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 FormationStar 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 high-mass 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 FormationSun’s Formation

Solar system compositionSolar system composition

Our solar system formed from a large gas cloud of H Our solar system formed from a large gas cloud of H and He, enriched by nearby supernova and nova (from and He, enriched by nearby supernova and nova (from dying stars)dying stars)

Radioisotopes found in rock samples and meteorites Radioisotopes found in rock samples and meteorites indicates the solar system is third-generation indicates the solar system is third-generation – Enriched by two preceding supernova eventsEnriched by two preceding supernova events

Composition of the original gas cloud was Composition of the original gas cloud was approximately: approximately: – 70% H70% H– 25% He25% He– 5% other stuff5% other stuff

Sun’s FormationSun’s Formation

Solar system compositionSolar system composition

5% other stuff came from the elements created 5% other stuff came from the elements created by previous stars and consists of atomic and by previous stars and consists of atomic and molecular material, as well as simple and molecular material, as well as simple and complex compoundscomplex compounds

Consists mostly ofConsists mostly of– Gases (OGases (O22, N, N22, CO, CO22, etc.), etc.)– Ices (water, COIces (water, CO22, ammonia, methane), ammonia, methane)– Silicates and oxides (rock)Silicates and oxides (rock)– Metals (mostly Fe, Ni)Metals (mostly Fe, Ni)– All of the other elementsAll of the other elements

Sun’s FormationSun’s Formation

Solar system compositionSolar system composition

Element abundances in solar system are determined byElement abundances in solar system are determined by Universe composition (75% H, 25% He)Universe composition (75% H, 25% He) Supernova enrichment (5% other stuff)Supernova enrichment (5% other stuff) Isotope stability and end productsIsotope stability and end products

Material abundances in the solar system are determined by:Material abundances in the solar system are determined by: Element abundances from original Big Bang and supernova Element abundances from original Big Bang and supernova

enrichmentenrichment Chemistry of element combinationsChemistry of element combinations Radioisotope decay and resulting changes in Radioisotope decay and resulting changes in

compounds/moleculescompounds/molecules

Most common materials are:Most common materials are: Gases - H, He, OGases - H, He, O22, N, N22, CO, CO22, Ar, Ar Ices - water, COIces - water, CO22, ammonia, methane, ammonia, methane Silicates - rockSilicates - rock Metals - mostly Fe, NiMetals - mostly Fe, Ni

Planet FormationPlanet Formation

Planet and moon compositionPlanet and moon composition

Makeup of the planets and their moons is determined by:Makeup of the planets and their moons is determined by: Solar nebula (original gas cloud) compositionSolar nebula (original gas cloud) composition Heating by the Sun Heating by the Sun

– More extreme closer to the SunMore extreme closer to the Sun

Makeup of the first clumps to coalesce in the planetary Makeup of the first clumps to coalesce in the planetary disk is determined by:disk is determined by:

First by electrostatic attractionFirst by electrostatic attraction Then by adhesionThen by adhesion

– IcesIces– DustDust

Then by gravitational attractionThen by gravitational attraction– Density-gravity profile Density-gravity profile – Most dense region is closest to the Sun, but it is also the first Most dense region is closest to the Sun, but it is also the first

to be swept out by the early solar windsto be swept out by the early solar winds

Planet FormationPlanet Formation

Planet and moon compositionPlanet and moon composition

Inner solar system dominated by:Inner solar system dominated by: Silicates - rockSilicates - rock Metals - mostly Fe, NiMetals - mostly Fe, Ni

Outer solar system dominated by:Outer solar system dominated by: Ices - water, COIces - water, CO22, ammonia, methane, ammonia, methane Gases - H, He, OGases - H, He, O22, N, N22, CO, CO22, Ar, Ar

The EndThe End