High Mass Stars

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High Mass Stars

• Dr Bryce

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Class notices

• Observing this week for groups 6 or 7

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Iron is dead end for fusion because nuclear
reactions involving iron do not release
energy (Fe has lowest mass per nuclear particle)
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Helium Capture

• High core temperatures allow helium to fuse with
  heavier elements

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Evidence for helium capture Higher abundances
of elements with even numbers of protons
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The end of the nuclear road

• Iron builds up in core until degeneracy pressure
  can no longer resist gravity
• Core then suddenly collapses, creating supernova
  explosion
• Nova means new, but this happening at the end of
  a stars life

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Supernova Explosion

• Core degeneracy pressure goes away because
  electrons combine with protons, making neutrons
  and neutrinos
• Neutrons collapse to the center, forming a
  neutron star

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Energy and neutrons released in supernova
explosion enable elements heavier than iron to
form, including Au and U
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Supernova Remnant

• Energy released by collapse of core drives outer
  layers into space
• The Crab Nebula is the remnant of the supernova
  seen in A.D. 1054

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Supernova 1987A

• The closest supernova in the last four centuries
  was seen in 1987

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Rings around Supernova 1987A

• The supernovas flash of light caused rings of
  gas around the supernova to glow

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Impact of Debris with Rings

• More recent observations are showing the inner
  ring light up as debris crashes into it

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Role of Mass

• A stars mass determines its entire life story
  because it determines its core temperature
• High-mass stars with 8MSun have short lives,
  eventually becoming hot enough to make iron, and
  end in supernova explosions
• Low-mass stars with never become hot enough to fuse carbon nuclei,
  and end as white dwarfs
• Intermediate mass stars can make elements heavier
  than carbon but end as white dwarfs

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• Life Stages of High-Mass Star
• Main Sequence H fuses to He in core
• Red Supergiant H fuses to He in shell around He
  core
• Helium Core Burning
• He fuses to C in core while H fuses to He in
  shell
• Multiple Shell Burning
• Many elements fuse in shells
• 5. Supernova leaves neutron star behind

Not to scale!
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The binary star Algol consists of a 3.7 MSun main
sequence star and a 0.8 MSun subgiant star.
Stars in Algol are close enough that matter can
flow from subgiant onto main-sequence star
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Star that is now a subgiant was originally more
massive As it reached the end of its life and
started to grow, it began to transfer mass to its
companion (mass exchange) Now the companion star
is more massive
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Star that started with less mass gains mass from
its companion Eventually the mass-losing star
will become a white dwarf
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Accretion Disks

• Mass falling toward a white dwarf from its close
  binary companion has some angular momentum
• The matter therefore orbits the white dwarf in an
  accretion disk

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Accretion Disks

• Friction between orbiting rings of matter in the
  disk transfers angular momentum outward and
  causes the disk to heat up and glow

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Nova

• The temperature of accreted matter eventually
  becomes hot enough for hydrogen fusion
• Fusion begins suddenly and explosively, causing a
  nova

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Nova

• The nova star system temporarily appears much
  brighter
• The explosion drives accreted matter out into
  space

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Two Types of Supernova
Massive star supernova Iron core of massive
star reaches white dwarf limit and collapses
into a neutron star, causing explosion White
dwarf OR binary supernova Carbon fusion
suddenly begins as white dwarf in close binary
system reaches white dwarf limit, causing
total explosion
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One way to tell supernova types apart is with a
light curve showing how luminosity changes with
time
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Nova or Supernova?

• Supernovae are MUCH MUCH more luminous!!! (about
  10 million times)
• Nova H to He fusion of a layer of accreted
  matter, white dwarf left intact
• Supernova complete explosion of white dwarf,
  nothing left behind

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A neutron star is the ball of neutrons left
behind by a massive-star supernova Degeneracy
pressure of neutrons supports a neutron star
against gravity