Death of Stars

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Death of Stars
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Why Do Stars Leave the Main Sequence?

• Running out of fuel

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Stage 8 Hydrogen Shell Burning

• Cooler core ? imbalance between pressure and
  gravity ? core shrinks
• hydrogen shell generates energy too fast ? outer
  layers heat up ? star expands
• Luminosity increases
• Duration 100 million years
• Size several Suns

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Stage 9 The Red Giant Stage

• Luminosity huge ( 100 Suns)
• Surface Temperature lower
• Core Temperature higher
• Size 70 Suns (orbit of Mercury)

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Lifecycle

• Lifecycle of a main sequence G star
• Most time is spent on the main-sequence (normal
  star)

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The Helium Flash and Stage 10

• The core becomes hot and dense enough to overcome
  the barrier to fusing helium into carbon
• Initial explosion followed by steady (but rapid)
  fusion of helium into carbon
• Lasts 50 million years
• Temperature 200 million K (core) to 5000 K
  (surface)
• Size 10 ? the Sun

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Stage 11

• Helium burning continues
• Carbon ash at the core forms, and the star
  becomes a Red Supergiant

• Duration 10 thousand years
• Central Temperature 250 million K
• Size gt orbit of Mars

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Deep Sky Objects Globular Clusters

• Classic example Great Hercules Cluster (M13)
• Spherical clusters
• may contain
• millions of stars
• Old stars
• Great tool to study
• stellar life cycle

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Observing Stellar Evolution by studying Globular
Cluster HR diagrams

• Plot stars in globular clusters in
  Hertzsprung-Russel diagram
• Different clusters have different age
• Observe stellar evolution by looking at stars of
  same age but different mass
• Deduce age of cluster by noticing which stars
  have left main sequence already

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Catching Stellar Evolution red-handed
Main-sequence turnoff
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Type of Death depends on Mass

• Light stars like the Sun end up as White Dwarfs
• Massive stars (more than 8 solar masses) end up
  as Neutron Stars
• Very massive stars (more than 25 solar masses)
  end up as Black Holes

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Reason for Death depends on Mass

• Light stars blow out their outer layers to form a
  Planetary Nebula
• The core of a massive star (more than 8 solar
  masses) collapses, triggering the explosion of a
  Supernova
• Also the core of a very massive stars (more than
  25 solar masses) collapses, triggering the
  explosion Supernova

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Light Stars Stage 12 - A Planetary Nebula forms

• Inner carbon core becomes dead it is out of
  fuel
• Some helium and carbon burning continues in outer
  shells
• The outer envelope of the star becomes cool and
  opaque
• solar radiation pushes it outward from the star
• A planetary nebula is formed

Duration 100,000 years Central Temperature 300
? 106 K Surface Temperature 100,000 K Size 0.1
? Sun
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Deep Sky Objects Planetary Nebulae

• Classic Example Ring nebula in Lyra (M57)
• Remains of a dead,
• exploded star
• We see gas expanding
• in a sphere
• In the middle is the
• dead star, a
• White Dwarf

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Stage 13 White Dwarf

• Core radiates only by stored heat, not by nuclear
  reactions
• core continues to cool and contract
• Size Earth
• Density a million times that of Earth 1 cubic
  cm has 1000 kg of mass!

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Stage 14 Black Dwarf

• Impossible to see in a telescope
• About the size of Earth
• Temperature very low
• ? almost no radiation
• ? black!

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More Massive Stars (M gt 8MSun)

• The core contracts until its temperature is high
  enough to fuse carbon into oxygen
• Elements consumed in core
• new elements form while previous elements
  continue to burn in outer layers

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Evolution of More Massive Stars

• At each stage the temperature increases
• ? reaction gets faster
• Last stage fusion of iron does not release
  energy, it absorbs energy
• ? cools the core
• ? fire extinguisher ? core collapses

Region of instability Variable Stars
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Supernovae Death of massive Stars

• As the core collapses, it overshoots and
  bounces
• A shock wave travels through the star and blows
  off the outer layers, including the heavy
  elements a supernova
• A million times brighter than a nova!!
• The actual explosion takes less than a second

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Supernova Observation
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Type I vs Type II Supernovae
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Type I Carbon Detonation

• Implosion of a white dwarf after it accretes a
  certain amount of matter, reaching about 1.4
  solar masses
• Very predictable used as a standard candle
• Estimate distances to galaxies where they occur

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Type II Core Collapse

• Implosion of a massive star
• Expect one in our galaxy about every hundred
  years
• Six in the last thousand years none since 1604

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SN1987A

• Lightcurve

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Whats Left?

• Type I supernova
• Nothing left behind
• Type II supernova
• While the parent star is destroyed, a tiny
  ultra-compressed remnant may remain a neutron
  star
• This happens if the mass of the parent star was
  above the Chandrasekhar limit

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More Massive Stars end up as Neutron Stars

• The core cools and shrinks
• Nuclei and electrons are crushed together
• Protons combine with electrons to form neutrons
• Ultimately the collapse is halted by neutron
  pressure, the core is composed of neutrons
• Size few km
• Density 1018 kg/m3 1 cubic cm has a mass of
  100 million kg!

Manhattan
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Formation of the Elements

• Light elements (hydrogen, helium) formed in Big
  Bang
• Heavier elements formed by nuclear fusion in
  stars and thrown into space by supernovae
• Condense into new stars and planets
• Elements heavier than iron form during supernovae
  explosions
• Evidence
• Theory predicts the observed elemental abundance
  in the universe very well
• Spectra of supernovae show the presence of
  unstable isotopes like Nickel-56
• Older globular clusters are deficient in heavy
  elements

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Neutron Stars (Pulsars)

• First discovered by Jocelyn Bell (1967)
• Little Green Men?!? Nope
• Rapid pulses of radiation
• Periods fraction of a second to several seconds
• Small, rapidly rotating objects
• Cant be white dwarfs must be neutron stars

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The Lighthouse Effect

• Pulsars rotate very rapidly
• Extremely strong magnetic fields guide the
  radiation
• Results in beams of radiation, like in
  lighthouse

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Super-Massive Stars end up as Black Holes

• If the mass of the star is sufficiently large (M
  gt 25 MSun), even the neutron pressure cannot halt
  the collapse in fact, no known force can stop
  it!
• The star collapses to a very small size, with
  ultra-high density
• Nearby gravity becomes so strong that nothing
  not even light can escape!
• The edge of the region from which nothing can
  escape is called the event horizon
• Radius of the event horizon called the
  Schwarzschild Radius

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How might we see them?

• Radiation of infalling matter

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Evidence for the existence of Black Holes

• Fast Rotation of the Galactic Center only
  explainable by Black Hole
• Other possible Black Hole Candidates
• Cygnus X-1 (X-ray source), LMC X-3
• Observational evidence very strong

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Black Holes The Center of Galaxies?

• IR picture of the center of the Milky Way

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Novae New Stars

• Actually an old star a white dwarf that
  suddenly flares up
• Accreted hydrogen begins fusing
• Usually lasts for a few months
• May repeat (recurrent novae)

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Review The life of Stars