Nucleosinteza stelaraNucleosinteza stelara este termenul colectiv pentru reactiile nucleare ce au loc in stele, pentru a construi nuclei ai elementelor mai grele decat hidrogenul. Procesele implicate au inceput sa fie intelese la inceputul sec. XX, cand s-a descoperit ca energia eliberata de reactiile nucleare era responsabila pentru longevitatea soarelui ca o sursa de caldra si lumina. Principala sursa producatoare de energie a soarelui o constitue fuziunea hidrogenului in heliu, care are loc la o temperatura minima de 3.000.000 K. In 1920, Arthur Eddington, pe bazele masuratorilor precise ale atomilor de F.W.Aston Francis William Aston a fost primul care sa sugereze ca stelele isi obtineau energia prin fuziunea nucleara. In 1928, George Gamow, a dat factorul Gamow, o formula cuantica-mecanica, ce dadea probabilitatea aducerii unor doi nuclei suficient de aproape pentru ca o forta nucleara suficient de mare ca sa depeasca bariera Coulomb. In 1939, intr-o lucrare intitulata “Productia energiei in stele”, Hans Bethe a analizat posibilitatile diferite pentru reactii in care hidrogenul este transformat in heliu. El a selectat doua procese care le-a crezut a fi sursa de energie in stele:- Lantul proton –proton- Ciclul Carbon-Nitrogen-OxigenReactii importanteArderea Hidrogenului- lantul proton-proton- ciclul Carbon-Nitrogen-OxigenArderea Heliului- Procesul triplu-alpha- Procesul alpha Arderea elementelor mai grele- Carbon- Neon- Oxigen - SiliconProductia elementelor mai grele decat Fe.Dezintegrarea Procesul PCaptura ProtonilorProcesul RpCaptura NeutronilorProcesul RProcesul SReacţiile termo-nucleare, care poartă numele de reacţii de fuziune în fizica nucleară, sunt cele care fac stelele să producă energie. În cele mai frecvente cazuri aceasta se găseşte sub formă de lumină, căldură şi radiaţii (de diferite frecvenţe).Reacţiile de fuziune din interiorul stelelor, nu pot fi reproduse decât în anumite condiţii, prezente numai pe corpurile cereşti de masă foarte ridicată: temperaturi de câteva milioane de grade Celsius şi presiuni

foarte mari (cauzate de forţa gravitaţională).Reacţiile termo-nucleare constau în fuziunea a două sau mai multe nuclee de hidrogen pentru a forma un nucleu de heliu, sau unul mai complex. În urma fuziunii, 0.7 la sută din masa hidrogenului este convertită în energie, ceea ce semnifică foarte mult.În stelele de masă scăzută, asemănătoare Soarelui, în urma proceselor de fuziune rezultă materiale relativ simple, cu un număr atomic mic, cum ar fi heliu(2) sau litiu (3).În urma fuziunii din stele cu o masă ridicată rezultă însă materiale mult mai complexe, cu un număr atomic mare.Fie mari sau mici, stelele care au o masă suficientă pentru realizarea reacţiilor termo-nucleare; pot „arde” un timp foarte îndelungat, deoarece la început toate sunt compuse în cea mai mare parte din hidrogen.Oameni de ştiinţă încearcă în continuare să realizeze procesul de „fuziune la rece”, o reacţie asemănătoare celei din stele, care să se poată realiza la temperaturi mai scăzute. Folosind un astfel de procedeu, apa ar putea devenii combustibilul de bază al planetei, un litru de apă echivalând aproximativ 500 de litri de benzină.

NucleosynthesisNucleosynthesis in the NewsNucleosynthesis Activities

A star's energy comes from the combining of light elements into heavier elements in a process known as fusion, or "nuclear burning". It is generally believed that most of the elements in the universe heavier than helium are created, or synthesized, in stars when lighter nuclei fuse to make heavier nuclei. The process is called nucleosynthesis.

Nucleosynthesis requires a high-speed collision, which can only be achieved with very high temperature. The minimum temperature required for the fusion of hydrogen is 5 million degrees. Elements with more protons in their nuclei require still higher temperatures. For instance, fusing carbon requires a temperature of about one billion degrees! Most of the heavy elements, from oxygen up through iron, are thought to be produced in stars that contain at least ten times as much matter as our Sun.

Our Sun is currently burning, or fusing, hydrogen to helium. This is the process that occurs during most of a star's lifetime. After the hydrogen in the star's core is exhausted, the star can burn helium to form progressively heavier elements, carbon and oxygen and so on, until iron and nickel are formed. Up to this point the process releases energy. The formation of elements heavier than iron and nickel requires the input of energy. Supernova explosions result when the

cores of massive stars have exhausted their fuel supplies and burned everything into iron and nickel. The nuclei with mass heavier than nickel are thought to be formed during these explosions.

More about nucleosynthesis...

Ker Than

for National Geographic News

Published January 28, 2010

Using the most powerful laser system ever built, scientists have brought us one step closer to nuclear fusion power, a new study says.

The same process that powers our sun and other stars, nuclear fusion has the potential to be an efficient, carbon-free energy source—with none of the radioactive waste associated with the nuclear fission method used in current nuclear plants.

(See "Radioactive Rabbit Droppings Help Spur Nuclear Cleanup.")

Thanks to the new achievement, a prototype nuclear fusion power plant could be operating within a decade, speculated study leader Siegfried Glenzer, a physicist at Lawrence Livermore National Laboratory in California.

Glenzer and colleagues used the world's largest laser array—the Livermore lab's National Ignition Facility—to heat a BB-size fuel pellet to millions of degrees Fahrenheit.

"These lasers are pulsed, and for a very short amount of time"—one ten-billionth of a second—"the power they produce is more than all the power generated by the entire electrical grid of the United States" at any given moment, Glenzer said.

The test confirmed that a technique called inertial fusion ignition could be used to trigger nuclear fusion—the merging of the nuclei of two atoms of, say, hydrogen—which can result in a tremendous amount of excess energy. Nuclear fission, by contrast, involves the splitting of atoms.

The laser demonstration means scientists are now much closer to triggering nuclear fusion in a controlled setting—something that's never been done before and which is necessary if fusion is to be harnessed for energy.

Nuclear's Nice Side?

Performing nuclear fusion in the lab requires enormous amounts of laser power, but if perfected, controlled fusion should generate ten to a hundred times more electrical energy than is used to spark the nuclear reactions. Nuclear fusion, after all, is what allows stars to burn for billions of years.

And fusion could be not only powerful but clean and green as well.

Not only does nuclear fusion not produce long-lasting nuclear waste, but fusion could potentially be used to chemically neutralize radioactive pollutants and has been "proposed as a cure to our nuclear waste problem," Glenzer said. Simply put, neutrons released by fusion could rearrange radioactive atoms so they aren't radioactive anymore.

(Related: "'Nuclear Archaeologists' Find World War II Plutonium.")

Nuclear fusion energy is also potentially carbon free, meaning it could be used to generate power without creating any more carbon dioxide gas, which contributes to global warming.

And while fossil fuels, such as oil and coal, and nuclear fission fuels, such as uranium, are limited resources, there's enough nuclear fusion fuel on, in, and around our planet "to power the Earth longer than the lifetime of the sun," Glenzer said.

(Related: "Cheap Oil to Last, 'Doomsday' Fears Overblown, Author Says.")

Gold Fusion

During the laser experiment, the fuel pellet was placed inside a solid-gold cylinder about the size of a pencil eraser, which was hit by multiple laser beams.

The gold cylinder absorbed the laser energy and converted it into thermal x-ray energy.

The x-rays then ricocheted inside the cylinder and struck the fuel pellet from all sides. As the pellet absorbed the x-rays, it heated up—eventually reaching about 60 million degrees Fahrenheit (33 million degrees Celsius)—then collapsed in on itself.

The experiment was designed only to test the lasers' ability to heat the cylinder efficiently. Made largely of plastics and helium, the fuel pellet was not filled with enough actual fuel—chemical variants of hydrogen called deuterium and tritium—to actually trigger nuclear fusion.

Actual fusion, Glenzer said, will occur sometime this year.

With a fully loaded fuel pellet, "the implosion will be like squeezing a soccer ball to the size of a pinhead," he added. "The center of that spherical ball will get so hot that nuclear fusion starts."

Nuclear Fusion Plant by 2020?

If successful, the upcoming nuclear fusion experiment will create two classes of energetic particles: alpha particles and neutrons.

"The neutrons escape and can be used to do things like heat up water"—which could potentially be used to produce steam to drive turbines in an electrical plant, Glenzer said.

"The alpha particles remain trapped [in the burning sphere] and continue to heat the fuel and make it burn," as happens in a star.

Scientists estimate that if they can get to the point where they can burn about five fuel pellets a second, a power plant could continuously generate up to a gigawatt of energy—about what the city of San Francisco is consuming at any given moment.

A working prototype of a such a plant could be built in a decade, Glenzer said.

Cheaper to Burn Cash?

Nuclear fusion researcher Michael Mauel is "very excited" about the recent experiment and said it shows the ignition method works as expected.

But "whether or not we'll have lasers imploding pellets to make fusion energy—it's way too early to tell," said Mauel, who was not involved in the study, which will be published in the journal Science tomorrow.

In addition to the considerable engineering challenges involved in ramping up the laser systems for wide-scale use, the cost of the fuel pellets will also have to come down, said Mauel, a Columbia University physicist.

"Each one of these costs between ten [thousand] and a hundred thousand dollars," Mauel said. To use the pellet method to generate nuclear fusion power, "they'll have to cost less than ten cents a piece."

Educational Brief

Subject: ACE Mission Topic: Cosmology and Stellar Evolution

http://oposite.stsci.edu/pubinfo/pr/96/22/A.jpg2

ACE MISSION The ACE (Advanced Composition Explorer) spacecraft was launched in August of 1997 to make observations that are being used to test current theories on the creation and evolution of the galaxy. The purpose of the ACE spacecraft is to sample the matter that comes near Earth from the Sun, the space between the planets, and the Milky Way galaxy beyond the solar system.

BIG BANGACE instruments are being used to verify modern theories concerning the origin of the universe. Observations of light coming from distant galaxies show a shift in the spectrum toward the lower frequencies (Doppler effect). This phenomenon is called the red shift and has generally been the evidence cited by astronomers when they suggest that the universe was once much closer together than it is now. This evidence indicates that the universe began ten to fifteen billion years

is currently at this stage in its evolution and is fusing hydrogen to create helium .

The nuclear fuel (of light elements) which feeds the fusion process eventually runs out, and the stars end the first chapter in their lives. When the fuel runs out, the outward force which balances the gravitational attraction decreases. At that time gravitational forces again pull the outer layers of the star together, and the same processes that started the fusion process early in the stars’ history begin again. This time the star expands even more than before, and it becomes a red giant. The star may expand to one hundred times its original equilibrium size. The amount of time between a star’s birth and its red giant stage is dependent upon the original mass of the star.

The next step also depends upon the original mass of the star. The Sun-like stars (stars having a mass

ago with a tremendous explosion they call the Big Bang.

Another important piece of evidence supporting the Big Bang was the discovery of cosmic microwave background radiation (CMB) in the 1960s by Arno Penzias and Robert Wilson. They discovered microwave radiation coming from all directions in the universe with equal intensity. They concluded that the radiation came from beyond our galaxy, from the universe as a whole.

A third standard used to establish the validity of the Big Bang hypothesis is the abundance of light elements in the universe. This standard is based on pioneering efforts by George Gamow and his collaborators. Their theory makes predictions about the density of baryons (neutrons and protons) and light elements three minutes after the Big Bang. ACE data is being used to test theories on how these elements were created and how they have evolved, and to give scientists a clearer picture of their abundance in our galaxy.

FUSION AND NUCLEOSYNTHESISThe elements we see all around us in the universe were created by nucleosynthesis. In this process, nuclear fusion occurs in stars. As fusion proceeds, lighter elements combine to produce heavier elements. Fusion also generates large amounts of energy (such as visible light) as matter is converted into energy. Einstein’s famous E = mc2 equation can be used to calculate the amount of energy released when a given mass is converted. The min the equation represents the nuclear mass defect. This mass defect is the difference between the mass of the stable nucleus that was produced during the process and the sum of the masses of its parent particles.

According to the theory, the light elements

approximately one-half to about three and one-half times the mass of the Sun) eventually deplete their nuclear mass to about 20% of what they had at birth. At that time they shrink to a white dwarf. This occurs as the inward pull of gravity again wins out over the decreasing outward pressure from the fusion process. These stars eventually cool and become cold and dark. They are often called black dwarfs.

Stars that began their history with masses ten or more times that of our Sun have quite a different fate. Similar to the Sun-like stars, these stars go through the red giant phase (they are called red supergiants) and then shrink to create forces which restart their nuclear furnace. But in this case the larger mass creates a larger gravitational pull and a larger number of internal collisions. The combined effect results in tremendously high temperatures capable of creating heavier atomic nuclei through fusion. The core eventually becomes mostly iron. Since the nuclear structure of iron does not allow its fusion to heavier elements (that would require the input of energy), fusion will cease. With the outward fusion pressure gone, the star goes through a rapid gravitational collapse (this is thought to occur in less than a second) and the temperature rises to 100 billion degrees. Since the nucleons (protons and neutrons) present in the star are being forced very close together, they create a tremendous repulsion of the positively charged nuclei from one another. This repulsion causes the star’s core to recoil into an unbelievable explosion. Scientists call this explosion a supernova. The fragments released from this supernova spew out in space to eventually form new stars, planets, and other celestial bodies. The nuclei of isotopes with masses heavier than nickel, created by this and previous supernovae, are thought to be distributed through the universe during these supernovae.

(hydrogen, helium, and lithium) were produced during the first few minutes. During the first three minutes after the Big Bang, the universe cooled from 10 E 32 K to 10 E 9 K. After this cooling took place, protons and neutrons that formed right after the Big Bang collided to produce deuterium (one proton combined with one neutron). The deuterium then either combined with an additional proton to create helium-3 and energy (gamma rays, which are high frequency electromagnetic radiation) or it combined with another deuterium to create helium-3 and a free neutron. A tritium (hydrogen-3, the heaviest isotope of hydrogen) and a free neutron are formed from the combination of two deuterium nuclei. The helium-3 would have combined with deuterium to form helium-4 and a proton, while the tritium would have combined with a deuterium to form helium-4 and a free neutron. The lithium -7 isotope could also have been created from the combination of a helium-3 and a helium-4.

This theory is collectively called Big Bang nucleosynthesis and makes the prediction that approximately 25% of the mass of the primordial universe (shortly after Big Bang nucleosynthesis) should have been helium, mainly helium-4. The abundance of helium-4 is slowly increasing as the universe ages due to its production in stars. ACE instruments are collecting data to test scientists’ theories concerning the abundance of helium isotopes and other elements.

Current theory suggests that the production of the heavier elements (from oxygen up through iron) only occurs on stars where temperatures are very much hotter than our Sun. These stars have temperatures above one billion degrees because they contain masses at least ten times that of our Sun.

Some of the remaining cores of these super giants (only the very largest) can form neutron stars. This can happen if the intense pressure from the gravitational attraction forces electrons to combine with nearby protons, thus forming neutrons. The other possibility for the heaviest cores is that the largest of the large may collapse with so much gravitational attraction that even light cannot escape. These massive collapsed stars are called black holes.

The ACE mission is important because of the ability of its instruments to study the origin and evolution of the elements. ACE detects many of the heavier isotopes which originated during the formation, evolution, and subsequent explosion of stars. The comparative number of different isotopes found in the galaxy is thought to be related to the life cycle of the massive stars.

It is important here to keep in mind that the material that the Earth and the rest of the solar system are made of has been changed and rearranged throughout the billions of years since their creation, so measuring its complete composition is difficult. ACE is measuring cosmic ray particles to draw comparisons between matter from the solar system (Sun, meteorites, planets, atmospheres of planets, the moon), comets, the local interstellar medium, and the galaxy. It is hoped that these comparisons will help answer some of the questions which astrophysicists have about the creation and evolution of the galaxy.

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STELLAR EVOLUTION AND ACEThe currently accepted theory of stellar evolution involves the following sequence of events. After the Big Bang, gravitation pulled together clouds of gas and dust to create giant clusters of matter. Continued contraction of these clusters eventually increased their temperature due to the interaction of colliding particles and the pressures created by the large gravitational attraction. As the temperature approached 15 million degrees, the electrons in the atoms were ripped off to create a plasma. (A plasma is the state of matter present when some or all an element's electrons have been separated from their nuclei.) Continued contraction occurred until the particles in the plasma moved with such high velocities (and therefore high energies and temperatures) that they began to fuse. The energy that was released eventually reached the surface of the cluster of matter, was released into space, and stars were born! It is believed that this fusion process produces enough energy to generate an outward pressure in the stars that reaches equilibrium with the inward pull of gravity. Our Sun

ACE home pageCosmicopia

CREDITS: Daniel Hortert GESSEP Program Pat Keeney GESSEP Program

Dr. Eric Christian

ACE Deputy Project Scientist

Dr. John Krizmanic

Astroparticle Physicist

Beth Barbier ACE Outreach Specialist

Nucleosynthesis

Introduction: As the main sequence fusion cycles (proton-proton and CNO)

transform more and more hydrogen to helium, one of the most likely possibilities of fusion would involve two 4He nuclei fusing to create a nucleus with an atomic mass of 8. However, there are no stable isotopes of any element with an atomic mass of 8. 8Be in particular has a lifetime of only 10-17 seconds! At the temperatures in which the proton-proton and CNO cycle occurs, 8Be will break apart before it is involved in any further fusion reactions. This has become known as the beryllium bottleneck, because it is 8Be's instability that prevents the heavier elements from being formed relatively immediately as helium is created.

The Triple-Alpha ProcessWhen a large enough amount of hydrogen

Red Giant: Betelgeuse

Original image courtesy of NASA.

is converted to helium within the core (in our sun, about 10% of its mass), the core may begin to collapse on itself, increasing the density and temperature. When the temperature rises above 100 million K, helium nuclei may be converted to carbon (12C) through a very high, and extremely improbable, energy reaction called the triple-alpha process (remember an alpha particle is really just a helium nucleus). This is because the temperatures are high enough to fuse two 4He into the extremely unstable 8Be at a large enough rate so that there is always a small amount of 8Be. In the short amount of time that a 8Be nucleus exists, it may fuse w ith another 4He producing an "excited" carbon isotope with an atomic mass of 12. These carbon nuclei in their "excited" state are unstable, but they may release a gamma ray before breaking apart, thus becoming the stable 12C nucleus. This usually begins occuring during the red giant phase of a star (you will learn more about that later), at which point the hydrogen fuel in the core has been used up, and the temperature rises enough to trigger the triple-alpha process.

Further Element FormationAfter that, atoms of even higher

mass may be created from the fusion of carbon with other

Supernova 1987A

Original image courtesy of NASA.

nucleons. For example:

13C + 4He 16O + neutron (n) 17O + 4He 20Ne + n 21Ne + 4He 24Mg + n

This process of creating the heavier elements is called nucleosynthesis. Elements up to iron may be created in this fashion as well as through a variety of other fusion reactions. Elements heavier than iron are formed through neutron capture, because the fusion of iron with other elements must absorb energy, rather than release it. This situation of neutron capture occurs during a supernova (more on this later), creating up to the heaviest of natural elements.

Nucleosynthesis of The Elements

This page is concerned with where the chemical elements come from, how the atomic nuclei are forged. It is a long story, largely deduced in the second half of the twentieth century, that ultimately and rather romantically says: We Are Stardust.

The Start

Current thinking is that the the universe erupted from the cauldron of the Big Bang some 13.7 billion years ago, as described on this Wikipedia timeline page.

The crucial period of baryionic matter formation (protons, neutrons, atoms) – the so called epoch of Big Bang Nucleosynthesis or BBN – lasted for only about seventeen minutes, from 3 to about 20 minutes from the beginning itself. As far as chemists are concerned, little else happened for several hundred thousand years after this crucial epoch.

During this 17 minute time period:

The quark soup cooled to an ionised plasma of photons, electrons, positrons, neutrinos, protons and neutrons.

Initially the temperature was so high that the protons and electrons combined into neutrons:                p  +  e       nEquilibrium meant that both protons and neutrons were present in large numbers.

The universe expanded and cooled to ~1010 Kelvin. At this temperature the nuclear chemistry changed and no more neutrons were formed. Free neutrons have a half life of 617 seconds and once they stopped being made their numbers, relative to the stable protons, started to decline.

When the universe had cooled to ~109 Kelvin there were 164 neutrons to every 1000 protons. At this lower temperature neutrons are able to combine/react with protons to form deuterium nuclei, 2H. In this bound state neutrons are stable to decay.

In nuclear chemistry terms, deuterium nuclei, 2H, are very reactive. For several minutes the deuterium nuclei, 2H, reacted by a variety of nuclear reactions to give a mixture of isotopes: 3He, 4He, 7Li, along with the primordial 1H and 2H.

The ratios of 1H, 2H, 3He, 4He and 7Li in the early universe can be measured, with considerable difficulty, and the numbers constrain the mass,

temperature and density conditions at this epoch.

The nuclear chemistry described above is confirmed by high energy physics experiments at CERN, the Stanford Linear Accelerator and a few similar establishments that can reproduce the conditions seconds after the Big Bang, albeit on a small scale. This science is part of the standard model of contemporary physics.

The plasma of the expanding universe continued to expand outwards, to cool to undergo any further nuclear chemistry.

After about 300,000 years the process of 'recombination' occurred. The expanding universe had been an optically opaque plasma of photons, free electrons and 1H, 2H, 3He, 4He & 7Li nuclei. But when the temperature fell to about 3000°, the electrons were able to combine with the atomic nuclei to form neutral atoms, and as a result the universe became optically clear. This thermal energy associated with recombination is origin of the cosmic background radiation.

Stellar Nucleosynthesis

This would be the end of the story, except that the rapidly expanding universe had a built in brake, gravity, which operated both globally and locally. The implications of gravity for the entire universe are still the subject of debate, but local effects are better understood. After about 100 million years gravity caused – and still causes – matter to collapse into bodies that become hot and light up the dark sky as stars.

Stars are hot and dense enough to burn hydrogen, 1H, to helium, 4He. There are several nuclear synthetic routes and various nuclei are formed as by-products, including:

13N,   13C,   14N,   15O,   15N,   12C,   16O, 17F   &   8Be

although these nuclei are either radioactive or are quickly consumed in the stellar furnace.

Stars evolve so that they have onion-skin like shells of

thermonuclear combustion with differing nuclear chemistry. The exact structure depends on the mass of the star.

For large stars, moving inwards:

The temperature in the stellar interior increases and more nuclear synthetic pathways become available producing:

20Ne,   23Na,   23Mg,   24Mg,   28Si,   31P,   31S,   32S,   & all the way up to   56Fe

Discussion

big bang nucleosynthesis

By the first millisecond, the universe had cooled to a few trillion kelvins (1012 K) and quarks finally had the opportunity to bind together into free protons and neutrons. Free neutrons are unstable with a half-life of about ten minutes (614.8 s) and

formed in much smaller numbers. The abundance ratio was about seven protons for every neutron. Before one neutron half-life passed nearly every neutron had paired up with a proton, and nearly every one of these pairs had paired up to form helium. By this time the universe had cooled to a few billion kelvins (109 K) and the rate of nucleosynthesis had slowed down significantly. By the time the universe was three minutes old the process had basically stopped and the relative abundances of the elements was fixed at ratios that didn't change for very long time: 75% hydrogen, 25% helium, with trace amounts of deuterium (hydrogen-2), helium-3, and lithium-7. Big Bang nucleosynthesis produced no elements heavier than lithium. To do that you need stars, which means waiting around for at least 200 billion years.

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we are all made of stars

More than ninety per cent of the universe is composed of hydrogen and helium. Both elements have been around since shortly after the beginning of the universe. Yet, hydrogen and helium together won't make anything as complex and as interesting as the earth, or a bacterium, or a refrigerator, or you and I. To do that we need carbon and oxygen and nitrogen and silicon and chlorine and every other naturally occurring element. Almost all the hydrogen and helium present in the universe today (and some of the lithium) were created in the first three minutes after the big bang. All of the other naturally occurring elements were created in stars.

Stars like the sun

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Details were discussed in the section on Fusion. The basic parts of the reaction are …

2(11H + 1

1H → 2

1H + 0+1e + 0

0ν) 0.4 MeV + 1.0 MeV

2(11H + 2

1H → 3

2He + 00γ ) 5.5 MeV

32He + 3

2He → 4

2He + 211H 12.9 MeV

Which overall yields …

4(11H) → 4

2He + 2(0+1e + 0

0γ + 00ν) 26.7 MeV

Stars heavier than the sun use carbon-12 as a catalyst.

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You need really massive stars for this — say 20 to 120 times the mass of the sun.

Really, really heavy stars do something different.

The Mass-5 and Mass-8 Bottlenecks. There are no stable isotopes (of any element) having atomic masses 5 or 8. But there is always a very small amount of beryllium-8 at any moment that is available to fuse with a third helium to produce carbon-12. This extremely improbable sequence is called the triple-alpha process because the net effect is to combine 3 alpha particles to form a carbon-12 nucleus. The triple-alpha process is not relevant in main sequence (normal) stars like the sun because their core temperatures are too low. However, in the red giant phase, after many stars have accumulated vast amounts of helium in their core, the central temperature can rise high enough (108 K) to initiate the triple-alpha process.

42He + 4

2 He + 92 keV

→ 8

4Be*

42He + 8

4Be* + 67 keV

→ 12

6C*

126C* →

126C + 0

0γ + 7.4 MeV

Overall

3(42He) → 12

6C + 00γ + 7.4 MeV

[magnify]

In order of increasing alpha number, the following forms of fusion take place …

Weak interaction freeze-outAt high temperature, the matter in the Big Bang consisted only of its most elementary constituents. When the temperature dropped below a few hundred MeV, ordinary nucleons (or baryons) could form: these are protons and neutrons since no heavier nuclei would have survived the high temperatures. In addition there are the light particles (leptons), such as electrons, neutrinos and photons. Neutrons and neutrinos interact with electrons and protons by means of the weak nuclear interaction. This is the interaction that is responsible for radioactive decays of unstable isotopes. When the temperature of the universe drops below about 1 MeV (or 10^{10}K), the weak interaction rate becomes slower than the rate of expansion of the universe. At this stage, about 1 second has elapsed of cosmic time since the Big Bang. Once the weak interaction have effectively halted, the residual number of neutrons (and neutrinos) is fixed. There is approximately one neutron remaining for every ten protons.

Primordial nucleosynthesisThe lifetime of a free neutron to decay is about ten minutes. However most neutrons do not have time to decay. After only about three minutes have elapsed, something else occurs. Neutrons interact with protons to form nuclei of deuterium, or heavy hydrogen. The deuterium soon gains another neutron to form tritium, which in turn rapidly absorbs a proton to form a helium nucleus of mass 4, consisting of two protons and two neutrons. There is no stable element of mass 5, nor of mass 8, so additional nucleosynthesis via He + p or He + He is generally not possible although trace amounts of one or two heavier elements, most notably lithium (of mass 7) do form. One finds that practically every neutron ends up in a helium nucleus. The Big Bang therefore predicts that there should be one helium nucleus for every ten protons, created in the first three minutes of the expansion. Approximately 25 percent by mass of the matter in the universe is now in the form of helium nuclei: the rest consists of protons. For the Sun helium is about 30%, since some of the hydrogen has already been processed through stars (including the Sun itself!), ie the solar material is not "primordial".

A Helium abundance of about 1/4 by mass turns out to be a

robust prediction of the Big Bang theory, and depends only on the fact that the very early universe passed through a high temperature, high density phase, much like the center of a star. This abundance is in fact just what we observe when we look at material which we believe to be close to primordial. Other important predictions include small amounts of deuterium and lithium, although the final abundances of these elements, deuterium especially, depend on the precise value of Omega_b. If the density of ordinary matter (baryons) is high, the early nucleosynthesis is efficient, and one makes essentially no deuterium. If the baryon density is low, however, one makes an amount of deuterium that is comparable to what is observed by astronomers in the absorption spectra of distant quasars.

HeliumHelium is synthesized inside stars by thermonuclear fusion. However, most stars, like the sun, are still burning hydrogen and so have made little helium, and certainly dispersed none of it. The synthesized helium is deep inside the stellar interior. Yet the universe indeed is observed to contain one helium atom for every ten atoms of hydrogen: by mass, it is about 25 percent helium. This is close to the case for the sun, it is as observed in solar cosmic rays, for interstellar gas in HII regions, and for hot stars, where the helium emission lines are excited. Moreover, when we compare stars which are metal-rich with metal-poor stars, one finds essentially the same helium abundance. There are metal-deficient galaxies which contain almost the same helium abundance. This confirms that helium has mostly not been synthesized along with the heavier elements, such as the metals, but was made prior to the formation of the first stars. The coincidence between observation and prediction of the helium abundance in the universe provides one of the major pieces of evidence for the Big Bang theory.

Deuterium and the baryon densityUnlike helium, deuterium is a very fragile element. It burns at a temperature of only 10^6 K, well below the temperature in the solar core. A considerable fraction of any primordial deuterium at the beginning of the galaxy would have been destroyed by the present time. This is confirmed by observation: interstellar clouds contain deuterium, as do protostars, stars which have not yet developed nuclear burning cores, whereas evolved stars have essentially no deuterium. By studying coulds of gas at very high redshift in the medium between galaxies one can infer the amount of deuterium

relative to that of hydrogen. Comparison with the Big Bang prediction requires one to choose a density that cannot exceed about a tenth of the critical density for closure of the universe, otherwise too little primordial deuterium would have been synthesized. There is no alternative to the Big Bang for synthesizing deuterium: stars destroy it rather than produce it. The significance of this result is that most of the matter in the universe must be non baryons, for example it could consist of weakly interacting neutral particles that did not participate in the nuclear reactions that led to deuterium production.