THERMONUCLEAR FUSION (HYDROGEN “BURNING”) Stars condense out of the gas and dust clouds in the Milky Way Galaxy. As they collapse into a spherical shape the interior heats up. When the temperature at the center reaches 10,000,000 K, nuclei of hydrogen undergo change converting into helium. This process is called thermonuclear fusion and is the same process that takes place at the core of a hydrogen bomb. This is the source of the star’s energy. The Sun has been burning its hydrogen for 5 billion years. It is believed that there is a sufficient supply of hydrogen in the Sun’s core for the burning to continue another 5 billion years.

Photosphere Thermonuclear Core T > 10,000,000 K Interior (T < 10,000,000 K) 10,000,000 K 15,000,000 K BASIC STRUCTURE – HYDROGEN-BURNING STAR The Sun’s thermonuclear core has a radius of 200,000 km. The density is 150,000 kg/m 3.

NOTE:Because of the 10,000,000+ K temperatures at the core of a hydrogen- burning star, all of atoms are completely ionized. That is, all of their electrons are stripped away from the nuclei. Thermonuclear Core All of the atoms are completely ionized. The nuclei are moving in the core within a “soup” of free, unattached electrons.

DURING A NUCLEAR REACTION THERE IS A LOSS OF MASS Mass before the reactionMass after the reaction

THE LOST MASS DOES NOT DISAPPEAR FROM THE UNIVERSE The lost mass re-appears as energy. In other words, part of the mass contained in the original two nuclei is converted into energy. This energy is what produces the intense EMR from the Sun.

E = Mc 2 Einstein’s famous equation tells exactly how much energy the lost mass becomes. Increase in energyDecrease in mass Speed of light

THE TOTAL ENERGY & MASS DOES NOT CHANGE Energy available in Nucleus 1 Energy available in Nucleus 2 Energy available in Nucleus 3 Energy released by the nuclear reaction The energy released by the nuclear reaction is primarily in the the form of EMR (i.e., γ-ray photons) although the kinetic energy of the nuclei is also a factor.

PROTON DECAY The subatomic particles protons (p + ) under certain conditions self-destruct shedding their positive charge in a process called proton decay. p+p+ e+e+ n0n0 υeυe Proton (p +) Neutrino (υ e ) Neutron (n 0 ) Positron (e + ) The neutrino is a sub-subatomic particle with little, if no, mass. It has no charge. The positron has the same mass as an electron, only it has a positive charge instead of a negative charge. It is an anti-electron. Proton Decay

POSITRON/ELECTRON ANNIHILATION The positron (e+) is an anti-electron. Whenever it comes into contact with an electron (e-) the mass of the two particles is completely converted into energy (γ-ray photon). e+e+ e-e- γ Positron/Electron Annihilation

NEUTRON DECAY In order for a neutron (n 0 ) to be stable it must be attached to a proton (p + ). Unattached neutrons decay within 12 minutes into a proton (p + ), electron (e - ), and a neutrino (υ e ). This process is called neutron decay. n0n0 p+p+ Stable neutron attached to a proton through the strong nuclear force. n0n0 e- p+p+ υeυe An unattached neutron will decay after 12 minutes.

IN THE HOT, DENSE THERMONUCLEAR CORE OF A STAR TWO PROTONS ARE SQUEEZED TOGETHER Ordinarily, the positively-charged protons collide, recoil, and emit a photon related to the energy exchange of the collision. Recall, this is an example of thermal radiation. However, the extreme temperature and density conditions found at the star’s core can overcome the repulsion of the two positively-charged protons, and force the protons together.

p+p+ Prior to the collision p+p+ After the collision, a thermal photon is produced with no physical change in the two protons. Photon p+p+ p+p+ Temperature (T < 10,000,000 K) Temperature (T > 10,000,000 K) p+p+ p+p+ Prior to the collision p+p+ n0n0 e+e+ υeυe After the collision, one of the protons decays (i.e., sheds its charge).

p+p+ p+p+ p+p+ p+p+ 1H11H1 1H11H1 2 He 2 υeυe p+p+ n0n0 e+e+ What happens to these particles? The proton (p + ) is stable and remains unchanged. The neutron (n 0 ) will attach with the proton and be stable, or after12 minutes decays into a proton, electron, and neutrino. The positron (e + ) will collide with an electron and undergoes annihilation. The neutrino passes right through the Sun, not interacting with any of the other nuclei or particles.

p+p+ Combines with a neutron to form 2 H 1. Collides with another proton to form a neutron, positron, and neutrino. n0n0 Decays into a proton, electron, and neutrino. Combines with a proton to form 2 H 1. e+e+ Annihilates with a electron to form a γ-ray photon. Remember: A stellar core is a “soup” of electrons with a density of 150,000 km/m 3. υeυe Passes through the star without interacting with any other particles. Remember: A neutrino is tiny sub-subatomic particle, small in comparison even to the nucleus of an atom (i.e., it passes right on through a nucleus without interacting with either the proton or neutron).

BOTTOM LINE Eventually the proton and neutron combine to produce 2 H 1, deuterium. p+p+ p+p+ p+p+ p+p+ 2 He 2 p+p+ n0n0 γ υeυe 2H12H1 Immediately absorbed by a nucleus in the high density core. A γ-ray photon is the same size as an atomic nucleus.

The newly produced deuterium ( 2 H 1 ) will most likely combine with another proton ( 1 H 1 ) p+p+ n0n0 p+p+ p+p+ n0n0 p+p+ 3 He 2 2H12H1 1H11H1 γ

The newly produced helium ( 3 He 2 ) will combine with a variety of nuclei including another helium ( 3 He 2 ) p+p+ n0n0 p+p+ p+p+ n0n0 p+p+ The 3 He 2 will combine with 1 H 1, of course, but the interaction is more likely with Another 3 He 2 given the larger size of the Helium nuclei compared to the hydrogen nuclei, especially as the helium content in the star’s core increases. p+p+ n0n0 p+p+ p+p+ n0n0 p+p+ 6 Be 4 3 He 2

p+p+ n0n0 p+p+ p+p+ n0n0 p+p+ The newly produced beryllium ( 6 Be 4 ) is a highly unstable isotope. It decays in a variety of ways including the following: p+p+ n0n0 p+p+ p+p+ n0n0 p+p+ 6 Be 4 4 He 2 1H11H1 1H11H1

Here is a summary of the thermonuclear reactions in the core of a star Important Note: These first two thermonuclear reactions have to take place before the third one can.

PROTON-PROTON CYCLE Net reaction of the Proton-Proton Cycle.

4 He 2 is a stable isotope of helium It does not “burn” at 10,000,000 K. Its “kindling temperature” is 100,000,000 K. Hydrogen burns at 10,000,000 K to produce helium. Neutrinos do not interact with the nuclei of atoms in the star’s core. They fly out of the core leaving the star near the speed of light 300,000 km/s. γ-ray photons are the same size as the nucleus of an atom. They are absorbed by the nuclei in the dense 150,000 kg/m 3 core of the star.