Transcript Slides

Star Formation
Formation of the First Materials
Big-Bang Event
 Initial event created the physical forces, atomic particle
building blocks, photons, dark matter, and dark energy
– Protons, neutrons, electrons, photons dominate atomic
universe
 Brief period of fusion transformed protons (H, p+) and
neutrons (no) into 75% H and 25% He (2p+ + 2no)
through high-energy collisions
 Collisions and fusion quickly cut off as density and
temperature dropped rapidly
Formation of the First Atomic Materials
Big-Bang event created:
 Mostly hydrogen (75%)
 Next is helium (25% - 1/3 of the
mass of atomic universe)
 Small amount (10-5) of
deuterium (2H or, or 2D, or p+ +
no)
 Collisions and fusion produced
an even smaller amount of 3He
(10-6)
 Brief fusion period also
produced a tiny amount of
lithium (10-10)
Star Formation
First stars
 Formed from original 2/3 H, 1/3 He universe
composition
 First stars were gigantic (100-500 times Sun’s
mass)
– Turbulence that thermal motion too high for small
stars to form
 Rapid fusion of core H into He also created
other fusion products
Star Formation
First stars
 Primary energy in all stars is generated by H → He
fusion (4p+→ 2p+ + 2no = 4H → 4He)
 When hydrogen in the core is exhausted, fusion ends
unless overlying mass is large enough to compress He
to high enough temperatures to fuse into Be
– If the star is massive enough, Be fusion is followed in rapid
sequence by the fusion production of C, O, Ne and so on until
iron is formed
– First stars formed after the Big Bang were the largest stars to
form
 500 times the Sun’s mass
 Fe formation initiates a cataclysmic end to fusion since
higher mass nuclei absorb energy (endothermic) in the
fusion process
Star Formation – Nuclear Fusion and Binding Energy
Nuclear binding energy = Δmc2
Mass difference between component particles (4p+) and
the resulting nucleus (4He) is Δm
For the helium nucleus (alpha particle) Δm = 0.0304 u
which gives a binding energy of 28.3 MeV
Stellar Energy
Fusion-fission binding
energy of nucleons

Lower-mass atoms
release energy in the
fusion process
(exothermic)
– Absorb energy in
the fusion process

Higher-mass atoms
release energy in the
fission process (Fe and
above)
– Absorb energy in
the fission process
(endothermic)
– Production of highmass nuclei in the
core of a star
terminates fusion
Elements Formed in Stars
End of energy production in a star’s core
 Fusion fuel exhausted
 Star’s core cools rapidly
 A. Small stars cool to form a white dwarf
 B. Large stars undergo rapid gravitational collapse
– Violent collapse creates implosion
– High-pressure, high-temperature conditions force nuclei into
neutron-rich mix
– Secondary fusion process (rapid process) initiated
– Violent rebound produces a supernova for large stars (>5 Mo)
 Less-explosive nova created in mid-mass stars (like the Sun)
– Material is blown away from the star’s core
Elements Formed in Stars
Following the termination of the fusion process
in a star
 Core implosion creates a secondary fusion
event
– Extreme pressures and temperatures force
electrons to combine with neutrons
 Neutron-rich core
 If this survives intact, a neutron “star” is formed
– Material blasted outward contains high-mass nuclei
 Secondary shell fusion
– High-energy neutrons blasted from the core
implosion are fused into departing material (neutron
enrichment)
 Generates high-mass nuclei
Stellar Fusion
Atomic material enrichment
 Fusion process inside of stars creates helium and
everything heavier
 Supernova responsible for much of atomic material
heavier than iron
 Nuclear furnace inside larger stars can also produce
heavy nuclei with the slow bombardment of nuclei by
neutrons (slow process of neutron enrichment)
 Atomic nuclei beyond helium are produced by
supernova and by large star cores, but not equally
Stellar fusion
Atomic
material
enrichment
 All elements
heavier than
He are
produced
inside stars,
but not in
equal
abundance
 Lower-mass
atomic
material is
more
abundant than
higher-mass
elements
Sun’s Formation
Solar system composition
 Our solar system formed from a large gas cloud of H
and He, enriched by nearby supernova and nova (from
dying stars)
 Radioisotopes found in rock samples and meteorites
indicates the solar system is third-generation
– Enriched by two preceding supernova events
 Composition of the original gas cloud was
approximately:
– 70% H
– 25% He
– 5% other stuff
Sun’s Formation
Solar system composition
 5% other stuff came from the elements created
by previous stars and consists of atomic and
molecular material, as well as simple and
complex compounds
 Consists mostly of
–
–
–
–
–
Gases (O2, N2, CO2, etc.)
Ices (water, CO2, ammonia, methane)
Silicates and oxides (rock)
Metals (mostly Fe, Ni)
All of the other elements
Sun’s Formation
Solar system composition
Element abundances in solar system are determined by
 Universe composition (75% H, 25% He)
 Supernova enrichment (5% other stuff)
 Isotope stability and end products
Material abundances in the solar system are determined by:
 Element abundances from original Big Bang and supernova
enrichment
 Chemistry of element combinations
 Radioisotope decay and resulting changes in
compounds/molecules
Most common materials are:
 Gases - H, He, O2, N2, CO2, Ar
 Ices - water, CO2, ammonia, methane
 Silicates - rock
 Metals - mostly Fe, Ni
Planet Formation
Planet and moon composition
Makeup of the planets and their moons is determined by:
 Solar nebula (original gas cloud) composition
 Heating by the Sun
– More extreme closer to the Sun
Makeup of the first clumps to coalesce in the planetary
disk is determined by:
 First by electrostatic attraction
 Then by adhesion
– Ices
– Dust
 Then by gravitational attraction
– Density-gravity profile
– Most dense region is closest to the Sun, but it is also the first
to be swept out by the early solar winds
Planet Formation
Planet and moon composition
Inner solar system dominated by:
 Silicates - rock
 Metals - mostly Fe, Ni
Outer solar system dominated by:
 Ices - water, CO2, ammonia, methane
 Gases - H, He, O2, N2, CO2, Ar
The End