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Star Formation
• Our Sun formed billions of years ago
• We see evidence that star formation is a constant process
Star Formation
• Star formation starts with Dark Dust Clouds
Relatively Dense, but very cool
Cloud starts to contract under its own gravity
Contraction causes heating
Nuclear Reactions begin
• Many things oppose the gravitational contraction
– Heat
– Rotation
– Magnetism
Stage 1: Interstellar Cloud
• DDC contracts
– Very large (tens of parsecs)
– Very massive (thousands of Sun masses)
– Very cool (~10K)
• Internal cloud pressure initially balances against gravity
• Something happens for it to become unstable
– Star explosion, formation
• Cloud begins to fragment
Stage 2: Collapsing Cloud
• The fragmented cloud is about 100x larger than
our solar system
• It is still very cool because it radiates away its
heat out into space
• The very center is the warmest part ~100 K
• Eventually the cloud will contract enough
(becoming dense enough) where it will trap most
of the heat generated
Stage 3: Fragmentation Ceases
• At this point the cloud is the size of our solar
– The gas at the center is very dense
– Radiation cannot escape
– Temperature increases to ~10,000 K
– This region is called a Protostar
• Protostar is still contracting
– Internal pressure cannot yet counteract gravity
Stage 4: A Protostar
• Protostar’s temperature increases
with contraction
– Reaches ~1,000,000 K
– (10,000,000 K needed for nuclear ignition)
• Has a luminosity 1000 times
greater than our Sun
– No nuclear reactions
– Very Large
– Releasing gravitational potential
energy as it shrinks
– Can plot it on H-R diagram
Stage 4: A Protostar
• Protostar’s temperature increases
with contraction
– Reaches ~1,000,000 K
– (10,000,000 K needed for nuclear ignition)
• Has a luminosity 1000 times
greater than our Sun
– No nuclear reactions
– Very Large
– Releasing gravitational potential
energy as it shrinks
– Can plot it on H-R diagram
Stage 5: Protostar Evolution
• The Protostar is very hot –
the gas is at high pressure
• But it still isn’t enough to
counteract gravity
• The Protostar continues to
• This is called the “T Tauri”
Stage 5: Protostar Evolution
• The Protostar shrinks to
about 10X the size of the
• Central Temp = 5,000,000 K
– Not enough for fusion
• Contraction slows
– Harder for gravity to compress
hotter gas
Stages 1 – 5
Stage 6: Newborn Star
• 10 million years have passed
• The Protostar has shrunk
down to almost 1 solar radius
• Its central temperature
reaches 10,000,000 K
• Fusion of Hydrogen begins
• It is still not on the main
Stage 7: The Main Sequence
• For 30 – 50 more million years, the new star continues to
• Eventually gravity is exactly balanced by the outward
pressure of the Nuclear reactions
• The star also exactly balances the energy it produces by
fusion and the energy it radiates out
• The core is at 15,000,000 K and the surface is at 6000 K.
• It will stay like this for 10 billion years
• This takes long? Full grown in 4 months!
Other Mass Stars
• How do stars of other
masses evolve?
• The basic steps are the
same, but they will settle
on the main sequence at
different points.
• But then why is main
sequence so broad?
• Stars have slightly
different compositions
Failed Stars
• Some of the gas clouds that could form stars
don’t. Why not?
– They contract under gravity
– They get hot
– Where does it go wrong?
• The gravity crushes the cloud, but not enough to
ignite thermonuclear fusion
• There just isn’t enough mass to create a large
enough gravitational force
Failed Stars
• These failed stars are called Brown Dwarfs
• Brown dwarfs are less than 0.08 solar masses,
– (80 times the mass of Jupiter)
– The lower limit for Brown Dwarfs is 12 Jupiter
– Anything less massive is just called a planet
• These objects eventually become cold balls of gas
• We estimate there are 100 billion of these in our
Very High Mass Clouds
So the lower limit for stars is ~ 0.08 Solar Masses
Is there an upper limit?
We thought it was about 100 solar masses
There is so much gravitational force that the
nuclear fusion begins quickly
• Then there is so much nuclear fusion that it
overcomes the force of gravity holding the star
• These stars blow themselves up early in their
Very High Mass Clouds
• In 2010 Astronomers announced that they found
a star with a birth weight of about 320 solar
• Much larger than models predict could be stable
• Current mass is about 265 solar masses
Main Sequence Stars
• How do main
sequence stars
know how big to
• What properties
• Will the star always
remain this way?
PVT Thermostat
• Stars have P, V, T, , and
 values
–  is the rate of nuclear
• They have a selfregulating feature to
balance the internal and
external forces
• If  then Q  T  P  V     
Stellar Evolution
• Stars have only two significant regions during
their evolution
• Core – where the fusion is taking place
• Envelope – an inert layer of gas
– There is no convection between core and envelope
• As Hydrogen gets turned into Helium, the He will
be inert (not hot enough to fuse)
• Later in the star’s life it runs low on Hydrogen
Stellar Evolution
The Active Sun
• The Sun seems steady and predictable
– The luminosity (energy output) is nearly constant
• The surface activity of the Sun is changing and
– Doesn’t affect the evolution of the Sun
– Greatly affects the Earth
• Sunspots are dark areas on the Sun’s surface
• Typically about 10,000 km across
• Umbra – dark center
• Penumbra – grey surroundings
• Due to temperature change
• Sunspots appear randomly, in different numbers
• However, there is an 11 year cycle
• Sunspots are caused by magnetic field lines blocking
• The magnetic field is locally strong and breaks through the
• If this happens so we see it at the side of the Sun, it is a
The Sun and Earth
• These factors affect the climate of the Earth
• Luminosity actually decreases slightly when
sunspots are absent
• “Maunder minimum” was a period of solar
inactivity from 1645 – 1715
• May be linked to the “Little Ice Age” in Europe
• We haven’t been studying these things long
enough to conclude the effects yet!
The Sun and Earth
The Sun and Earth
The Active Sun
• Prominences eject hot solar
– They follow magnetic fields
– Gas cools and falls back into the
• Releases massive energy
– 1025 Joules
– 1 Billion years of Earth’s energy
The Active Sun
• Solar Flares are more
energetic than
• Caused by rapid release
of magnetic energy
• Equivalent to millions of
100-megaton bombs
• Gas breaks free of
magnetic field lines and
escapes into space
The Active Sun
• Coronal Mass Ejections are
a release of a “magnetic
bubble” of gas
• This gas interferes with the
normal solar wind and can
interact with the Earth’s
magnetic field
• Imparts a large amount of
energy and disrupts
Nuclear Fusion
• The Sun (and all other stars) convert mass directly into
energy according to Einstein’s equation:
E = mc2
Energy = Mass X (speed of light)2
• The mass of individual atoms is very small, and subatomic particles (protons, neutrons) is even less
• Usually measured in atomic mass units (u) instead of
Nuclear Fusion
• 1 atomic mass unit = 1.66053886 × 10-27 kilograms
• Converting 1 amu directly to energy:
• E = mc2 = (1.66x10-27 kg)(3x108 m/s)2 = 1.5 x10-10 J
• Doesn’t seem like a lot, does it?
• One kg of mass transformation equals 9x1016 J!
• The United States uses approximately 4,000,000 Giga
Watt hours per year. (1.5x1019 J)
• That is only 167 kg of mass turning into energy!
• Hiroshima bomb only turned 1 gram of mass into Energy
Nuclear Fusion
• Similar electrical charges repel
• Protons are both positively charged
• However, if they have enough energy they can
overcome the repulsive force
Nuclear Fusion
• Protons fuse to Deuterons, Deuterons fuse with protons
to form Helium-3, Helium-3 fuses to form Helium-4
Nuclear Fusion
• Mass of a Hydrogen atom (proton): 1.008178u
• Mass of Helium-4: 4.003976
• Does the mass of Helium-4 equal the sum of its
parts (4 Hydrogen atoms)?
4.032712 = 4.003976 ?
• No. The parts are more than the whole. Where
did the mass go?
Nuclear Fusion
• The missing amount of mass is called the Mass Defect
– The Mass Defect is converted directly to energy
• This energy represents the binding energy of the nucleus
• For low atomic masses, a larger nucleus is more stable
and therefore lower energy
– Fusion releases energy
• For large atomic masses a smaller nucleus is more stable
– Fission releases energy
Nuclear Fusion
• By looking at the luminosity of the Sun, we can
calculate that it gives off 3.9x1026 W
• The mass difference between He and 4 H is
0.028736 amu
• This equals an energy of 4.3x10-12 J
• At this rate 600 million tons of Hydrogen must be
converted into Helium every second
Stellar Evolution
• Because there is no convection
between the core and envelope,
there is no Hydrogen re-supply
• The Helium builds up in the core
• With less nuclear reactions:
The temperature goes down
The pressure goes down
Gravity starts to win vs. pressure
The star contracts
Stellar Evolution
• As the star contracts, it will
heat up
– (Gravitational PE conversion)
– Temp well above 10 Million K
– Not hot enough for HE fusion
• 100 Million K
• Hydrogen shell of the core
burns fast and furious
– Increased energy causes star to
“puff” outward
Stellar Evolution
Stage 8 Star
• The star’s surface
temperature drops
– Luminosity increases slightly
– Radius increases by ~3x
• On its way to becoming a
Red Giant
– Will take ~100 million years
– Envelope expands due to
increasing gas pressure while
core is shrinking
Stellar Evolution
Stage 9: The Red Giant Branch
• The star grows to about 100x its main sequence size
– The surface temperature stays constant
– Cooler surface is becoming opaque to interior radiation
– Luminosity is very high because of its size
• While the outer shell has grown, the core continues
to shrink
– It is mostly non-burning Helium “ash”
– No pressure to counteract gravity
– Temperature increases to above 100 Million K
Stellar Evolution
He  4 He  8 Be
Be  He  C
Stage 10: Helium Fusion
• Two Helium atoms collide and
create Beryllium-8
– Triple Alpha Reaction
• Beryllium-8 is very unstable
– Usually breaks apart in 10-12 s
• Because of high density in core, Be-8 will fuse with a Helium
nucleus to form Carbon-12
Stellar Evolution
The core is developing layers like an onion
Hydrogen is fusing in outer core shell
Helium is fusing in middle layer
Inert Carbon ash is
building up in its core
– Not hot enough to fuse
into another element
• Gravity is continuing to
compress the core
– The core is getting hotter
Stellar Evolution
• The core will shrink until it reaches “electron
– Ionized electrons are swimming around in the core
• At the degeneracy point gravity has contracted the
core so much that individual electrons “touch”
– At this point gravity can’t contract it any more
• The core is supported by this electron “pressure”
– Temperature continues to increase
Stellar Evolution
• The core temperature is rising
• The PVT Thermostat should result in a pressure
increase, increasing the volume of the star
• However the pressure is fixed
– Pressure of the electron degeneracy
– This breaks the PVT Thermostat
• Helium fusion is running rampant
– Dumps a lot of energy into the core: Helium Flash
Stellar Evolution
• Helium Flash is an explosion that expands the
star’s core outwards
• How can the explosion overcome gravity?
• The Helium Flash creates a lot of photons
– Results in a large “Radiation Pressure”
– This pressure overcomes gravity
• Core becomes “normal” again, pressure – gravity
equilibrium is re-established
Stellar Evolution
• Hydrogen and Helium
continue fusing
• As a result of the Helium flash
the core cools
• This reduces energy output
• It moves on the H-R diagram
• Where it goes depends on
• 20-30% escapes
Stellar Evolution
• The carbon core
continues to get larger
• Hydrogen and Helium
burning gets more intense
• Temperatures get hotter
and hotter
Stage 11
• Star expands back out to
the Giant Branch
Stellar Evolution
• Gravity cannot compress the carbon core enough
to raise the temperature enough to fuse Carbon
– 600 Million K required
• Pressure is so high that the electrons are
degenerate again
• Density is 1010 kg/m3
– A grape would weigh 1000 kg
• The big problem is that the core no longer
produces energy
Stellar Evolution
• Outer layers are expanding due to heat convection
from the core; the core is contracting
• As the core runs out of fuel, it heats up and blasts
out the envelope forming a Planetary Nebula
• The name has nothing to do with planets
Stages 13 and 14
• What is left at the center of the nebula is a White Dwarf
• It is super-massive, and hot only due to stored energy
– About the size of Earth
– Mass is about half that of the Sun
• Its heat will eventually dissipate away into space
• At that point it will be a black dwarf
• It is a cold, dark, massive portion of space
Death Path of a Star
Stellar Evolution
Higher Mass Stars
• The main difference between a star like our Sun
and a star of higher mass (~ 2.5 times greater) is
that they can produce higher temperatures
• More mass means greater gravity
• Greater gravity means greater core compression
• Greater compression means hotter temperatures
• Hotter temperature means easier fusion
Higher Mass Stars
• Above 2.5 solar masses helium fusion happens
easily, because core does not become degenerate
• There is no Helium Flash
• Above 8 solar masses the temperature of a
contracted core is hot enough to fuse Carbon (and
heavier elements)
Stellar Explosions
• Low mass stars end in
a fizzle
• Higher mass stars can
explode dramatically
– Novae
– Supernova Type I
– Supernova Type II
Stellar Explosions
• Need a white dwarf and Evolving Red Star
– Stars revolve around their common center of mass
• White dwarf draws mass
off of companion star
– Accretion disk forms
– Friction causes particles to
fall inwards
– Disk gets hot, gives off
Stellar Explosions
• As gas falls onto surface of white dwarf:
Becomes degenerate
Hydrogen starts to fuse
Heats gas up even more
Causes more reactions
So many photons are created that there is an explosion
• Accretion disk gets blown off of white dwarf
– Great increase in luminosity
– Can re-occur relatively quickly
Stellar Explosions
Supernova Type I
• These detonations are “hydrogen poor”
• The process is essentially the same as a Nova
• Chandrasekhar Limit is the difference
– 1.4 Solar masses
– Carbon Fusion begins instantly, everywhere
– NOTE: Collapse of Core due to gravity
– Star detonates
– No remnants are left behind
Stellar Explosions
Supernova Type II
• Stars with mass greater than ~8 solar masses
• Star evolves as usual
• The “onion” layers inside
the star build to Iron
• Due to high mass, fusion of
these heavy elements is
Stellar Explosions
Fusion Energy Release
• Energy is released by fusing atoms as long as mass is
converted to energy
• From hydrogen to iron, the
mass per nucleon decreases
as atomic number increases
• Above iron, mass per
nucleon decreases
• Fusion cannot continue
Nuclear Binding Energy
Stellar Explosions
Making heavy elements
• How do the plentiful elements heavier than iron form if
fusion is an endothermic process at that point?
• Neutrons are a byproduct of
other nuclear reactions
• Neutrons (no electric charge)
are absorbed by a heavy
• Creates an isotope
Stellar Explosions
Neutron Capture
• The isotopes created are usually unstable and decay in
time (minutes to years) – Beta decay
• When this happens in
evolving stars, the process is
relatively slow so it is called
the s-process
• This explains the synthesis
of non-radioactive elements
Stellar Explosions
Back to the Type II
• As the high mass star runs out of fuel, the large
(3X+ the mass of the Sun) core collapses under
the force of gravity
• Core shrinks  heats up  becomes degenerate
• Core gets so hot that photons get enough energy
to destroy an iron nucleus
• The core is now a soup of protons, neutrons, and
Stellar Explosions
Photoelectron Dissociation
• Electrons in this soup are moving near the speed
of light
• Fast enough to smash into a proton and result in a
neutron (Reverse Beta decay)
• Neutrinos are created in this decay, and there are
so many they blast off the envelope of the star
Stellar Explosions
• The blast can be a billion times brighter than our Sun
• It will outshine the galaxy it is in
• In 1054 Chinese Astronomers witness the Supernova
explosion we know as the Crab Nebula
• It outshone Venus
• It could be seen in the day
• This lasts for ~ a month
Stellar Explosions
• What happens to the star
after the supernova
explosion? Is there
anything left?
• In a Type II Supernova
just the envelope is blown
off, the core remains
• What that core becomes
will depend on its mass
Stellar Explosions
• If the remaining core is
between 1.4 and ~3 solar
masses, the neutrons are
compressed to the
• The result is a Neutron Star
• They are approximately 10 –
20 km in diameter
• One teaspoon would weigh
a billion tons!
Black Hole Sun
• A single off of Soundgarden’s ’94 album Superunknown
• If the star has a core with a mass greater than about 3
solar masses, the Supernova remnant may become a
Black Hole
• Above about 3 solar masses gravity crushes the degeneracy
pressure and the core collapses to infinite density
• This will make the gravity immediately near the star huge
g 2
Black Holes
• By compressing a large
amount of mass into such
a small space, the gravity
is so large that:
– Light passing by is
“gravitationally Doppler
– Space time is significantly
– The escape velocity is
faster than the speed of
Black Holes
• Schwarzschild Radius: how small you would have to
squeeze a mass down to for it to become a black hole
rs  2
• Event Horizon: the point near the black hole where you
cannot escape its gravitational pull
– You cannot see “events” beyond this horizon because the light
cannot make it to you
Black Holes
If light can’t escape a black hole, then how do we find one?
• We have to look for the gravitational effects:
– Look for a binary star system where you can only see
one visible object
– Look for a binary star system where one star is
accreting mass to the other one
– Measure the effect on other nearby stars
– Look for binaries that are black hole-black hole or
black hole-neutron star
Chapter 16 Test
10 Multiple Choice
Some from text
5 Free Response
Answer 4
The mass of a single Hydrogen atom is 1.008178 amu.
The mass of one Helium atom is 4.003976 amu. Four
Hydrogen atoms are fused together to produce one
Helium atom.
How much energy is released by the fusion of 4 H atoms into
1 He atom?
The Sun produces 3.9 x 1026 Joules of energy every second
by converting Hydrogen to Helium. If an oil tanker carries
4.6x1033 Hydrogen atoms in its oil, then how many tankers
would the Sun need to consume every second?
Chapter 16 Test
• Nuclear reactions occur within the core of a star at
tremendous temperatures and pressures. Explain what
occurs in the core on an atomic level. What happens to
an atom made up of many protons, neutrons, and
electrons? What causes the particles to fuse together?
How is energy released and where does it come from?
• Describe the properties of the Radiative Zone of the Sun.
This zone is cooler than the core, how does this affect
the atoms found there? How is energy transmitted
through this zone?
Chapter 16 Test
• Describe what happens in the Convective Zone of
the Sun. What is the dominant form of heat
transfer (and describe its properties)? How
quickly does energy transfer through this zone?
• What is the significance and main properties of
the Photosphere? What is the main method of
energy transport?