The initial fusion of 2 protons actually involves positron decay which converts one of them into a neutron: a WEAK process.

eHHH 211 Q=0.42 MeV

Recall there is no 2-proton state!

Such weak processes are of course characterized by low cross sections (and exceedingly low rates).

This is, however, a prerequisite step to the nuclear fusion in the sun.

because only once there is sufficient deuterium:

pHHH 2

3

11

2

11

2

1Q=4.0 MeV

Even with the assumed age of the universe (from the Hubble expansion) first estimates by Alper (Gamow’s graduate student)

concluded there had not been enough time to build up the concentration of 2H needed to allow that very next step.

Meanwhile, in parallel, Gamow was trying to determine whether the heavy elements found in the universe could have been produced by the big bang itself.

4

8

42

4

22

4

2BeHeHe Q=-91.9 keV

8Be exists as a resonance decaying with 10-16 sec. Its formation requires 91.9 keV kinetic energy shared between the initial states.

If this model of stellar energy production is correct

we expect

1. Large abundance of low Z elements, trace amounts of heavy Z.

2. Large relative abundance of the light even-Z elements; small relative abundance of odd-Z elements;

3. Very little, if any of the elements between He and C (Li, Be, B) which were not produced by these reactions.

4. An enhanced abundance of Fe, the fusion end product.

What is the world’s most abundant element?

Oxygen O 46.6

Silicon Si 27.7

Aluminum Al 8.1

Iron Fe 5.0

Calcium Ca 3.6

Sodium Na 2.8

Potassium K 2.6

Magnesium Mg 2.1

Subtotal 98.5

Titanium Ti 0.4

Hydrogen H 0.1

Phosphorus P 0.1

Manganese Mn 0.1

Sulfur S 0.05

Carbon C 0.03

Total 99.3

CRUSTAL ABUNDANCE OF THE ELEMENTS (by percent weight)

Average composition of clean, dry air at the Earth's surface

Constituent SymbolMolecular Weight

Molecular fraction

Fraction by mass

Nitrogen Oxygen Argon Carbon dioxide Neon Helium Methane Krypton Nitrous oxide Hydrogen Ozone

N2 O2 Ar CO2 Ne He CH4 Kr N2O H2 O3

28 32 40 44 20 4 16 84 44 2 48

78.09 % 20.95 % 0.93 % 320 ppm 18 ppm 5.2 ppm 2.9 ppm 1.1 ppm 0.5 ppm 0.5 ppm 0.01 ppm

75.5% 23.2 % 1.3 % 486 ppm 12 ppm 0.7 ppm 1.6 ppm 3.2 ppm 0.8 ppm 0.03 ppm 0.02 ppm

Water, moisture H2O 18 - -

Source: Garrels, MacKenzie and Hunt: Chemical cycles. 1975

Distribution of Elements in the Human Body (by weight)

Element Atomic no. Percentage Role

oxygen 8 65.0 cellular respiration, component of water

carbon 6 18.5 basis of organic molecules

hydrogen 1 9.5 component of water & most organic molecules, electron carrier

nitrogen 7 3.3 component of all proteins and nucleic acids

calcium 20 1.5 component of bones and teeth, triggers muscle contraction

phosphorus 15 1.0 component of nucleic acids, important in energy transfer

potassium 19 0.4 min positive ion inside cells, important in nerve function

sulfur 16 0.3 component of most proteins

sodium 11 0.2 main positive ion outside cells, important in nerve function

chlorine 17 0.2 main negative ion outside cells

magnesium 12 0.1 essential component of many energy-transferring enzymes

iron 26 trace essential component of hemoglobin in the blood

copper 29 trace component of many enzymes

molybdenum 42 trace component of many enzymes

zinc 30 trace component of some enzymes

iodine 53 trace component of thyroid hormone

Solar system massesSun 1.981030 kgJupiter 1.901027 kgEarth 5.981024 kg

Absorption “lines”

• First discovered in spectrum of Sun (by an imaging scientist named Fraunhofer)

• Called “lines” because they appear as dark lines superimposed on the rainbow of the visible spectrum

Sun’s Fraunhofer absorption lines

(wavelengths listed in Angstroms; 1 A = 0.1 nm)

The Solar Spectrum

Emission line spectra

Insert various emission line spectra here

The optical emission line spectrum of a young star

Emission line images

Planetary nebula NGC 6543(blue: Xrays)

Green: oxygen; red: hydrogen

Orion Nebula

Spectra of ions

• Emission lines from heavy ions -- atoms stripped of one or more electrons -- dominate the high-energy (X-ray) spectra of stars

• Ions of certain heavier elements (for example, highly ionized neon and iron) behave just like “supercharged” H and He

Wavelength (in Angstroms)

Neon Iron

Hydrogen is so common, astronomers identify different types of ISM clouds by the form of hydrogen they contain. The five basic types of interstellar cloud: diffuse H-I clouds, molecular clouds, the Warm Inter-cloud Medium (WIM), H-II regions, and hot coronal gas.

Component Temperature Density State (K) (atoms/cm3) H-I clouds 50 - 100 1 - 103 neutral atoms Molecular clouds 20 - 50 103 - 105 molecules WIM 103 - 104 0.01 partially ionized H-II regions 104 102 - 104 mostly ionized Coronal gas 105 - 106 10-4 - 10-3 fully ionized

The Interstellar Medium

21-Centimeter Radiation21-Centimeter Radiation

• Much of interstellar space consists of gaseous atomic hydrogen

• Astronomers observe interstellar regions by a“21-centimeter line”

• This requires no light, only enough hydrogen to produce a detectablesignal

•Wavelength of 21-centimeter radiation is much larger than the typical size of interstellar dustparticles

Sub-Millimeter Array on Mauna Kea

H-I Clouds and 21-cm Radiation The roman numeral indicates the degree of ionization: I: not ionizedII: singly ionized - one electron removedIII: doubly ionizedPortions of the ISM composed mainly of neutral hydrogen are called "H-I clouds” - cold (T = 50 - 100 K), and dense (1 to 1000 atoms/cm3).

Detecting H-I clouds directly was not possible until the use of radio telescopes (mid-1950s). Electromagnetic interactions between electron and proton can flip the electron spin: emitting or absorbing a 21-cm wavelength photon (in the radio spectrum). Such a long wavelength easily penetrates most intervening material. The prevalence of hydrogen gas along with the penetrating power its 21-cm radiation it emits allows astronomers to map out the spiral structure of the Milky Way.

Milky Way in Visible Light

Milky Way in 21 centimeter light of neutral hydrogen.

1. Abundance of low Z, trace amounts of heavy Z.

2. Larger abundance of even-Z; small abundance of odd-Z.

3. Very little between He - C (Li, Be, B) not produced by these reactions.

4. Enhanced abundance of Fe, the fusion end product.

July 1969 Apollo 11 astronauts trap cosmic ray particles on exposed aluminum foil, returned to earth for analysis of its elemental & isotopic composition. With no atmosphere or magnetic field of its own, the moon’s surface is exposed to a constant barrage of particles.

March 3, 1972 Pioneer 10 launched -on its flyby mission, studies Jupiter's magnetic field and radiation belts.December 1972 Apollo 17’s lunar surface cosmic ray experiment measured the flux of low energy particles in space (foil detectors brought back to Earth for analysis.

October 26, 1973 IMP-8 launched. Continues today measuring cosmic rays, Earth’s magnetic field, and the near-Earth solar wind from a near-circular, 12-day orbit (half the distance to the moon).

October 1975 to the present GOES (Geostationary Orbiting Environmental Satellite) An early warning system which monitors the Sun's surface for flares.

1977 The Voyager 1 and 2 spacecraft are launched. Each explored acceleration processes of charged particles to cosmic ray energies.

August 31, 1991 Yohkoh spacecraft launched - Japan/USA/England solar probe - studied high-energy radiation from solar flares.

July 1992 SAMPEX (Solar Anomalous and Magnetospheric Particle Explorer) in polar orbit. By sampling inter- planetary & magnetospheric particles, contributes to our understanding of nucleosynthesis and the acceleration of charged particles.

July 1992 IMAX (Isotope Matter-Antimatter eXperiment) balloon-borne superconducting magnetic spectrometer measured the galactic cosmic ray abundances of protons, anti-protons, hydrogen, and helium isotopes.

August 25, 1997 Advanced Composition Explorer (ACE) was launched!

Hydrogen (H) 1 1.00 640

Helium (He) 2 6.8 10-2 94

Lithium, beryllium, boron 2.6 10-9 1.5

Carbon, Nitrogen, Oxygen 1.2 10-3 6

Iron (Fe) 26 3.4 10-5 0.24

All heavier atoms 1.9 10-6 0.13

ElementAtomic

Number (Z)

Solar SystemComposition

(relative number of atoms)

PrimaryCosmic Ray

Flux(particles/m-2 sec)