Basic Atomic Structure and Nuclear Formation Challenge

• Matter composed of atoms (neutrons, protons in nucleus + electrons in shells)
• Electron arrangement simple: positively charged nucleus attracts electrons automatically
• Nuclear formation difficult: protons positively charged, repel each other
• Strong nuclear force keeps protons together when extremely close
• Requires hundreds of millions of degrees temperature to overcome repulsion

Big Bang Nucleosynthesis

• Early universe had required high temperatures briefly after Big Bang
• High matter density everywhere created plasma (subatomic particle soup)
• No nuclei initially, just nuclear constituents mixed together
• Plasma expansion and cooling allowed particles to stick together
• Created first atomic nuclei, which then captured electrons
• Produced hydrogen, helium, few other elements up to atomic number 4
• Process called "Big Bang nucleosynthesis" occurred few minutes after Big Bang
• Rapid expansion limited formation to lightest nuclei only

Stellar Nucleosynthesis

• Heavier element formation required patience (few hundred million years)
• Light nuclei collected under gravity, formed first stars
• Gravitational pressure increased temperature in stars
• Temperature became sufficient to fuse small nuclei into larger ones
• Nuclear fusion creates energy, makes stars hot and shine
• Stellar fusion can proceed up to atomic number 26 (iron)
• Iron most stable element with largest binding energy
• Fusion releases energy until iron; beyond iron requires energy input

Neutron Capture Process

• Elements heavier than iron created via neutron capture
• Fusion processes create free neutrons
• Neutrons (no electric charge) easily enter atomic nuclei vs protons
• Inside nucleus: neutron decays → proton + electron + electron anti-neutrino
• Creates heavier elements, many initially unstable isotopes
• Unstable nuclei decay until reaching stable configuration
• Slow neutron capture occurs in old stars by chance over time
• Stars eventually run out of fuel, die, some collapse and explode
• Supernovae distribute nuclei inside/outside galaxies
• Some lighter elements created by cosmic ray splitting of heavier elements

Rapid Neutron Capture and Heavy Element Production

• Slow stellar neutron capture insufficient for Earth's heavy element abundance
• Rapid neutron capture requires extreme high-pressure, neutron-rich environment
• Initially thought to occur in supernovae, but calculations showed insufficient neutron production
• Observational evidence contradicted supernova theory:
  • Heavy elements in dwarf galaxies would require so many supernovae
  • Would have blown apart small galaxies (but we observe them intact)
• Current theory: heavy elements produced in neutron star mergers
  • Neutron stars are supernova remnants containing lots of neutrons
  • Actually super-dense nuclear plasma, not discrete nuclei
  • Collisions create conditions for rapid neutron capture
  • Can produce all heavy elements found on Earth
• Recent neutron star merger light analysis supports hypothesis (light contains evidence of heavy elements)

Unstable Elements and Laboratory Creation

• Heaviest periodic table elements missing/unstable
• Decay into smaller nuclei in timeframes: few thousand years to microseconds
• Stellar-produced heavy elements long gone
• Properties known only through laboratory creation
• Created by shooting smaller nuclei at each other with high energy

Island of Stability

• Long-standing nuclear physics hypothesis
• Heavy nuclei with specific neutron/proton numbers should have lifetimes up to ~100,000 years
• Not yet created in laboratories
• Called "island of stability" - appears as island on graph
  • One axis: number of protons
  • Other axis: number of neutrons
• Exact location unclear, predictions have shifted over time
• Currently believed to require pushing more neutrons into heaviest previously produced nuclei

Complexity from Simplicity

• Remarkable that all observed complexity built from only 3 ingredients:
  • Neutrons
  • Protons
  • Electrons