Stellar Evolution and our Sun
(Song “The Sun” from “Severe Tire Damage”, They Might be Giants (band name))
Today we begin at the beginning, with the origin of the solar system.
I will describe briefly the Big Bang theory, and the chemical reactions that created the
Fundamental Particles
elements
p = protons
n = neutrons
e- = electron
describes any set of atoms with same # of protons
1836 x wt. of emass = 1.672 x 10-27 kg
free neutron has mean lifetime of 750 s
decays to p + emass = 1.674 x 10-27 kg
mass = 0.910 x 10-30 kg
Mass of the atom is mostly in the nucleus
Nucleus is 4 orders of magnitude smaller than electron cloud
Electrons are 1/1800 mass of protons and neutrons
 =  mesons (muons)
also called mesotron
subatomic particle, +, - or neutral charge
mass about 200 x e- kg
mean lifetime 10-7 s
e+ = positive electron = +
positively charged electron
 = muon neutrino elementary particle without charge
spin = ½
mass is variable
 = muon antineutrino
e = electron neutrino
e = electron antineutrino
elements we see in the solar system today.
To do this, we need to focus on some geochemistry...
Review of Atomic Structure, and Radioactive Decay
Volume of the atom is mostly the electron cloud but mass is in nucleus.
Therefore, the nucleus mostly determines the basic properties of the atom.
Z = # of protons, or atomic number
in a neutral atom, Z = number of electrons
N = # of neutrons in the nucleus
A = mass # = Z + N
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nuclide = element with variable # neutrons
e.g. 12C has 6 protons and 6 neutrons
13
C has 6 protons and 7 neutrons
isotope = same Z, different N and A (54Fe, 56Fe)
isobar = same A, different Z and N (56Fe, 56Co)
isotone = same N, different Z and A (56Fe, 57Co, 58Ni)
You can make as many isotopes as you want, but they won’t all be stable!
Radioactive Decay
Most atomic nucleii are stable
264 stable nuclides out of > 2000 known nuclides
but >99.999% of all atoms are stable
Bi (Z = 83) is the most massive stable element
Release of particles results in alpha and beta radiation
Release of electromagnetic waves results in gamma radiation
Alpha decay Z = -2, A = -4
result is a new element with 2 fewer protons + neutrons
 particle is a 4He nucleus
235
Examples:
U 231Th + 
147
Sm  143Nd + 
Beta decay Z = +1, A = 0
net reaction is neutron + proton + electron + antineutrino
 particle = electron
234
Examples:
Th  234Pa + e- + 
87
Rb 87Sr + e- + 
Electron capture
Z = -1, A = 0
result is new element with 1 less proton, 1 more neutron
40
Example:
K + e- = 40Ar + 
Now let’s go back to thinking about the origin and evolution of our solar system...
now that we know the vocabulary, we can describe the origin of the universe very well. (This is
the subject matter of cosmology.)
About 15 billion years ago, any two points in the observable universe were arbitrarily close
together; the density of matter at that moment was infinite. From that “singularity”, about the
size of a dime, matter began.
Something occurred that flung all matter away from that point in all directions: the Big Bang.
At the initial instant, (10-43 seconds) density of matter was 1075 toms/km3 & about 0.01 cm across
A good analogy for ths initial expansion:
Suppose a bear removes a hive from a beehive to get at its honey.
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Bees will rush off in all directions
Any given bee will observe his neighbors to be moving away from each other.
Some bees will be angrier, and will fly faster.
Swarm of bees steadily spreads out.
At any given time, the fastest bees will be farthest away from the starting point.
So the velocity at which the bee travels must relate to the distance he travels.
Velocity (bee) = distance/ time elapsed since bear stole the hive
So, we can date the universe by calculating the time elapsed since any two galaxies (that are now
receding) were in contact.
Age = relative distance/relative velocity
H0 (Hubble constant at this time) = current rate of expansion of the universe
usually units of km/s/Mpc (megaparsecs)
reciprocal, 1/H0 = age of the universe
v (velocity) = H0 x d (distance)
Galaxies don’t obey this perfectly because:
1. There are gravitational tugs between them
2. Accuracies are limited by our ability to measure H0
We measure distances of objects we know from other methods
E.g., relationship between luminosity and distance
AND velocity may not be constant
I.e. space is curved
See figures of open vs. closed universe
Our universe as dominated by very energetic photons (-rays) and ’s, ±some p, n,  (mu mesons)
and e-‘s. These interacted with the radiation field that was present
The Big Bang – created H, 2H (D), 3He, and 4He by hydrogen fusion
fireball cools very quickly
p production occurs down to T = 7 x 1012 K
 occur down to T = 8 x 1011K (10 s after Big Bang)
e- can be made down to T = 4 x 109 K (= 4-10 s after Big Bang)
e- production is quenched below that T
Critical reactions that happen in the Big Bang include:
H + 1H 2H + e+ + 
H + 1H 3He + 
P+nD+
D + D 3He + n
D + D 3He + p
1
2
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H 3He + e- + e
p + p  D + e+ + e
D + n 3H + 
D + p 3He + 
3
He + 3He 4He + 2p + 
3
Proton-proton chain reactions involve fusion of H and He
2(p + p 2D + e+ + e)
2(2D + p  3He + )
3
He + 3He 4He + 2e+ + 2e
4p 4He + 2e+ + 2e
Notice that there’s nothing much heavier than He, though elements as high as 6Li and 7Be are
formed in small amounts
afterglow is cosmic microwave background radiation
Penzias and Wilson (1964) – Nobel Prize 1978!
Now there are lumps that form in the cosmic background
Giant Molecular Clouds (GMC’s)
total mass up to a million solar masses
diameter 50-300 ly across
dense (100-10,000 molecules per cc – still 105 x less than air in this room!)
cold (10-50K)
turbulent motions
threaded with magnetic fields
each galaxy contains about 1012 clouds that will become stars
Gas:
by number: 90% H, 9% He, 1% everything else
by mass: 74% H, 25% He
gases can be detected by spectroscopy, emission, and scattering
nearly 100 different molecules, with H, C, N, O, S etc.
Dust:
Very small, comparable to wavelength of visible and near-IR light
one particle per 200 m cube (1 grain per 1013 cm3)
Gels form larger molecules – atoms combine on grain surface
After 10,000,000 y, develops denser regions at core(cloud core)
Dense areas in each cloud become sites for gravitational collapse...
Cloud gets bigger and bigger...
Competing Phenomena: gravity, temperature/pressure, and magnetism
gravity: by Newton’s Law of Universal Gravitation, everything attracts
T & P: anything with T radiates, creating pressure
heat = molecular motion, which creates pressure
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i.e., hot surrounding plasma exerts compressive force
magnetic fields resist collapse, but large gravity can overcome it
Eventually the gas cloud gets too big! and you get gravitational collapse
Jeans Instability – “just enough” for gravity to be > T, P, and magnetic fields
material falls into the center of the core, pulled by gravity
material compresses, heats
gravitational energy converted to heat, density increases
Cloud starts out with angular momentum mostly in core (in form of turbulence), spinning slowly
as core collapses, it spins more rapidly (like an ice skater)
Because angular momentum must be conserved
“something big that’s spinning becomes something small spinning faster”
i.e., conservation of momentum!
If star is rotating, it has angular momentum, so the collapse forms a disk
“stuff at the top collapses toward center, stuff at edges moves out...”
non-equatorial orbits collide and cancel up/down motion, so you get a spinning disk!
Tends to flatten into a disk, at equatorial plane
Matter infalls, forms massive protostar in about 106 years
gas is 77% H, 21% He, and interstellar dust
distribution is non-uniform
mean density = 10-36 g/m3, or 1 H atom/m3
A DISK forms because centrifugal force opposes gravity in directions perpendicular to the
axis of rotation but doesn’t affect motion parallel to the rotation axis – so you get a disk rather
than a sphere!
Material in the disk migrates into a primitive solar nebula
Initially, all the angular momentum is in the center
inner materials transfer angular momentum outward in the disk
ultimately, the star itself is spinning very slowly
angular motion contained in the planets, which collide
planets have 99% of angular momentum
planets and sun form from same spinning disk
therefore, they all spin in the same direction!
Protostar forms in about 106 years
still heating up due to release of gravitational energy
density and radiation increase, and the collapse slows down
star has 99.9% of mass
reaches 7 million K, fusion ignites, you have a young star!
center of the cloud reaches the ignition point of nuclear reactions
star lights up (becomes luminous)
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All stars are self-luminous, fusion-powered, and form elements by nucleosynthesis
about 18 million degrees F
Stars use different fuels over their lifetimes, and live until their fuel supplies are exhausted.
Main sequence of stable H-burning begins (lasts for the lifetime of the star).
Initiates chain of nucleosynthesis (see below).
Every second, Sun converts 500 million metric tons of H to He, and 5 million metric tons of
excess material is converted into energy.
Condensation occurs in the cooling nebula:
Nebula contains "condensable" substances
Different compounds condense at different temperatures as it cools
Temperature affects the composition
"refractory compounds" melt last and condense first minerals >1500K
Fe/Ni alloy, Mg silicates, enstatite (MgSiO3) ~1400K
Na & K silicates, feldspars (Aluminum silicates)
chemical reactions: oxidizing Fe begins, formation of olivine <500K
carbons
ices (water, ammonia, methane) of H, O, C, N (1, 3_5 by abundance) 200_500 K
unshielded water ice only stable beyond asteroid belt
in terms of solid materials: ( by chart in 5_5 of Hartmann)
refractories
metal_rich dense silicates
silicate rocky material
most of that is familiar already, Earth, meteorites, moon...
below 1000, lower density silicates
carbonaceous silicates
dirty ice Accretion
All these particles slowly become planets
collide_and_stick (nearly parallel motions)
up to km_sized planetesimals, then in about 10000 years, 500_ 1000km
after that, smaller bodies fragment, larger bodies eat them
denser materials in the inner solar system form terrestrial bodies
less dense materials form cores of giant planets (abundant ice makes larger), then trap gas
dust grains are formed __ 0.001 mm (micron) blobs of minerals
dust grains begin to clump together, to 0.1 mm or so fluffy clumps (IDP)
as clumps grow, gravitational interactions grow more important
most pieces do not escape __ collisional accretion
gravitational collapse
Late stages
Gravitational interactions and pressure from the stellar wind sweeps clean
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Lots of craters are left as "sweeping" occurs
----------------------------------------Details of this process-------------------------------------------------Catalytic Carbon Cycle (occurs near 18 million K)
Gets C, N, and O from raw material of star
C + H  13N + 1.95 MeV
N  13C + + + e + 1.50 MeV
13
C + H  14N + 7.54 MeV
14
N + H  15O + 7.35 MeV
15
O 15N + + + e + 1.73 MeV
15
N + H  12C + 4He + 4.96 MeV
15
N + H  16O
16
O + H  17F
17
F  17O + + + e
17
O + H  14N + 4He
12
13
Notice that these reactions produce heat, so the star heats up.
T reaches 100 million K
Now you have enough energy to make heavier elements:
Alpha Process (alpha particles = He nuclei) forms elements up to 56Fe
C + 4He  16O + 
16
O + 4He  20Ne + 
20
Ne + 4He  24Mg + 
40
Ca + 4He  44Ti
44
Ca + 4He  48Ti
48
Ti + 4He  52Cr
52
Cr + 4He 56Fe
12
All these reactions release energy of some form; this helps keep the temperature inside the star
very high.
You run out of energy to make anything bigger that Fe (in fact the Fe-making reaction requires
energy rather than creating it).
Three remaining processes to make heavier elements:
S-process = slow neutron addition
H and He react with heavier elements in star:
1
H + 20Ne 21Na
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21
Na 21Ne + + + 
Now, if 21Ne encounters an alpha particle,
21
Ne + 4He 24Mg + n, creating a neutron
As soon as the neutron collides with another particle, it deposits its energy; if the target is a big
mass, it will absorb both the energy and the neutron.
So, you slowly add neutrons to unstable isotopes until eventually you decay to a stable isotope...
thus the name, slow neutron addition!
S-process accounts for 75% of the isotopes heavier that Fe
R-process
New isotopes form when stable isotopes are bombarded by a flood of neutrons created by
explosions within the supernova.
Adding neutrons to stable isotopes, creates unstable isotopes that don’t have time to release 
particles before being hit again, so they move up and left on the periodic chart, filling in
elements not created by the s process.
P -process
new isotopes created by flux of protons (rather than neutrons)
may be caused by flux of high energy photons
poorly understood!!!!
Eventually, the dissipation process caused the outermost disk to expand, and the innermost
part of the disk to condense to form the Sun
Sun evolves first.
Dust in the disk forms km sized bodies: planetesimals (104 y)
In inner regions of solar system, too hot for organic molecules or ice,
so you get only silicate grains aggregating > terrestrial planets
HIGHER density
In outer regions of the solar system, organics and ice grains condense.
Large planets (Saturn and Jupiter) gain large, dense cores,
can capture H and He from solar nebula by gravity
LOWER density
Planets forms by gradual accretion of solid material by collisions
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Interstellar grains are dust and ice 0.01 - 10 m in size
Caused by collisions between particles
Velocities must be <10 m/s (or they bounce off)
It takes 1000 orbits to grow to pebble size
It takes 300 million 10 km size objects to make Earth
It takes 4 b 10 km size objects to make the core of Jupiter!
Planets attain final masses by about 100,000,000 y.
As planetesimals gain mass through accretion of particle, their mutual gravitational attractions
cause lots of collisions:
Proto-Mars collides with proto-Earth, knocks off the Moon from the mantle
Some debris is still left from the first stage of accumulation: forms comets
By 1 b.y., debris is cleared from planetary zones
debris enters large gravitational spheres, and scatters planetesimals
(Esp. Jupiter and Saturn)
Comets and icy stuff get scattered in (volatiles on terrestrial planets; water on Moon)
Some rocky stuff gets scattered out by Earth, Mars
Asteroid belt between Jupiter and Saturn maybe never accumulated a planet due to disruption of
gravitational pull of Jupiter...
---------------------------------------------END gory
details...-------------------------------------------------
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All Cultures have Mythology Relating to the Sun...
Lugh – Celtic
born in a crystal tower, raised by the god of the sea, killed his grandfather
Re – Egpyt
creator, portrayed with a hawk head and a fiery disk on his head
one of two figures in illustration with an orange disk is him
the other figure is Harakhte, god of the heavens
Tonatiuh – Aztec
central Mexico
figure shows 4 cycles of creation/destruction (4 suns) on edges
central figure is Tonatiuh, the 5th sun
Huitzilopochtli – also Aztec
blue man with head decorated with hummingbird feathers
Apollo – Greek and Roman
son of Zeus
god of sun, logic, and reason
could fortell the future
also a musician and a healer
Apollo (at far left)
Maui - Polynesia
trickster hero of polynesian mythology
small but courageous
stole his wife from another man because he had a bigger penis...
Liza - west African Fon people
god of day, heat, work, and strength
Inti -Peru
represented at Machu Pichu in shadow clock
Malina - Inuit people of Greenland
the Moon is her brother Anningan, who raped her so she ran away
he’s forever chasing her, sometimes catches her (at eclipses)
10 Chinese suns
originally there was a 10 day week, one day for each sun
Sons argued, so their father sent an archer to scare them
the archer shot nine of them before running out of arrows – so only one sun remains
archer was banished to Earth to become a human
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The Story of Our Sun
made up of several layers; no distinct borders separating them
Core
center of the Sun
thermonuclear energy core
the only part of the sun that actually makes energy
about 1/4 of Sun’s radius
T = 18 million degrees F (10 millions K)
Radiative Zone
where most of the harmful gamma rays bounce around dissipating energy
until they become less harmful forms of energy
out to about 0.71 Sun radii
T = 9 million degrees F (2 million K)
T still hot enough for electrons and H nucleii to join into H atoms
photons are being absorbed, so at outer edges of zone, heat does not transfer out
Convection Zone
energy flow dominated by convection
this region is very dense, so it takes a long time for photons to travel through it
(about 170,000 yrs form energy to travel 696,000 km from core to surface)
Energy flows out at 50 cm/hours (20x slower than snail!)
solar material rises and falls due to heating and cooling
10,000 degrees F (5,810 K)
Photosphere (“sphere of light”)
what you see when you look at the Sun; most of the visible white light
also emits electromagnetic radiation at many other wavelengths
e.g., radio waves, UV, X-rays, and gamma rays
single, thin layer of gas
about 516 miles deep
heated from below by escaping photons, T decreases as you go outward
here the energy created by nuclear fusion escapes into space
dominated by absorption line spectra
T = 20,000 degrees F (11,366 K)
So hot that gas is a plasma; i.e., electrons no longer are bound to nucleii
gas made up of charged particles: protons and electrons
thus, easily influenced by strong solar magnetic fields that pass through it
granulation: blotchy pattern with 1000 km “granules” caused by convection of gas
granule size = Texas + Oklahoma
like a boiling pot of water
about 4000 granules at any given time
sunspots: irregularly-shaped regions, size 20-40,000 km (size of Earth)
Places where hot gases of photosphere are bathed in concentrated magnetic field
Last about 2 months
Solar flares occur in sunspot groups
used to help demonstrate that Sun does not rotate as a rigid body
rotation period at equator is 27 ½ days
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rotation period at poles is 33 days
locations vary on a 11 year cycle, produced by 22 years cycle in magnetic field
described by Babcock’s magnetic dynamo model
22 year cycle caused by convection and differential rotation
solar flares
sudden, localized transient increases in brightness that occur near sunspots
Mostly X-ray wavelength energy!
Chromosphere (“sphere of color”)
the lower part of the sun’s atmosphere
10-4 less dense than photosphere
visible only during a total solar eclipse, when the Moon blocks light from the
photosphere
dominated by emission line spectra
T average same as photosphere, though it INCREASES with altitude (opposite)
spicules = vertical spikes that are jets of gas,last only 15 minutes
may reach 4-9 thousand km high!
About 300,000 spicules at any given time
Corona
upper layer of sun’s atmosphere
non-uniform density
only 10-6 as bright as photosphere – about same as full moon
about 3 millions degrees F (1.67 million K) – 100x hotter than Sun’s surface!
Location of solar loops = immense coils of hot gas that move when hit by solar flares
heat comes from the friction of loops moving in a high viscosity coronal gas
(Just like brakes heat up on a car from friction!)
Coronal mass ejections: 1013 kg of material ejected into solar wind!
Those T’s do allow escape of some corona gas, which becomes solar wind
Our sun will die in about 5 billion years because it will heat up:
current T = 18 million degrees F
burn-up T = 180 million degrees F
Sun will expand to envelope Mercury and Venus, and vaporize Earth...
It’ll fuse 4He into larger atoms for a few years (100 million) until it runs out, during which it will
continue to shine.
Then it will become a white dwarf (shining but not producing heat), them a black dwarf (huge
black chunk of carbon floating in space...)
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Missions to the Sun
Pioneer 5 _ USA Solar Monitor _ (March 11, 1959)
Space probe is now in a solar orbit.
Pioneer 6 _ USA Solar Probe _ 63.4 kg _ (December 16, 1965 _ Present)
The Probe is still transmitting from solar orbit.
Pioneer 7 _ USA Solar Probe _ 63 kg _ (August 17, 1966 _ ?)
Solar_orbiting probe was recently turned off.
Pioneer 8 _ USA Solar Probe _ 63 kg _ (December 13, 1967 _ Present)
Solar probe is still transmitting from solar orbit.
Pioneer 9 _ USA Solar Probe _ 63 kg _ (November 8, 1968 _ March 3, 1987)
Still in solar orbit. Died on March 3, 1987.
Skylab _ USA Space Station _ (May 26, 1973)
Skylab, which was America's first space station, was manned for 171 days by three crews during
1973 and 1974. The space station included the Apollo Telescope Mount (ATM), which astronauts used to
take more than 150,000 images of the Sun. Skylab was abandoned in February 1974 and re_entered the
Earth's atmosphere in 1979.
Explorer 49 _ USA Solar Probe _ 328 kg _ (June 10, 1973)
Solar physics probe placed in lunar orbit.
Helios 1 _ USA & West Germany Solar Probe _ 370 kg _ (December 10, 1974 _ 1975)
Solar probe is in a solar orbit; came within 47 million kilometers of the Sun.
Solar Maximum Mission _ USA Solar Probe _ (February 14, 1980)
The Solar Maximum Mission (SMM) was designed to provide coordinated observations of solar
activity, in particular solar flares, during a period of maximum solar activity. The spacecraft suffered an
on_orbit failure. A repair mission on STS_41C in 1984, during which shuttle astronauts rendezvoused
with SMM, was successful. SMM collected data until Nov. 24, 1989, and re_entered on Dec. 2, 1989.
Yohkoh _ Japan/USA/England Solar Probe _ (August 31, 1991)
This spacecraft studied high_energy radiation from solar flares.
Helios 2 _ USA & West Germany Solar Probe _ (January 16, 1976)
Solar probe came within 43 million kilometers of the Sun.
Ulysses _ USA & Europe Solar Flyby _ 370 kg _ (October 6, 1990)
The Ulysses spacecraft is an international project to study the poles of the Sun and interstellar
space above and below the poles. It used Jupiter as a gravity assist to swing out of the ecliptic plane and
onward to the poles of the Sun. The Jupiter flyby was on February 8, 1992. The first solar polar passage
was in June 1994. The spacecraft passed the solar equator in February 1995 and passed over the north
pole in June 1995.
SOHO _ Europe Solar Probe _ (December 12, 1995)
The main scientific purpose of SOHO (Solar and Helispheric Observatory) is to study the Sun's
internal structure, by observing velocity oscillations and radiance variations, and to look at the physical
processes that form and heat the Sun's corona and that give rise to the solar wind, using imaging and
spectroscopic diagnosis of the plasma in the Sun's outer regions coupled with in_situ measurements of the
solar wind. SOHO will be put into a "halo orbit" around the L1 Lagrange point __ the point 1.5 million
kilometers (932,000 miles) away from us at which13
the gravitational pull of the Earth balances that of the
--------------------------------------------------------------------------------------------------------------------Sun.
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Genesis _ USA Solar Wind Sample Return _ 30 July 2001
The primary objective of the Genesis mission is to collect samples of solar wind particles and
NOW ON TO THE AGE OF THE SOLAR SYSTEM...
The energy, or heat produced by these reactions drives dynamic planets! And it gives us a way
to work out the age of the solar system:
Let’s take the system of 87Rb decaying to 86Sr
Half-life = 48.8 x 109 years
 = 1.42 x 10-11
All the dating techniques are based on the fact that a radioactive parent isotope (in this exercise,
87
Rb) decays to a stable daughter isotope (here 87Sr). These equations all derive from the basic
formula:
N=N0e-t,
where:
N = the number of atoms remaining,
N0 = the starting number of atoms, and
 = a decay constant.
So, this equation can be re-stated as:
D = D0 - N(et-1),
where D is the amount of daughter isotope present today,
N is again the amount of parent isotope present today,
and D0 is the amount of starting daughter.
For this exercise, we are interested in the reaction of 87Rb breaking down by radioactive
decay to form 87Sr. These isotopes are tricky to measure directly, but they are easy to measure
as ratios relative to another isotope, 86Sr. Thus, we have the equation:
This equation should remind you of the analogous equation for a straight line, which is:
y = b + xm
(same thing as y = mx + b)
In other words, to determine the age of this rock, all we have to do is determine m, which is the
slope of the line in a given plot, and from that we can solve for et - 1.
87
86
Sr  87 Sr 

0
Sr  86 Sr 
87
Rb  t
(e  1)
Sr
86
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