Fate of Stars

1
Fate of Stars

• Low to Medium Mass stars
• grow to red Giant then blow off most of mass
• High Mass stars
• supernove
• Note in both cases a remnant is left
• Type of remnant depends upon mass

2
Einsteins Gravity

• Before continuing need to discuss Einsteins
  theory of Gravity - General Relativity
• We know Newtons law of gravity
• But as for why one body would attract another,
  Newton stated, I make no hypothesis
• Need to go to General Relativity to explain this
• very complex theory

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Einsteins Gravity

• But basic idea is that time and space are united
  into one concept spacetime
• So bodies want to move in straight lines in
  spacetime
• but mass warps or curves the space such that
  they cant (see p. 417 fig S3.16)
• (Think of a mass on a rubber sheet)
• So objects move in straight lines on curved
  surfaces
• called Geodesics (think of lines of longitude on
  a globe)

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Einsteins Gravity
5
Equivalence Principle

• One cannot tell the difference locally between an
  acceleration and the Gravitational field

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Equivalence Principle and bending of Light
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Equivalence Principle and bending of Light

• This bending of star light has been seen by the
  sun
• done in 1919 by Eddington by using an eclipse

8
The Bizarre Stellar Graveyard

• 3 possible final states for stars
• (sometimes called Compact Objects)
• White Dwarfs
• result if star has mass less than 1.4 Msun
• Neutron Star
• result if star has Mass between 1.4 to 3 Msun
• Black Hole
• result if star has mass greater then 3 Msun

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The Bizarre Stellar Graveyard
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White Dwarfs

• Inert core after H burning
• (after red Giant stage)
• about size of earth with 1 Msun
• Held apart by Electron degeneracy
• a quantum effect caused because 2 electrons can
  not be in the same quantum mechanical state
• known as Pauli principle
• so two forces acting on the star
• gravity (inward)
• electron degeneracy pressure (outward)

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White Dwarfs
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White Dwarfs

• Odd fact - as WD becomes more massive it shrinks
• due to gravity
• But there is a limit to how much mass the
  electron degeneracy can support
• Mass must be less then 1.4 Msun
• or electrons will collapse and form a neutron
  star
• known as Chandrasekhar limit

13
White Dwarf in Binary System

• Remember most stars are in binary systems
• If white dwarf is close enough mass from the
  other star can accrete onto it
• May provide the dead white dwarf with a new
  energy source
• called a dwarf nova (just due to friction)
• But may even give enough mass to re-ignite
  H-burning (could be 100,000 Lsun)
• Before modern day could not distinguish between
  Nova and supernova (hence the name)
• Today call these Type 1 A Supernova

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White Dwarf in Binary System
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White Dwarf Supernova

• What if the accreted mass exceeds the
  Chandrasekhar limit (1.4 Msun) ?
• The electron pressure can no longer overcome the
  force of gravity
• results in tremendous energy release
• Energy comparable to that of a massive star
  supernova
• peak luminosity 10 billion Lsun

16
Neutron star

• Even more dense then white dwarf
• Mass between 1.4 and 3 Msun with R 10 Km
• electrons and protons are merged together to form
  neutrons
• Held apart by Neutron degeneracy
• a quantum effect caused because 2 neutrons can
  not be in the same quantum mechanical state
• (similar to W D except electrons are replaced by
  neutrons)
• so two forces acting on the star
• gravity (inward)
• Neutron degeneracy pressure (outward)

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Neutron star
18
Pulsars

• A type of NS where mater is falling on the NS
• The NSs spin its magnetic field causes the
  matter to be beamed out like a lighthouse
• First seen in 1967 by radio astronomers
• seen as repeating sources,
• so termed LGM (for Little Green Men) - a joke
• as time goes on the pulsars loose energy and slow
  down
• May vary spin as fast as 625 per sec or 1 rev per
  sec

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Pulsars
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Binary Pulsar as a test for GTR

• One famous Binary Pulsar found by Taylor and
  Hulse in Crab Nebula in 1974
• observed period of system found it decreases in
  time
• This is in agreement with Einsteins theory of
  Gravity (General Relativity)
• Found that loss of energy could be explained by
  loss of gravitational radiation
• (just like electromagnetic radiation but much
  weaker since it is due to gravity)
• this won them the Nobel prize in 1996.

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Binary Pulsar in Close systems

• Like WD, NS in close systems may have matter
  falling onto them due to their neighboring star
• May emit X-rays
• 100,000 time the energy emitted by our sun
• Gain in energy causes rotation rate to increase
• also like white dwarf, this accretion may cause
  star to nova
• called x-ray bursters

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Black Holes
23
Black Holes

• What if star has mass greater then 3 Msun Then
  not even neutron degeneracy pressure can equal
  gravity.
• Turns out nothing (we think) can keep star from
  collapsing onto itself
• Keeps on collapsing until it is a point of
  infinite density (since no volume)
• point called singularity
• Called a Black Hole

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Black Holes
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Black Hole Tidal Forces

• As one falls into a Black Hole
• the tidal forces become incredibly
• Large
• The force at your feet is larger then at your
  head so one becomes elongated.
• Also one is squeezed on both sides

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Event Horizon

• Around the singularity is a sphere from which no
  light can escape
• called the Schwarzschild radius or event horizon
• so more massive the BH the larger the EH

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Crossing Event Horizon

• This event horizon is a point of no return
• If one crosses it you can not get out
• As you approach, the singularity the tidal forces
  increase, pull you apart.
• As to when the tidal forces pull you apart this
  depends on size or mass of BH.
• Small BH has huge tidal forces at EH
• Supermassive BH may not see tidal forces until
  you are closer to singularity
• But outside observer never sees you cross the EH,
  due to redshift.

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Crossing Event Horizon
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Observational evidence for BH

• Cant see a BH, but may see its effects on
  surrounding objects
• May see accretion of gas onto BH.
• Good BH Candidate Cygnus X-1
• Binary System
• Period of companion star is indication of a very
  massive unseen object
• Black Hole is best guess for that companion
• BH mass 20 -30 Msun
• Huge x-ray source

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Observational evidence for BH
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Supermassive Black Holes at the Core of Galaxies
?
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Hawkings No Hair theorem

• Steven Hawking has a proposal about observable
  properties of a BH
• No Hair Theorem a black hole only has
• Mass
• Charge
• Angular Momentum
• All other information is lost !!!
• Current field of research- is this true, what
  happens to information

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Hawking Radiation

• Black Holes aint so black after all
• In 1974 Hawking looked virtual pair production at
  the event horizon.
• He found that one of the particles might escape
  and thus become a real particle. In order to
  conserve energy the BH must loose mass equal to
  the energy of the particle produced.

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Gamma Ray Bursts

• In 1960s US spy satellites observed high energy
  gamma rays coming from extra solar sources
• Not known what source is
• But do know
• seem to be distributed evenly in sky
• since such high energy, must be local
• May be NS -NS collision (but why isotropic, not
  favored) or extremely high energy SN but unclear

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Gamma Ray Bursts