Stellar Evolution: Birth

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Stellar Evolution Birth Death of Stars
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Star Birth and Life

• Stage 1 Stars are born in a region of high
  density Nebula made of gas, mostly hydrogen, and
  dust.
• Nebulae begin to condense into a huge globule of
  gas and dust and contracts under its own gravity.
  
• Orion Nebula M42

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Protostars

• Stage 2. A region of condensing matter will
  begin to heat up and start to glow forming
  Protostars. If a protostar contains enough
  matter the central temperature reaches 15
  million degrees centigrade.
• This image is the outflow (red)and protostar.

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15 Million Kelvin

• Stage 3. At this temperature, nuclear reactions
  in which hydrogen fuses to form helium can
  start.

5-step process of stellar fusion 1. Nuclear
fusion. Gravity gas pressure (equilibrium) 2.
Out of fuel. 3. Fusion stops, temperature drops.
4. Core contracts (gravity pulling atoms in).
5. Increased temperature (more atoms, more
collisions) and density in the core reinitiates
nuclear fusion, equilibrium is achieved, and the
cycle begins again at step 1.
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Main Sequence Star

• Stage 4. The star begins to release energy,
  stopping it from contracting even more and
  causes it to shine. It is now a Main Sequence
  Star

Name the nearest main sequence star to Earth.
Sun.
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Main Sequence

• Stage 5 - A star of one solar mass remains in
  main sequence for about 10 billion years, until
  all of the hydrogen has fused to form helium.

Look at the diagram on the right. There are
essentially two sections of a star the core
(where fusion occurs), and an outer gaseous
shell.
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Red Giant.

• Stage 6 - The helium core now starts to contract
  further and reactions begin to occur in a shell
  around the core.
• Stage 7 - The core is hot enough for the helium
  to fuse to form carbon. The outer layers begin to
  expand, cool and shine less brightly. The
  expanding star is now called a Red Giant.

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Planetary Nebula.

• Stage 8 - The helium core runs out, and the outer
  layers drift of away from the core as a gaseous
  shell, this gas that surrounds the core is called
  a Planetary Nebula.

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White Dwarf

• The remaining core (thats 80 of the original
  star) is now in its final stages.
• The core becomes a White Dwarf as the star
  eventually cools and dims. When it stops shining,
  the now dead star is called a Black Dwarf.
• Picture a brown dwarf (center) compared to
  Jupiter left) and the Sun (right).

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Massive Stars - The Life of a Star of about 10
Solar Masses

• Massive stars burn brighter and end life more
  dramatically
• Carbon core contracts further and reaches high
  enough temperatures to burn carbon to oxygen,
  neon, silicon, sulfur and finally to iron.
• The high density core resists further collapse
  causing the infalling matter to "bounce" off the
  core.
• This sudden core bounce (which includes the
  release of energetic neutrinos from the core)
  produces a supernova explosion (brighter than a
  whole galaxy of a billion stars).

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Supernova

• Supernova remnant
• Supernova explosion

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Neutron Stars

• If the progenitor mass (mass of a star when H
  ignition occurs) is around ten times the mass of
  the Sun, the neutron star core will cool to form
  a neutron star
• Neutron stars are potentially detectable as
  "pulsars", powerful beacons of radio emission

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Black Holes

• If the progenitor mass is larger than 10 times
  the mass of the sun, then the resultant core is
  so heavy that not even nuclear forces can resist
  the pull of gravity.
• The core collapses to form a black hole.
• 2 Types of stellar black holes are thought to
  form.

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Stellar Black Holes

• A black hole does not have a surface in the usual
  sense of the word.
• There is simply a region, or boundary, in space
  around a black hole beyond which we cannot see
  the event horizon.

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2 Types of Stellar Black Holes

• There is strong evidence for two types of black
  holes
• Mid-mass A newly discovered type of black hole
  that has a mass of 500 - 1,000's of Suns.
• Super massive Black holes with a mass of a
  million or more Suns located in the centers of
  galaxies.

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Mid-mass Black Holes

• Stellar black holes with masses 500 to 1,000 or
  so Suns

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Supermassive Black Holes

• Contain the mass of many millions of stars are
  thought to lie at the center of most large
  galaxies.
• Evidence comes from optical and radio
  observations which show a sharp rise in the
  velocities of stars or gas clouds orbiting the
  centers of galaxies.
• Recent research, including results from Chandra
  suggests that galaxies and their black holes do
  not grow steadily, but experience seasonal
  activity.

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Supermassive Black Holes