Be-7 and also releasing a gamma photon. The Be-7 nucleus then collides through a positron, releasing a neutrino and forming Li-7. This in turn fuses through a proton, splitting to release two He-4 nuclei. A rarer event is the ppIII chain by which a Be-7 nucleus developed as over fuses v a proton to kind B-8 and release a gamma photon. B-8 is unstable, experience beta positive degeneration into Be-8, publication a positron and a neutrino. Be-8 is additionally unstable and splits right into two He-4 nuclei. This process only contributes 0.02% the the Sun"s energy. These forms are summarised as:

CNO Cycle

Stars through a mass of around 1.5 solar masses or much more produce many of their energy by a different type of hydrogen fusion, the CNO cycle. CNO stands for carbon, nitrogen and oxygen together nuclei the these facets are associated in the process. As its surname implies, this procedure is cyclical. It requires a proton to fuse v a C-12 nuclei to begin the cycle. The result N-13 cell nucleus is unstable and also undergoes beta positive decay to C-13. This climate fuses with one more proton come from N-14 which in turn fuses through a proton to provide O-15. Being stormy this experience beta positive decay to form N-15. Once this fuses through a proton, the resultant nucleus instantly splits to type a He-4 nucleus and a C-12 nucleus. This carbon cell nucleus is climate able to initiate an additional cycle. Carbon-12 thus acts like a atom catalyst, that is necessary for the process to proceed yet ultimately is not used up by it.


Why walk the CNO cycle dominate in higher-mass stars? The answer needs to do v temperature. The an initial stage that the pp chain requires two protons fusing with each other whereas in the CNO cycle, a proton has to fuse through a carbon-12 nucleus. Together carbon has actually six protons the coulombic repulsion is better for the very first step of the CNO cycle 보다 in the pp chain. The nuclei therefore require better kinetic power to get over the stronger repulsion. This way they have to have a greater temperature come initiate a CNO fusion. Higher-mass stars have actually a more powerful gravitational traction in your cores which leads to higher core temperatures.

The CNO cycle becomes the chief resource of power in stars that 1.5 solar masses or higher. Main point temperatures in these stars space 18 million K or greater. As the Sun"s main point temperature is around 16 million K, the CNO cycle accounts for only a minute portion of the full energy released. The family member energy developed by each procedure is presented on the plot below.


Calculating the Sun"s key Sequence Lifespan

As we have already seen, the Sun has a key sequence lifespan of around 10 exchange rate (1 × 1010) years. How do astronomers calculate such a value? A an initial order approximation because that this value is surprisingly easy to derive.

You will recall that the fixed of a helium-4 nucleus is slightly less than the sum of the four separate protons essential to kind it. In atom physics, the masses dealt with are so tiny that the atomic mass unit or amu is used rather of the kilogram where 1 amu = 1.66 × 10-27 kg. A proton has a mass of 1.0078 amu so four protons include up to 4.0312 amu. A helium-4 nucleus has a massive of 4.0026 which method that the massive defect, the difference in between the two full masses, is 0.0286 amu or just 0.7%. Indigenous equation 6.2:

E = Δm c2 for this reason substituting in values givesE = 0.0286(1.66 × 10-27)(3 × 108)2 ∴ E = 4.3 × 10-12 J

The production of every helium cell core releases just a small amount of energy, 10-12 J i m sorry does no seem a lot. We understand though measurement the the Sun"s luminosity is 3.90 × 1026 J.s-1. To develop this quantity of energy, large numbers the helium, (3.90 × 1026)/(4.3 × 10-12) = 9 × 1037, need to be created every second. Every second, 600 million lots of hydrogen fuse to type 596 million tons of helium. This means 4 million lots of issue is destroyed and also converted into energy each second.

The high temperature essential for hydrogen combination is only discovered in the core region of the Sun. This comprises only about 10% that its total mass. The power potentially available from this massive of hydrogen is roughly:

Etotal = (mass defect every He cell core produced) × c2 × (mass the H in core) ∴ Etotal = 0.0071(9 × 1016)(0.1 × 2.0 × 1030) = 1.28 × 1044 J

Given the the Sun"s power output is at this time 3.90 × 1026 J. S-1 and assuming the it will be roughly constant for its key sequence lifespan, climate the sunlight has enough core hydrogen for around 10 billion years. Together it is currently about about 5 billion year old this way it is fifty percent way v its main sequence life.

Energy transport in a Star

We have actually now watched how energy is developed in a star such together the Sun. How, though, walk this power escape indigenous the star? 2 processes, radiation and also convection, pat a critical role.

The Sun"s internal comprises three main regions. The core, only 25% of the Sun"s diameter, a radiative zone prolonging from the main point to 70% the the diameter and also the outer region where convection procedures dominate.

High-energy gamma photons created in the core do not escape quickly from it. The high temperature plasma in the core is around ten time denser than a dense metal top top Earth. A photon have the right to only take trip a centimeter or so on mean in the core before interacting with and scattering native an electron or positive ion. Each of these interactions changes both the energy and travel direction the the photon. The direction a photon travel after an interaction is arbitrarily so sometimes it is reflected earlier into the core. However over many successive interactions the net impact is the the photon progressively makes its way out from the core. The course it bring away is called a random walk. Photons lose energy to the electrons and ions through each interaction producing a variety of photon energies. This procedure is recognized as thermalisation and results in the characteristic blackbody spectrum that forms the continually background spectrum of stars.

Interactions between ions and electrons additionally produce many added photons of miscellaneous energies. These also contribute come the blackbody spectrum.

The electrons and also nuclei created in blend reactions additionally carry kinetic power that they deserve to impart to various other particles through interactions, raising the thermal power of the plasma. Neutrinos created by the various blend and degeneration reactions take trip out native the main point at practically the rate of light. Lock are efficiently unimpeded by the dense matter in the main point of main sequence stars. They carry away around 2% that the full energy.

See more: Jimmy Kimmel Books Cubs’ Kris Bryant On Jimmy Kimmel, See His Registries

The external 30% of the sun is at lower temperature and also density 보다 the inside parts. Here, convection currents room responsible for transporting power to the surface. Deep cells, 30,000 km across are responsible because that supergranulation. The cell just listed below the photosphere are just 1,000 km across and space responsible for the granulation seen on the surface of the sunlight as in the image below.