Protostars or Young Stellar Objects (YSO's) ... 'Live hard

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Life Cycles of Stars
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The Hertzsprung-Russell Diagram
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(No Transcript)
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How Stars Form

• Collapsing gas and dust cloud
• Protostar - mostly infrared

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Main Sequence Stars

• Brown Dwarf (L, T, Y)
• Red Dwarf (M)
• Normal Star (O, B, A, F, G, K)

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All Objects Exist Because of a Balance Between
Gravity and Some Other Force

• People, Planets-Interatomic Forces
• Normal Stars-Radiation
• White Dwarfs-Electron Repulsion
• Neutron Stars-Nuclear Forces
• Quark Stars?
• Black Holes-No Known Force

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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)

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Mass, Luminosity, Lifetime
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Before Stars Form

• Pre-stellar cores
• Protostars
• Pre-main sequence star (PMS)
• Planet system formation. 

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Protostars or Young Stellar Objects (YSOs)

• Class 0 (T lt70K) 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 it.
• Class II (T 650-2880K) T Tauri stars. Massive
  expulsion of material       
• Class III(T gt 2880K) PMS stars

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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

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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 dont
  last long

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Wolf-Rayet Star
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How Stars Die

• Main Sequence Stars Brighten With Age
• The More Massive a Star, the Faster it Uses Fuel
• Giant Phase
• White Dwarf
• Supernova
• Neutron Star - Pulsar
• Black Hole

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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 down
• Star becomes redder and larger (Red Giant)

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Live hard, die young, leave a good looking
corpse
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Peeling off to the Giant Phase
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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

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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

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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 atoms

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Iron and Beyond

• Build from C to Fe by fusing successively heavier
  atoms
• Cant 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 Unstable
• n ? p e (Beta Decay)

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The S-Process
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Building Atoms
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The R-Process

• There are nuclei the s-process cant make
• The process is slow
• Precursors break down before next neutron hits
• Stops at Bi and Pb. Where do U and Th come from?
• The r-process piles neutrons on faster than atoms
  can decay
• Occurs in Supernovae

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The End Fate of Medium-Size Stars

• Core reaches limits of its ability to sustain
  fusion
• Fusion shells sputter and become unstable
• Star expels outermost layers as Planetary Nebulae
• Inert core left as white dwarf
• Dwarf has such tiny surface area it takes
  billions of years to cool
• Coolest (oldest?) known 3900 K

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Tiny Stars

• Red Dwarfs are tiny but have huge sunspots and
  violent flares
• They have convection throughout their interiors
• 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

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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

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Shell Structure of Massive Star

• 4H gt He
• 3He gt C
• He C gt O, Ne
• Ne He, C gt O, Mg
• 2O gt Si
• 2Si gt Fe

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Core Collapse

• Fe core collapses to neutron star in milliseconds
• 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

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Historical Supernovae

• 185 - Chinese
• 1006 - Chinese, one European record
• 1054 - Chinese, European, Anasazi?
• 1572 - Tychos Star
• 1604 - Keplers Star
• 1885 Andromeda Galaxy
• 1987 - Small Magellanic Cloud (170,000 l.y.)

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Remains of SN 1054 (Crab Nebula)
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Life (Briefly!) Near a Supernova

• Suns Energy Output 90 billion megatons/second
• Lets 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

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Now Assume the Sun Goes Supernova

• Brightens by 10 billion times
• 1010 25 magnitudes
• Our 4 tons of explosive becomes 40,000 megatons
• Equivalent to entire Earths nuclear arsenal
  going off one km away - every second
• This energy output would last for days

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Neutron Stars and Pulsars

• Mass of sun but diameter of a few km
• Rotate at high speed
• Sun 1,400,000 km gt 10 km
• Rotation speeds up 140,000 x
• 28 days gt 17 seconds
• Pulsars infalling matter emits jets of radiation
• Millisecond pulsars probably spun up by
  accretion, or merger of neutron stars

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How a Pulsar Works
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Black Holes

• Singularity gravity but no size
• Event horizon (Schwarzschild radius) no
  information can escape
• Detectable from infalling matter, which emits
  X-rays
• Quantum (atom-sized) black holes may exist
• Cores of galaxies have supermassive black holes

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Black Hole
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Probably Not
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Planetary Systems

• Protoplanetary Disks
• Accretion of Planets
• Expulsion and Migration of Planets
• About 400 extrasolar planets known
• Our Solar System may be unusual?

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Protoplanetary Disks in Orion