Stellar Fusion

FUSION:hydrogen to helium

• Our Sun is 75% H2, 24% He, and 1% heavier elements.

• The Sun produces energy by FUSION: by combining hydrogen atoms to make helium.

E = mc2

STEP 1: 1H + 1H -> 2H + +

STEP 2: 2H + 1H -> 3HeSTEP 3: 3He + 3He -> 4He + 2 1H

OVERALL: 4 1H -> 4He + 2+ + energy

How much energy? About 600 million kcal per mole of helium produced! How long would that

“fuel” YOU?

helium to carbon• While the Sun fuses H to He there is a

balance between gravity pulling the Sun’s contents inward and the energy produced pushing the Sun’s contents outward.

• When most of the Sun’s H fuel is exhausted the balance will tip, and the Sun will contract. This will heat up the core until its hot enough to “ignite” the helium. The Sun will then expand to a new size as it fuses He into C.

more E = mc2

STEP 1: 4He + 4He -> 8BeSTEP 2: 4He + 8Be -> 12C

OVERALL: 3 4He -> 12C

This process produces less energy than the fusion of H into He. This can be seen in the stars, which are

red giants. “Red hot” is cooler than our Sun’s “yellow hot” … or massive star’s “blue hot”.

Nuclear Binding Energy

Stars can produce energy by fusion only up to 56Fe … beyond that fusion is

“endothermic.”

Fusion <- -> Fission

(make elements > 56Fe)

NebulaeCrab Nebula:

supernova seen

in AD 1054

Supernovac. 1870 on Earth - the most recent in our galaxy

Black Hole

X-ray”center”

artificial fusion

• Produces more energy per pound of fuel than fission

• Creates far less radioactive waste products

• Uses much cheaper fuel (H) than fission (U or Pu)

• So ….. WHY NOT?