Nuclear Forces

The power behind Stars

Fundamental Forces

• Gravity– Attractive force governed by mass

• Electromagnetism– Attractive or repulsive force that is governed

by electric charge

• Weak nuclear– A force that governs nuclear decay (fission)

• Strong nuclear– A force that governs the fusion of nuclear

particles

Weak Nuclear

• This force guides the breakdown of large atomic nuclei such as Uranium

• The three types of decay are known as – Alpha decay (α) - a helium nucleus is released– Beta decay (β) - an electron and a neutrino are

released– Gamma decay (γ) - a gamma ray is released

(most intense form of light

Strong Nuclear

• This force guides the fusion of sub atomic particles

• The energy released powers the stars• The fusion takes place despite the

repulsive force of the positive electromagnetic charge

• Hydrogen fuses with neutrons until one neutron breaks down into a proton and it becomes a helium nucleus

Strong Nuclear

• Nuclear fusion of Hydrogen to Helium occurs spontaneously at a temperature of 10,000,000 C°

• However, the next step from Helium to Lithium and Beryllium does not occur until the temperature reaches 20,000,000 C°

• This explains why stars spend so much time on the Main Sequence

Where does the energy come from

E=mc2

Mass changes into Energy

• The Sun produces enough energy in one second to power the City of Chicago for about one million years

• The Sun will do this for more than 10 billion years

Question?

• How can fusion (fusing atomic nuclei) produce energy and fission (breaking down atomic nuclei) also produce energy

• The break even point is Fe (iron)

• All fusion reactions up until Fe are exothermic (they release energy) but after Fe the fusion reactions are endothermic (they require energy to be put in)

The End of Big Star’s Lives

• Stars cannot produce elements heavier than Fe by natural processes because the formation of Fe steals energy from the core and cools it below the fusion temperature

• However, as heavier elements are produced at the core, the density becomes incredible

Supernova

• As the core density increases, the neutrons begin to form dense mini neutron stars start to form and dissipate

• When one of the mini neutron stars survives it eliminates the core pressure and the outer layers of the star’s core collapse toward the center.

• Since the core is only a little bigger than the Earth, the collapse takes less than one second

Supernova

• When the surface of the core hits the very center, a temperature of more than 1 billion degree forms every naturally occurring element and beyond

• The rebounding material travels toward the surface of the star at the speed of light

• Even at this speed, it takes about 30 to 50 minutes to reach the surface

Now we see the light!

• The impeding explosion can’t be seen until the core material reaches the surface where it obliterates the surface and blasts all of it heavier elements into space.

• This is where all of the element heavier than He come from including the Carbon in your cell walls and DNA, to the Fe in your blood, to the Calcium in your bones, and more importantly the Uranium that keeps our tiny planet warm