Class 18 : Evolution of stars and GRBs continued

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Class 18 Evolution of stars and GRBs (continued)

• Finish talking about 10Msun star
• neutron stars
• Evolution of a 50Msun star
• Black holes
• Hypernovae
• Gamma-Ray Bursts (GRBs)
• Observational characteristics of a GRB?
• Possible explanations

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Evolution of 10Msun star (cont.)

• Get shell or onion structure
• No more energy available when core becomes iron.
• Catastrophic core collapse
• Core turns into neutron star
• Rest of star ejected in a supernova explosion

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Core-collapse (type-II) supernovae

• Very powerful explosion
• 1044 J released as radiation
• 100? more released in a neutrino pulse

SN1987a (LMC)
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Cas-A remnant
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Neutron stars

• The remnant of the SN explosion
• Typical mass of 1.5 Msun and radius of 10km
• made of densely packed neutrons (1018kg/m3)
• Extremely properties
• Very strong gravity on surface
• Very strong magnetic fields on surface
• Can spin very quickly (hundreds of times per
  second) gives rise to pulsars.

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Evolution of a 50Msun star

• Starts off as before
• Forms from gas cloud
• steady H-burning (for 2-3 million years)
• When hydrogen runs out, get whole succession of
  nuclear reactions
• get onion structure (like 10Msun case)
• Core collapses and drives Supernova
• Core tries to form neutron star
• but too much mass
• Neutron star crushes under its own weight
• Undergo complete collapse get black hole

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Anatomy of a Black Hole

• Singularity central point containing all of
  the mass. Known laws of physics break down.
• Event horizon point of no return.
  Everything within this radius is dragged to the
  singularity by enormous gravity

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18th Century ideas

• Idea of an object with gravity so strong that
  light cannot escape first suggested by Rev. John
  Mitchell in 1783
• Laplace (1798) A luminous star, of the same
  density as the Earth, and whose diameter should
  be two hundred and fifty times larger than that
  of the Sun, would not, in consequence of its
  attraction, allow any of its rays to arrive at
  us it is therefore possible that the largest
  luminous bodies in the universe may, through this
  cause, be invisible.

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• Curved spacetime around a star

From web site of UCSD
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Hypernovae

• Possible that very massive stars may undergo a
  hypernova
• 100? more powerful explosion than a supernova.
• Can be seen across vast distances
• May be responsible for Gamma-Ray Bursts

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II Gamma-Ray Bursts

• Flashes of Gamma-Rays discovered by military in
  1960s
• Occur about once per day
• Evenly distributed on the sky
• Last 0.01-1000 seconds
• Mysterious until recently we
• Didnt know how far they are,
• or how powerful they are.

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• Problem
• Gamma-ray telescopes give blurry images!
• Difficult to say where GRB was located.
• Very many possible objects located in error box

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• Still dont know anything about short GRBs
• Have recently located long-GRBs
• They are in distant galaxies!
• Must be very powerful
• 100-1000? observed energy of a supernova
• What could make such an explosion?

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• Possibility I
• Hypernova of a very massive star
• forms black hole
• Possibility II
• Collision of 2 neutron stars
• also forms black hole