thermonuclear fusion (hydrogen “burning”) stars condense out of the gas and dust clouds in the...
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THERMONUCLEAR FUSION(HYDROGEN “BURNING”)
Stars condense out of the gas and dust clouds in the Milky WayGalaxy. As they collapse into a spherical shape the interior heatsup. 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 processthat takes place at the core of a hydrogen bomb. This is the sourceof 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’score for the burning to continue another 5 billion years.
PhotosphereThermonuclear 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/m3.
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 completelyionized. The nuclei are movingin the core within a “soup” of free,unattached electrons.
DURING A NUCLEAR REACTION THERE IS A LOSS OF MASS
321 NucleusNucleusNucleus
Mass before the reaction Mass after the reaction
0)( 321
321
MMMM
MMM
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 = Mc2
Einstein’s famous equation tells exactly how much energy the lost mass becomes.
2cME
Increase in energy Decrease in mass
Speed of light
THE TOTAL ENERGY & MASSDOES NOT CHANGE
EcMcMcM 23
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Energy available in Nucleus1
Energy available in Nucleus2
Energy available in Nucleus3
Energy released by thenuclear reaction
The energy released by the nuclear reaction is primarily in thethe form of EMR (i.e., γ-ray photons) although the kinetic energyof 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+
e+
n0
υe
Proton (p+)
Neutrino (υe)
Neutron (n0)
Positron (e+)
The neutrino is a sub-subatomicparticle 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.
eenp 0Proton 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).
2)( cMMEnergyMM PositronElectronPositronElectron
e+
e-
γ Positron/Electron Annihilation ee
NEUTRON DECAYIn order for a neutron (n0) 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 (υe). This process is called neutron decay.
n0
p+
Stable neutron attached to a proton through the strong nuclear force.
n0
e-
p+
υe
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+
Prior to the collision
p+
After the collision, a thermal photon is produced with no physical change in the two protons.
Photon
p+ p+
Temperature (T < 10,000,000 K)
Temperature (T > 10,000,000 K)
p+p+
Prior to the collision
p+
n0
e+
υe
After the collision, one of theprotons decays (i.e., sheds itscharge).
eneHHeHH 01
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1
p+p+
p+
p+
1H11H1
2He2
υep+ n0 e+
What happens to these particles?
The proton (p+) is stable and remainsunchanged.
The neutron (n0) will attach with theproton and be stable, or after12 minutes decays into a proton, electron, and neutrino.
The positron (e+) will collide with anelectron and undergoes annihilation.
The neutrino passes right through theSun, not interacting with any of theother nuclei or particles.
p+
Combines with a neutron to form 2H1.
Collides with another proton to form a neutron, positron, and neutrino.
n0
Decays into a proton, electron, and neutrino.
Combines with a proton to form 2H1.
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/m3.
υePasses 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 LINEEventually the proton and neutron combine to produce 2H1 , deuterium.
p+p+
p+
p+2He2
p+
n0
γ
υe 2H1
Immediately absorbed by a nucleus in thehigh density core. A γ-ray photon is the same size as an atomic nucleus.
eHHH 12
11
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The newly produced deuterium (2H1) will most likely combine with another
proton (1H1)
p+
n0 p+
p+
n0
p+
3He2
2H1
1H1
γ
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11
12 )*( HeHeHH
The newly produced helium (3He2) will combine with a variety of nuclei including another helium (3He2)
p+
n0
p+
p+
n0
p+
The 3He2 will combine with 1H1, of course,but the interaction is more likely with Another 3He2 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+
n0
p+
p+
n0
p+
6Be4
3He23He2
p+
n0
p+
p+
n0
p+
The newly produced beryllium (6Be4) is a highly unstable isotope.
It decays in a variety of ways including the following:
p+n0
p+
p+
n0
p+
6Be4 4He2
1H1
1H1
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46 *)( HHHeHHHeBe
Here is a summary of the thermonuclear reactions in the
core of a star
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HHHeHeHe
HeHH
HHH e
Important Note: These first two thermonuclear reactions have to take place before the third one can.
524
5226
42)2(42)(26
22)(222)(26
22222)(2
222)(2
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e
e
ee
ee
e
e
HeH
HHeH
HHeHeH
HeHHH
HHHHH
HHH
PROTON-PROTON CYCLE
Net reaction of the Proton-Proton Cycle.
524 24
11 eHeH
4He2 is a stable isotope of heliumIt does not “burn” at 10,000,000 K.Its “kindling temperature” is 100,000,000 K.
Hydrogen burns at 10,000,000 Kto produce helium.
Neutrinos do not interact withthe nuclei of atoms in the star’score. They fly out of the coreleaving the star near the speedof light 300,000 km/s.
γ-ray photons arethe same size asthe nucleus of anatom. They areabsorbed by the nuclei in the dense 150,000 kg/m3
core of the star.