The Interior of the Sun

! The temperature of the core is about 15 million K!

! It also has very high density.

! Under these conditions, nuclear reactions occur, which power the Sun.

! All solar energy is generated in the core ! The core is about 10% of the Sun’s diameter

Energy from the Sun! Chemical reactions (like fire, metabolism) involve an

atom’s electrons. Eg. ! 2 H2 + O2 = 2 H2O

! Nuclear reactions involve an atom’s nucleus.... ! They are much more powerful. ! They create new elements!! All of the Sun’s energy comes from nuclear reactions. ! (so the Sun is not “on fire”)

! Since the Sun is mostly made of Hydrogen (H), the reactions involve H nuclei.

Energy from the Sun! Atoms in the Sun’s core have lost their

electrons. (they are ionized)

! A Hydrogen nucleus = 1 proton

! If four Hydrogen nuclei come together, they can fuse, forming a Helium nucleus.

! This fusion reaction creates a tremendous amount of energy

Atom Ion

Nuclear Power can be generated two ways:

! Break up a big nucleus ! (eg. Uranium or Plutonium)

! Combine small nuclei ! (eg. Hydrogen)

Fission Fusion

Both types have been used by people on Earth.

Nuclear WeaponsNuclear fission was discovered in 1939, during World War II The U.S. built Fission bombs (using U & Pu) and destroyed Hiroshima & Nagasaki at the end of the war.Since then, more powerful fusion bombs have been built (Hydrogen bombs)

Trinity Test, New Mexico, 1945First Nuclear Bomb

Hydrogen (fusion) bomb, Pacific Ocean, 1962

Nuclear weapons testing contaminated Earth’s atmosphere with radiation. So, in 1963, nations agreed not to explode them above ground.

Nine countries still have nuclear weapons

(US & Russia have 7,000 each)

America’s nuclear weapons are managed by the Department of Energy.

Rick Perry (2017-2019)lost on DTWS

Dr. Stephen Chu(2009-13): Won Nobel Prize in Physics

US Energy Secretaries:

Nuclear Power …….can also be used to generate electricity.Current, nuclear reactors use controlled fission, to do this, generating considerable nuclear waste, and (on rare occasions) melting down.

Nuclear fusion reactors are still experimental...but may one day provide copious clean energy, with little waste.

If we can figure out how to make controlled fusion, we will have a reliable, and “clean” source of energy.

“Tokamak”, an experimental nuclear fusion reactor San Onofre Fission Plant, CA

Fusion in the Sun! For Fusion to happen, 4 Hydrogen nuclei (protons) must

combine to become Helium

! But Helium has 2 protons and 2 neutrons.

! Can protons turn into neutrons?

! ... Only if they also produce a positively charged particle.

! Positive electrons, called “positrons” are an example of antimatter!

! The sequence of steps to produce Helium is called the proton-proton chain

The Proton-Proton Chain

positr

on

(antim

atter!

)

photo

n

(light

energy

)

Energy from the Sun: Nuclear Fusion

Start with:

4 Hydrogennuclei

End up with:

1 Helium nucleus

4 Hydrogens weigh more than one Helium….

Where Stars Get Their Energy! The end product of nuclear fusion is the Helium nucleus ! This Helium is about 1% lighter than the 4 Hydrogens that

made it up. ! Where did that mass go? ! Albert Einstein discovered that mass (m) can transform into

energy (E).

E =mc2

! c is the speed of light ! c = 3 x 10 8 m/s so c2 = 9 x 10 16 m2/s2!!!!!

! A small amount of mass can produce a lot of energy

Energy from Mass Example ! When Hydrogen fuses into Helium, 1% of the mass becomes energy. ! Question: If 1 gram of matter disappears, how many Joules of Energy will

be produced. ! ! 1 Joule = kg m2 / s2 . (eg: a 100 Watt light emits 100 Joules per second)

! In nuclear reactions, mass is converted into energy

E =Mc2

M is the mass lost. (not the total mass), in kilograms (kg) E is the Energy produced, in Joules (J)

! M =1 gram = 0.001 kg

c2 = 9 x 10 16 m 2/s 2 (Note: use m/s here!)

E =Mc2

E = (0.001 kg) x ( 9 x 10 16 m 2/s 2)

E = 9 x 1013 kg m2 / s2 = 9 x 1013 Joules of energy

This is equal to the energy:

! of burning 1 million gallons of gasoline! ! exploding 25,000 tons of dynamite (25 kilotons of TNT)

All from one gram of mass disappearing!

Nuclear Fusion in the Sun

! The Sun converts 4 million tons of matter to energy every second!

! This energy is equal to 2 billion hydrogen bombs ! (1 Hydrogen bomb ~ 2 Megatons of TNT)

! The Sun has been shining for 5 billion years, all this time converting Hydrogen to Helium.

! Fortunately, the Sun has enough Hydrogen “fuel” to keep shining for 5 billion more years.

The Sun: Summary

! Size & Mass ! Interior: Core, Convection Zone ! Atmosphere: Photosphere, Corona, Chromosphere ! Sunspots: 11 year cycle. ! Energy source: nuclear fusion of hydrogen to

helium

Fusion… powers every star

StarsChapter 11:

Optional: 11:12-11:13