Post-Key Sequence Stars

What happens as soon as a main sequence star runs out of hydrogen in its core? The answers to this take us along the next phase of stellar evolution. Similar to a lot of stperiods in a star"s life, the specific post-main sequence is mainly dependent on its mass. We will certainly begin by looking at what happens to a a one-solar mass star favor our Sun and also then discover what happens to higher-mass stars.

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Evolution of a Solar-Mass Star off the Main Sequence

One-Solar Post-Key Sequence Evolution.

Stars such as our Sun relocate off the main sequence and up the red large branch (RGB), fmaking use of hydrogen right into helium in hydrogen shell burning. A very brief helium flash sees the start of helium core fusion and also the star moves along the horizontal branch (HB). Once shell temperature is sufficient, helium shell burning starts and also the star moves up right into the asymptotic giant branch (AGB).

Moving Off the Key Sequence - Red Giant Branch

A star remains on the major sequence as lengthy as tright here is hydrogen in its core that it can fusage right into helium. So far we have assumed that a star on the main sequence maintains a consistent energy output. In reality, as a primary sequence star eras its luminosity rises slightly, leading to it widening and its outer layer cooling. This describes why the main sequence is a large band also fairly than a narrowhead line - stars move up and to the appropriate on this band also as they age.

At some point the hydrogen fuel in the core runs out and fusion stops, shutting off the exterior radiation press. Inward gravitational attraction causes the helium core to contract, converting gravitational potential energy into thermal energy. Although fusion is no longer occurring in the core, the climb in temperature heats up the shell of hydrogen bordering the core till it is hot enough to start hydrogen fusion, creating more power than once it was a main sequence star. This so-called shell-burning causes some interesting effects.


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The brand-new, boosted radiation pressure actually reasons the outer layers of the star to expand also to maintain the push gradient. As the gas expands it cools, just as a spray can feels colder after usage as the gas has been released. This development and also cooling reasons the effective temperature to drop. Convection transports the power to the outer layers of the star from the shell-burning region. The star"s luminosity eventually increases by a variable of 1000 × or so. During this stage of growth, the star will relocate up and to the best on the HR diagram alengthy the Red Giant Branch (RGB). A G (V)-class star might finish up as a high-K or low-M luminosity class III huge.

A red gigantic screens extremes of thickness. The outer envelope is grossly extfinished and also thus at a thickness below that of a vacuum on Earth. It is only weakly organized by gravitational pressure to the rest of the star and also conveniently ejected. Mass loss from a huge is frequently about 10-7 solar masses per year, compared via just 10-17 solar masses per year currently for the Sun. Whilst the envelope is tenuous and cool, the contracted helium core is incredibly dense. It is just about one-3rd its original dimension. Electrons within the core create a degenerate electron gas, they are packed tightly together in a volume governed just by the uncertainty principle. In this state it no longer behaves as a suitable gas.


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When the Sun becomes a red huge its radius will certainly be around 0.5 AU, that is about 100 × its existing dimension.

Helium "Burning" and also the Helium Flash

Hydrogen fusion in the shell produces more helium. This gets dumped onto the core, adding to its mass, resulting in it to heat up even even more. When the core temperature reaches 100 million K, the helium nuclei now have sufficient kinetic energy to overcome the solid coulombic repulsion and also fuse together, creating carbon-12 in a two-stage process. As three helium nuclei, likewise well-known as alpha particles, are provided it is referred to as the triple alpha procedure. Subsequent fusion via an additional helium nucleus produces oxygen-16 nuclei. This process is the major resource of the carbon and also oxygen uncovered in the Universe, consisting of that in our bodies.


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The triple alpha process for post-main sequence stars. Two helium nuclei (alpha-particles) fuse to develop a beryllium-8 nucleus. This is unstable and generally decays ago right into two H-4e nuclei within a portion of a 2nd but given the high number of He-4 nuclei in the core will sometimes collide with one before it has had a chance to degeneration. This produces a carbon-12 nucleus and releases a gamma photon. The C-12 nucleus subsequently might fusage via one more He-4 nucleus to develop oxygen-16 and a gamma photon. Neon-20 may also be formed by oxygen nuclei fmaking use of via helium but only negligible quantities are created.

In stars through mass less than around 2-3 solar masses the triple alpha process initiates in a issue of minutes or hrs. Once the temperature is hot sufficient for helium fusion in one component of the core, the reactivity quickly spreads throughout it because of the behaviour of the electron degenerate gas. This sudden onset of helium core fusion (or "burning") is dubbed the helium flash.


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The Horizontal Branch

The energy released by the helium flash raises the core temperature to the point wbelow it is no much longer degeneprice. It for this reason starts to behave actually aacquire as an ideal gas so deserve to expand and cool. Energy transfers bring about a hotter external layer of the star however a smaller sized overall size. The increase in reliable temperature and decrease in surchallenge location are such that the luminosity stays approximately constant. Such a star tracks across to the left alengthy the horizontal branch on the HR diagram. HB stars have actually helium core-burning and hydrogen shell-burning.

A solar-mass star has actually enough helium fuel for core-burning to last for about 100 million years.

The Asymptotic Giant Branch

Ultimately all the helium in the core has fsupplied into carbon and also oxygen and so the core contracts aacquire. Carbon and oxygen nuclei have actually more prolots in them than helium does so the coulombic repulsion is better. The temperature required to fuse these into heavier nuclei have to be also greater than the 100 million K required for He fusion. In stars of 8 solar masses or much less there is inadequate gravitational power to generate the temperatures compelled. No more core fusion deserve to thus take place. The core contractivity does but generate sufficient heat for the surrounding layer of helium to start fmaking use of, that is helium shell burning starts. Energy from the helium-burning consequently heats up surrounding unused hydrogen which additionally starts shell burning.

The giant star expands aget, perhaps up to 1.5 AU, tantamount to the orbit of Mars. It is now an asymptotic large branch star (AGB), occupying the upper-ideal percentage of the HR diagram. A one-solar mass AGB might have actually a luminosity 10,000 × that of our present Sun. Mira (ο Ceti) is an instance of an AGB star.


Outer layers of AGB stars are only weakly organized by gravity. The helium-burning shell is not dense sufficient to be degenerate so helium flashes take place through a runameans temperature increase. The resulting increased reaction rate generates a large power release or thermal pulse for a pair of hundred years. Throughout this phase nuclei within the helium-burning shell have the right to be synthesised right into heavier nuclei via the capture of neutrons and radioactive beta decay. This so-dubbed s-process (s for slow, in comparikid via the rapid, r-process that occurs in even more enormous stars) produces facets as hefty as bismuth with 83 protons. AGB stars might produce a thermal pulse eexceptionally 10,000 - 100,000 years.

Large convection currental fees in AGB stars carry product created in the thin helium-burning shell as much as the surconfront. These heavier nuclei are detected in the star"s spectrum which hence gives an insight on what is happening deep within the star. Similar to RGB stars, the radiation pressure tends to blow amethod much of the tenuously-organized external layer. The rate of mass loss is an order of magnitude higher though founding at around 10-6 solar masses per year. As the star evolves up the AGB branch, pulsations increase the price of mass loss up to about 10-4 solar masses per year.

The ejected product comprises a mixture of aspects including carbon and also oxygen dredged up from within the star. Carbon-wealthy molecules develop dust and also soot pposts that tend to shroud the actual star. As the cloud increases it cools yet the dust absorbs and also re-emits the radiation from the star at longer wavelengths. AGBs are for this reason regularly more luminous in the infrared than visible wavebands. The expanding cloud deserve to additionally be observed at radio wavebands.

AGBs such as Mira are intrinsic variable stars through durations of months or a couple of years. They can vary by approximately 10 magnitudes. Mira"s radius differs by a variable of two throughout its oscillations.

What happens to solar-mass stars once fusion is completed is disputed on the following web page.

High-Mass Post-Key Sequence Evolution.

Evolution of high-mass stars off the main sequence is an affiliated process and one still not completely construed. Such stars are rare and also have extremely brief lifespans relative to lower-mass stars. Supergiants such as Betelgeusage, Deneb, Rigel and Antares are some of the the majority of prominent stars in our skies and visible over huge ranges because of their excessive luminosities. This area gives a simple outline of the stages.

High-mass stars consume their core hydrogen at prodigious prices so may only endure on the primary sequence for millions quite than billions of years. Once this fuel is offered up, the core contracts as a result of gravity and heats up. This triggers helium-burning in the core. Unfavor lower-mass stars, this helium fusion (triple-alpha process) starts gradually rather than in a helium flash. In moving off the primary sequence, the effective temperature of the star drops as its external layers expand. The decrease in temperature balances the raised radius so that the as a whole luminosity remains basically constant. Evolutionary tracks for these massive stars for this reason move horizontally throughout the supergigantic region of the HR diagram as shown on the diagram over. The energy liberated by helium fusion in the core raises the temperature of the bordering hydrogen shell so that it also begins fmaking use of.

The size of these supergiants is enormous. Betelgeuse, thmust be between 13 and 17 solar masses, is so big that its envelope would certainly extfinish past the orlittle of Jupiter if it replaced our Sun. Its angular dimension is so huge that it can be directly imaged by the HST.


Betelgeuse is a red supergiant. The bappropriate yellow spot at the bottom of the star is thought to be a hotspot because of a huge convection cell.

In stars of 5 solar masses or higher, radiation press quite than gas press is the leading force in withstanding collapse. The mass is big enough that the gravity acting on the core after helium-burning is sufficient to create temperatures of 3 × 108 K where fusion of carbon through helium to create oxygen dominates. A star of 8 solar masses or even more can go on to synthesise also heavier elements in the core.

Gravitational core contraction after all the core helium is provided up geneprices a temperature of around 5 × 108 K at which point carbon nuclei fuse together to develop sodium, neon and magnesium. Production of magnesium releases a gamma photon, that of sodium releases a proton and also neon produces a helium nucleus. Once all the core carbon is consumed, even more collapse pushes temperatures to about 109 K. At this temperature, reactions that release gamma photons, such as 16O + 4He → 20Ne + γ, may be reversed by a procedure dubbed photodisintegration. Helium nuclei released using this process have the right to fusage through other neon nuclei to produce magnesium.

Once the neon is used up, core contractivity rises the temperature to 2 × 109 K where two oxygen nuclei fuse to form silsymbol. This in turn may undergo photofragmentation to develop magnesium and also helium nuclei that then fuse with other silsymbol nuclei to create sulfur. Comparable stages of reactions watch sulhair produce argon and also argon synthesise calcium. Eventually aspects such as chromium, manganese, iron, cobalt and nickel might be created. Ultimately the silicon in the core is converted, as silicon-burning, into iron with final core temperature getting to around 7 × 109 K. The core area of a superhuge thus resembles the layers of an onion via a thick iron core bordering by shells of silsymbol and also sulfur, oxygen and carbon, helium and also an outer shell of hydrogen as shown in the diagram listed below.


The onion-prefer layers inside a superlarge in the final stperiods of its life. Successive layers correspond to the various elements developed by fusion, through a dense core of iron at the centre.


Nucleosynthesis of aspects above helium is less efficient so that each successive reactivity produces much less power per unit mass of fuel. This means that the reactions occur at greater rates so that radiation push balances gravity. Whilst a enormous star might spfinish a few million years on the major sequence, its helium core-burning phase might be a couple of hundred thousand years. The carbon burning phase lasts a few hundred years, neon-burning phase a year, oxygen-burning fifty percent a year and also the silicon-burning just a day.

These substantial stars evolve extremely rapidly as soon as they relocate off the primary sequence. Statistically they are extremely low in numbers as they are less likely to form than lower-mass stars and their lifetimes are so brief anymeans. As we shall check out in a later section, they also make dramatic exits.

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Low-Mass Post-Main Sequence Evolution

As disputed formerly, low mass stars consume their core hydrogen at much reduced prices than stars such as our Sun. Their main sequence lifespans are tens to thousands of billions of years. Once they have consumed their core hydrogen, gravitational core collapse reasons the core to heat up. For stars via less than 0.5 solar masses but, their is inadequate mass to generate the temperatures require for the helium in the core to begin fusing. A brief period of hydrogen-shell burning sees its luminosity climb as with higher-mass stars. Unable to release energy from helium fusion, H-shell burning does not last lengthy. The star"s luminosity conveniently decreases and also the star cools dvery own. Its evolutionary track crosses ago throughout the primary sequence and also dvery own.