Fusion Within StarsS T E L L A R N U C L E O S Y N T H E S I S

• Stellar Nucleosynthesis: The formation of heavy elements by the fusion of lighter nuclei in the hearts of stars.

• Atom: “The smallest component of an element having the chemical properties of that element” *1+

• Proton: A positively charged subatomic particle. • Electron: A negatively charged subatomic particle. • Positron (e+): The antimatter equivalent of an electron. • Electron Neutrino(νe ): Elementary particles with almost no

mass, created as a byproduct of neutron/proton transformation

• Nucleon: “a constituent (proton or neutron) of an atomic nucleus”*2+

• Isotopes: “Different forms of atoms of the same element. They have the same number of protons in their nuclei but a different number of neutrons (the same atomic number but different atomic weights).”*2+

^ An atom and it’s components (we assume)

1H AKA Protium; the most common H isotope, consists of a single proton in it’s nucleus.²H AKA Deuterium; has a neutron and a proton in it’s nucleus. ³H AKA Tritium has a proton and 2 neutrons.

• Fusion happens because the star’s gas is compressed and heated under huge gravitational processes.

• This allows matter within to overcome the ColoumbBarrier, the force that pushes like charges apart.

• Hydrogen nuclei fuse into helium. The output helium has a smaller mass than the input hydrogen, the "missing" mass appears as energy, according to Einstein's famous equation E = mc^2. More energy is released than is expended.

• The net result of building one new helium atom is the release of

8 8E = 0.0477 x 10^(-27) kg * (3 x 10 m/s) * (3 x 10 m/s)E = 4.3 × 10-12 Joules. [3]

• 4 × 1026 watts a second; we would need 2.5 × 109 of our largest (5000W) power plants to put this out in a year!

At equilibrium temperature, energy pushing OUT balances gravity pulling IN

• If central temperature drops, energy generation decreases

• Gas pressure pushing outwards decreases

• Gravity pulls outer layers inwards

• As the outer layers press the core inwards, central temperature rises back towards its original value ...

•As the outer layers press the core inwards, central temperature rises back towards its original value ...•Fusion rates increase, with more energy produced•Gas pressure pushing outwards increases•Causes the outer layers to move back towards their original position•Same happens inversely if the star expands too much

• This is how a star remains the same for aeons.

• When the star cannot retain this equilibrium anymore it will change in size and pressure.

• This causes it to fuse different elements as it finds equilibrium again.

• If it cannot generate a state of equilibrium again, gravity wins and it dies!

• The two main cycles, P-P and CNO fuse hydrogen to helium, and compete with each other. These occur in main sequence stars, such as the Sun.

• The next two cycles convert helium to carbon, these are the alpha and triple alphaprocesses

[4]

• After helium density becomes too low stars begin the carbon burning process, increasing in size to become a red giant.

• Once the Carbon density falls too low, gravitational forces fail. Stars of a size 4 to 8 times our sun eject most of their shell, leaving behind a white dwarf core

• White dwarfs can pack mass comparable to the sun’s into an area 1,000,000 times smaller.

• White dwarfs produce no new fusion reactions and cool into black dwarfs over millions of years. It’s thought many are comprised of a Carbon and oxygen lattice underneath their crust, essentially being made of diamond![5]

•Supergiants are stars with enough mass to create pressure and temperatures to fuse more and more elements.• Fusion in larger stars than this is more complex and is a cumulative process. Scientists don’t completely agree on what happens...

•The core cools and the star condenses in size. More mass in a smaller space = more force exerted upon the atoms within; neon burning can begin.

• During neon burning, oxygen and magnesium accumulate in the central core while neon is consumed. After a few years the star consumes all its neon and the core cools down again. Again, gravitational pressure takes over and compress the central core, increasing its density and temperature until the oxygen burning process can start.[6]

• In about six months to a year the star fuses oxygen, creating a new helium core. This core is inert because it is not hot enough. When all oxygen is consumed, the core cools and contracts. This contraction heats it up to the point that the silicon burning process ignites.

Silicon burning• Silicon burning is the last process that can

occur in stars before a supernova occurs.

• It’s very short, often no longer than a day.

• Very hot ;2.7–3.5 billion kelvin (GK)[7]

• It produces heavy elements that require more energy input to build than is released upon their creation

• Mainly Iron is produced at the end of this process

• The star now has nothing to burn and contracts inward…

Supernovae• Since the core isn’t exerting enough gravitational force

anymore the many outer layers escape in a huge explosion of released energy

• The energy released here creates an environment in which the heaviest elements can be created

• This is a process known as neutron capture ; “a nuclearreaction in which a target nucleus absorbs a neutron (uncharged particle), then emits a discrete quantity of electromagnetic energy (gamma-ray photon). The target nucleus and the product nucleus are isotopes, or forms of the same element. Thus phosphorus-31, on undergoing neutron capture, becomes phosphorus-32.”*8+

• Many of them are very unstable and decay very quickly into more stable elements.

• Those that remain are dispersed into space, seeding it.

What remains of the star?

• The Chandrasekhar limit is the maximum mass which can be supported against gravitational collapse by electron degeneracy pressure. It is about 1.4 solar masses.

• “If it has more mass than the Chandrasekhar limit, it will collapse to form a neutron star or black hole, a process with the potential to initiate a supernova.” *8+

ITS PHYSICS TIME!

Proton –Proton Fusion Cycle

• Involves only isotopes of hydrogen and helium; it takes place in low-mass stars like our Sun at about 15 million Celsius. Two protons fuse together. There are 4 stages of this phase.

+Step 1: Two H nuclei fuse into 2H, releasing a positron and a neutrino as one proton changes into a neutron. This positron annihilates with an electron, the mass energy carried off by two gamma ray photons.

+Step 2: the 2H produced in the first stage can fuse with another hydrogen to produce a light isotope of helium, 3He

[9]

+Step 3: From here there are three possible paths to generate helium isotope 4He.

In pp1 helium-4 comes from fusing two of the helium-3 nuclei produced; the pp2 (dominant at temperatures of 14 to 23 MK) and pp3 (dominant if the temperature exceeds 23 MK) branches fuse 3He with a pre-existing 4He to make Beryllium-7. In the Sun, branch pp1 takes place with a frequency of 86%, pp2 with 14% and pp3 with 0.11%. There is also an extremely rare pp4 branch.

Pp4: We’ve never witnessed it before but we assume 3He interacts directly with a proton to make 4He.

Comparing the mass of the final helium-4 atom with the masses of the four protons reveals that 0.007 or 0.7% of the mass of the original protons has been lost. This mass has been converted into energy, in the form of gamma rays and neutrinos released during each of the individual reactions. The total energy we get in one whole chain is 26.73 MeV

• Comparing the mass of the final helium-4 atom with the masses of the four protons reveals that 0.007 or 0.7% of the mass of the original protons has been lost. This mass has been converted into energy, in the form of gamma rays and neutrinos released during each of the individual reactions.

Thank you for your

time!

References• [1]http://www.linguasphere.org/dictionary/n-4812-atom.html accessed

13/11/07• [2] http://wordnet.princeton.edu/perl/webwn?s=nucleon accessed 13/11/07• [3]http://spiff.rit.edu/classes/phys230/lectures/stellar_energy/stellar_energy.

html accessed 13/11/07• [4] http://www.seasky.org/cosmic/assets/images/starlife.jpg 13/11/07• [5]http://imagine.gsfc.nasa.gov/docs/science/know_l2/dwarfs.html accessed

13/11/07• [6] http://www.wisegeek.com/what-is-neon-burning.htm accessed 13/11/07 • [7]

http://webs.mn.catholic.edu.au/physics/emery/hsc_astrophysics_page4.htmaccessed 15/11/07

• [8]http://www.wisegeek.com/what-is-the-chandrasekhar-limit.htm accessed 16/11/07

• [9] http://hyperphysics.phy-astr.gsu.edu/hbase/astro/procyc.html accessed 16/11/07

• [10] http://www.chemie.de/lexikon/e/Proton-proton_chain_reaction accessed 16/11/07