Download Image Credit: NASA,ESA, HEIC, Hubble

The Most Astonishing Fact About the
Universe: The Life Cycle of Stars
The Sun
Andrew Rivers,
Northwestern University
Range of star properties dramatized: Which is the closest star in this picture?
Alpha Centauri and companions Beta and Proxima Centauri. The stars are at
similar distances; Proxima is closest, but is M-class and therefore far fainter.
Where does a star’s energy
come from?
• How much energy?
– 3.901026 Watts for the sun
– Largest earth power plants=5 109 Watts.
• Possibilities
– Chemical reactions?
• Most efficient: 2H + O = H20 +energy.
• Sun would last 18,000 years.
– Gravitational “settling”.
• If Sun still contracting, convert PE to KE to light
• 30 million years worth of energy!
• Contradicts biology, geology, astronomy
Solution: Nuclear Fusion
• Energy by combining light nuclei like
hydrogen to make heavy nuclei.
– In the sun 4 Hydrogen nuclei are fused into a
helium nucleus: Efficiency 0.08%
• Lifetime 10 billion years= O.K.
Why is fusion of two nuclei difficult?
long range, repulsive
Coulomb force
Short range,
attractive strong
nuclear force
Charged nuclei must get close enough, then SN
force can overcome Coulomb repulsion
How do two positive nuclei get close
enough for fusion?
Like charges repel
If traveling fast enough,
they get close
Attractive SN force
overcomes repulsive
Coulomb force
Foundation Principle 1: Power from Fusion
P-P chain, Fusion in Main Sequence stars
http://www.cartage.org.lb/en/themes/sciences/Astronomy/
Binding Energy Curve
Iron (Fe) most
stable element
As more nucleons
are in nucleus, they
are more tightly
bound by SN force
Energy released
through Fusion
Foundation Principle 2: Hydrostatic Equilibrium Balance
Fusion in core =
outward pressure
http://www.cartage.org.lb/en/themes/sciences/Astronomy/
Image credit: NASA / CXC / M. Weiss.
Hydrostatic Equilibrium: outward photon pressure from fusion reactions
in core balances inward gravitational pressure.
Mass Luminosity Law
Principle: Hydrostatic Balance
Principle: Fusion in Core
Larger mass stars have greater
internal pressure from gravity
Fusion rate must be greater for more
massive stars in order to balance
greater inward gravity pressure.
More massive stars have a greater
fusion burn rate (luminosity)
Why are more massive stars brighter?
Conclusion: Though more massive
stars have more nuclear fuel (M),
they have greater burn rates (L) and
therefore have shorter lifetimes
Tool of the Trade: Hertzsprung-Russell Diagram
Purpose: classify stars, explore stellar evolution
Where do we expect to find
stars on this plot? Anywhere?
In some places and not others?
Regions of the HR diagram, from
http://zebu.uoregon.edu/~soper/Stars/hrdiagram.html
Recall: Stars are approximate Blackbodies
A Star’s color depends on its surface temperature. Cooler=redder, hotter=bluer
A cool, bright star
Some faint, hot stars
HR diagram “scatter-plot of the nearest stars.
http://www.vb-tech.co.za/astronomy/From http://zebu.uoregon.edu/~soper/Stars/hrdiagram.html
25 closest stars, from
“O” Stars
“M” Stars
Physical Basis for Stellar
Evolution
• Hydrostatic equilibrium
– Inward gravitational pressure and outward
pressure (usually radiation pressure) must balance
in any stable star
– Gravity, though the weakest force is always
attractive and omnipresent.
• If there is no outward pressure, the star must collapse.
• “The War Against Gravity”
Why should stars have a “life
cycle”?
• Only set amount of Hydrogen gas to use in
nuclear fusion.
– Must find some other way to counteract gravitational
pressure
• Initial mass determines how quickly fuel will burn
(The luminosity) to maintain equilibrium
– L~M3.5 More massive stars, even with more fuel to
start should burn quicker
Life-story of a Sun-like Star, Stage 1: Formation
from a molecular cloud/solar nebula
Stage 4
Stage 1
Molecular cloud
Stage 2
Stage 3
Protostar
Main-sequence
Red giant
Stage 5
Stage 6
Planetary
nebula
White dwarf
• Raw material for star formation strewn
throughout the galaxy
– Giant Molecular Clouds. Cool clouds of H2
(molecular hydrogen) and some CO.
– When do they collapse?
• Pressure wave from Supernova or or collision between
clouds or other compressing event..
• Gas must be slow moving (little KE) to collapse
Observation: Dust in Molecular Clouds Blocks Visible Radiation
Reddening: red wavelengths
pass through dust more
readily than blue.
Star forming regions often
cold (easier to collapse)
therefore they do not radiate
visible light (dark clouds).
Molecular Cloud Barnard 68
Observation: Molecular Clouds from
the Spitzer Infrared Space Telescope
Optical
Longer wavelength IR not blocked by dust
How Can We See Molecular Clouds?
The Observation: Carbon Monoxide emission line in the Milky Way
Emission Line Spectrum
The Physics:
Molecules have
rotational transition,
releasing radio
waves
Life Story Stage 2: Protostar
Stage 1
Molecular cloud
Stage 2
Protostar
• Fusion has not yet begun, but clump of gas has
condensed.
– Radiates in red and infrared (Temperature is 2000 to
3000K)
– Optical light is blocked by dust
– Infrared gets through (wavelength large compared to
size of dust grains.
– Protostars only around for short time (few million
years)
Protostar stage: forming planets
Proto-planet accretion
To final solar system
Proto-planets
Condensing into
dust particles which
build up
Collapsing gas &
dust cloud
“pancaking”
Artist’s rendition of a forming proto-planetary disk with newborn planets
Life Story, Stage 3: The Main Sequence
Stage 1
Molecular cloud
Stage 2
Stage 3
Protostar
Main-sequence
• As cloud collapses, temp rises in the core until
fusion is possible
– When fusion “turns on” the protostar becomes a
star.
– The stars stay on the main sequence for 90% of
their lifetimes
– Stars form in groups (open clusters)
Q: What do all stars on
the main sequence
have in common?
A: PP-Chain. All are
burning Hydrogen into
Helium in their cores.
NGC 3603 Star Birth Sequence
Q: Why are the stars we
see in this newborn
cluster all blue?
Consider the Blackbody
spectrum. Which stars are
we biased to see?
Sun Formation Life-story told by HR diagram
Evolution of the sun onto the Main Sequence
Pleiades
Image Credit & Copyright: Rogelio Bernal Andreo
Q: Which cluster is older?
Hyades
HR Diagram of Pleiades Cluster
Bright, blue O stars
Faint, red M stars
Sun Life Story Stage 4: Red Giant Expansion
Stage 1
Molecular cloud
Stage 2
Stage 3
Protostar
Main-sequence
Stage 4
Red giant
• Star runs out of hydrogen fuel in
core
– Gravity doesn’t go away
– Star collapses and heats up
– Core inert, Shell burning begins
Artist’s rendition of Earth’s future
with red giant Sun
• Fusion is rapid because the shell layer is
still compressing
• Luminosity of star increases
• Radius of star increases, becoming giant
Shell Hydrogen
burning after core of
Hydrogen has been
exhausted.
New source of fusion means the
core of the star becomes hotter
causing the star size to grow.
• As core temp rises, Helium atoms eventually
reach speeds required for Helium fusion
• Sudden onset  Helium flash
– Extra outward pressure from the core star expands
– Hydrostatic equilibrium means that the star will then
contract
– Variable stars: the pulsations are not damped, but
periodic.
Hydrogen shell burning
Temp rises, Helium
ignites in core
Triple Helium fusion process
Q: Why is He fusion
harder, than H fusion,
requiring greater
temperatures?
Life Story, Stage 5: Red Giant rundown
Stage 1
Molecular cloud
Stage 2
Stage 3
Protostar
Main-sequence
Stage 4
Red giant
Stage 5
Planetary
nebula
• What happens when no more Helium?
– Star center compresses, Burn Carbon + Helium= Oxygen
• Planetary Nebula
– Low mass stars  the increased number of photons
from core push on outer carbon/silicon in layers 
puff off the star
– Much of mass returned to Interstellar Medium and is
heated by the star
Ring Nebula as seen by the
Hubble Space Telescope.
Image Credit: NASA, ESA, and C.R. O'Dell (Vanderbilt
University)
Cats Eye Nebula
Image Credit: NASA,ESA, HEIC, Hubble Heritage Team
Pictorial story of the evolution of Sun from proto-star birth to white dwarf cool-down.
Story of the sun as told on an HR diagram.
Note that the longest time in the normal
evolution is on the Main Sequence
An HR diagram story of the evolution of the sun after the main sequence.
Theoretical evolution of a star cluster: Birth to Main Sequence
Theoretical evolution of a star cluster: Main Sequence to Red Giants
Theoretical evolution of a star cluster: Red Giants and the Turn-off Point
Stage 4B: The Alternate Ending
Red Giant rundown, high mass
Stage 2
Molecular cloud
Protostar
Stage 3
MS Blue Giant
• Running out of fuel
Red giant
– Star center compresses
– Burn Carbon + Helium to get Oxygen
– Shell burning with other reactions
• Reactions become less efficient.
– Hydrogen burning most efficient
– Other reactions less so
– Iron=most stable element, no energy available
• But Gravity remains!
Shell burning in a star at the end of its days (
Freedman Geller & Kaufmann, Universe)
Recall: Binding Energy Curve
Iron (Fe) most
stable element
As more nucleons
are in nucleus,
they are more
tightly bound by
SN force
Energy released
through Fusion
Less tightly bound as
nuclei get bigger,
short range SN not as
effective
Image credit: Wikipedia user Sakurambo.
Supernova
Molecular cloud
Stage 4
Stage 2
Protostar
Stage 3
MS Blue Giant
Red giant
Supernova
• Iron core No more reactions can produce
energy to hold out core
• Star begins to collapse due to gravity
• What stops the collapse?
– Electron degeneracy pressure
– Electrons resist when we try to place them in the same
place (not the same thing as electrostatic repulsion)
– As soon as the collapsing core reaches the density
where electrons “see” each other, the star becomes
stable and stops collapsing
• Inner layers are still moving inward
and hit the “solid wall” of the new
white dwarf and….
• Bounce!
• The supernova emits as much energy
per second as all stars in the galaxy
combined (for awhile).
Supernova 1987A: Closest supernova during
age of telescopes (in Large Magellanic Cloud)
Before…..
After!
Supernova 1994D outshines
its entire host galaxy
Life Story end-game: White Dwarf stars
Stage 4
Stage 1
Molecular cloud
Stage 2
Stage 3
Protostar
Main-sequence
Red giant
Stage 5
Stage 6
Planetary
nebula
White dwarf
• A white dwarf star remains after the supernova
and as a product of evolution of low mass stars
• Theoretical calculations  the maximum white
dwarf = 1.4Msun
– Chandrasekar mass
• What then?
The Old open cluster NGC 6971. The current HR
diagram reveals age and stages of evolution.
Fighting the War Against
Gravity
• Normal Stars
– Fusion Hydrogen  Helium +energy. Outward pressure
from escaping photons balances gravity
• Massive stars, after hydrogen
– Fusion of heavier elements, pressure is balanced same as
above
– Iron is the most stable, no energy available to maintain
equilibrium through fusion
• White Dwarf stars
– Gravitational force balanced by “electron degeneracy
pressure”
Degeneracy pressure
• Fermions (electrons,
protons, neutrons) cannot
occupy the same energy
level
• If all available energy states
are filled, an electron must
go to a higher energy level
– this takes work! The gas
resists compression
A White Dwarf Star
Summary: Life-story of Stars
Stage 1
Molecular cloud
Stage 2
Stage 3
Protostar
Main-sequence
Life story of sun told as a track
on the H-R diagram
Stage 4
Red giant
Stage 5
Stage 6
Planetary
nebula
White dwarf
High mass stars have different HR tracks and can make
heavier elements before going supernova.
What is this picture?
Jocelyn Bell
Burnell, first to
notice a radio
source, identified
as a pulsar
Fast rotating neutron star generates
strong magnetic field and a “flashlight
beamed” periodic source of radio
waves, first known as LGM’s
Pulsars can rotate at a rate of
approx 1000 times per second
Alternative Ending: Neutron Stars
A rapidly rotating neutron star lies at the heart of the crab nebula
The Stellar
Evolution Cycle
Summary of star evolution stages
Grand Unification
We are
stardust!
All heavy elements are cooked in stars
Spread
New stars
through
formgalaxy
from the
by rich
supernovae
ashes