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Life Cycles of Stars
The Hertzsprung-Russell Diagram
How Stars
• Collapsing gas
and dust cloud
• Protostar mostly infrared
Main Sequence Stars
• Brown Dwarf (L, T, Y)
• Red Dwarf (M)
• Normal Star (O, B, A, F, G, K)
All Objects Exist Because of a Balance
Between Gravity and Some Other
People, Planets-Interatomic Forces
Normal Stars-Radiation
White Dwarfs-Electron Repulsion
Neutron Stars-Nuclear Forces
Quark Stars?
Black Holes-No Known Force
Mass, Luminosity, Lifetime
Luminosity = Mass3.5 (Solar Units)
Lifetime = Mass/Luminosity = 1/Mass2.5
Mass = .1: Lifetime = 316 (3160 b.y.)
Mass = .5: Lifetime = 5.7 (57 b.y.)
Mass = 1: Lifetime = 1 (10 b.y.)
Mass = 10: Lifetime = .003 (3 m.y)
Mass = 50: Lifetime = .000057 (570,000 yr)
Before Stars Form
• Pre-stellar cores
• Protostars
• Pre-main sequence star (PMS)
– Planet system formation.
Protostars or Young Stellar Objects
• Class 0 (T <70K) Emits in microwave range
because of opaque surrounding cloud
• Class I (T = 70-650K) Emits in infrared. Star still
invisible but can detect warm material around
• Class II (T = 650-2880K) T Tauri stars. Massive
expulsion of material
• Class III(T > 2880K) PMS stars
Early Stars and Planets
(Class 0) Early main accretion phase
(Class I) Late accretion phase
(Class II) PMS stars with protoplanetary disks
(Class III) PMS stars with debris disks
Super-Massive Stars
• Stars beyond a certain limit radiate so much
that they expel their outer layers
• W stars (Wolf-Rayet stars) are doing this: T
Tauri on steroids
• Upper limit about 100 solar masses
• More massive stars can form by merger but
don’t last long
How Stars Die
• Main Sequence Stars Brighten With Age
• The More Massive a Star, the Faster it Uses
• Giant Phase
• White Dwarf
• Supernova
– Neutron Star - Pulsar
– Black Hole
Leaving the Main Sequence
Helium accumulates in core of star
Fusion shuts down
Star begins to contract under gravity
Core becomes denser and hotter
Nuclear fusion resumes around helium core
Outer layers puff up enormously but cool
• Star becomes redder and larger (Red Giant)
hard, die
leave a
off to the
Later Lives of Giants
• Inert helium core begins to fuse helium to
carbon and oxygen
• Contraction of core stops
• Outer envelope contracts and heats up
• Red Giant becomes Yellow Giant
• Helium core runs out of fuel
• Helium fusion shell on outside of core,
hydrogen fusion above
• Star loops between red and yellow on H-R plot
Making the Elements
Heavy nuclei: Energy from Fission
Light Nuclei: Energy from Fusion
Both end at Iron: Most stable nucleus
Stars can generate H-Fe through Fusion
How do we get beyond Fe?
Two processes
– S-Process (Slow) in Red Giants
– R-Process (Rapid!) in Supernovae
Beyond Helium
He + particle = mass 5: not stable
He + He = Mass 8: Not Stable
The Mass 5-8 Bottleneck
Sometimes three He collide to make C
Li, Be, B rare in Universe
– Destroyed in Stars
– Created by spallation - knocking pieces off heavier
Iron and Beyond
• Build from C to Fe by fusing successively
heavier atoms
• Can’t Build Beyond Fe by Adding Protons
– Repulsion of nuclei = Charge1 x Charge2
– He + C = O: Repulsion = 2 x 6 = 12
– Fe + p = Co: Repulsion = 26 x 1 = 26
• Can Add Neutrons Until Atoms Become
– n  p + e (Beta Decay)
The S-Process
Building Atoms
The R-Process
• There are nuclei the s-process can’t make
– The process is slow
– Precursors break down before next neutron hits
– Stops at Bi and Pb. Where do U and Th come
• The r-process piles neutrons on faster than
atoms can decay
• Occurs in Supernovae
The End Fate of Medium-Size Stars
• Core reaches limits of its ability to sustain
• Fusion shells sputter and become unstable
• Star expels outermost layers as Planetary
• Inert core left as white dwarf
• Dwarf has such tiny surface area it takes
billions of years to cool
• Coolest (oldest?) known: 3900 K
Tiny Stars
• Red Dwarfs are tiny but have huge sunspots
and violent flares
• They have convection throughout their
• Interiors uniform in composition
• Do not accumulate helium in core
• Can use much more of their hydrogen up
• Never fuse He to C
• Lifetimes longer than age of Universe
Exploding Stars
• Nova
– White dwarf attracts matter from neighboring star
– Nuclear fusion resumes on surface of star
– Many novae repeat at decade or longer intervals
• Type I Supernova
– White dwarf attracts matter from neighboring star
– White dwarf core resumes fusion
• Type II Supernova
– Collapse of massive single star
Shell Structure of Massive Star
4H –> He
3He –> C
He + C –> O, Ne
Ne + He, C –>
O, Mg
• 2O –> Si
• 2Si –> Fe
Core Collapse
• Fe core collapses to neutron star in
• Remaining star material falls in at up to 0.1c
• Nuclei beyond Plutonium created
• Star blows off outer layers
• We see the thermonuclear core of the star
• Much of the light is from radioactive nickel
Historical Supernovae
185 - Chinese
1006 - Chinese, one European record
1054 - Chinese, European, Anasazi?
1572 - Tycho’s Star
1604 - Kepler’s Star
1885 – Andromeda Galaxy
1987 - Small Magellanic Cloud
(170,000 l.y.)
Remains of SN 1054 (Crab Nebula)
Life (Briefly!) Near a Supernova
• Sun’s Energy Output = 90 billion
• Let’s relate that to human scales. What
would that be at one kilometer distance?
• 90 x 1015 tons/(150 x 106km)2 = 4 tons
• Picture a truckload of explosives a km away
giving off a one-second burst of heat and
light to rival the Sun
Now Assume the Sun Goes Supernova
• Brightens by 10 billion times
– 1010 = 25 magnitudes
• Our 4 tons of explosive becomes 40,000
• Equivalent to entire Earth’s nuclear arsenal
going off one km away - every second
• This energy output would last for days
Neutron Stars and Pulsars
• Mass of sun but diameter of a few km
• Rotate at high speed
– Sun 1,400,000 km –> 10 km
– Rotation speeds up 140,000 x
– 28 days –> 17 seconds
• Pulsars: infalling matter emits jets of radiation
• Millisecond pulsars: probably “spun up” by
accretion, or merger of neutron stars
How a Pulsar Works
Black Holes
• Singularity: gravity but no size
• Event horizon (Schwarzschild radius): no
information can escape
• Detectable from infalling matter, which emits
• Quantum (atom-sized) black holes may exist
• Cores of galaxies have supermassive black
Black Hole
Probably Not
Planetary Systems
Protoplanetary Disks
Accretion of Planets
Expulsion and Migration of Planets
About 400 extrasolar planets known
Our Solar System may be unusual?
Protoplanetary Disks in Orion