Stellar Lifecycles

1
Stellar Lifecycles

• The process by which stars are formed and use up
  their fuel.
• What exactly happens to a star as it uses up its
  fuel is strongly dependent on the stars mass.

The Orion Nebula - Birthplace of stars
2
A Star is Born

• Stars form from huge, cold, clouds of gas and
  dust.
• At some point this cloud collapses on itself.
• Its own gravity causes clumps of material to
  form. These clumps heat up as material continues
  to fall upon them.
• Eventually temperatures are high enough in the
  center of these clumps to allow nuclear fusion
  reactions to occur.
• Often several large clumps can form within the
  cloud. Clusters of stars can all form at the same
  time from the same cloud.

A cluster of massive, hot blue stars have formed
still surrounded by clouds of gas that may form
new stars.
3
Main Sequence Stars

• Once nuclear fusion has begun, pressures in the
  core grow high enough to stop the stars from
  collapsing any further. It is then in Hydrostatic
  Equilibrium.
• These are now Main Sequence stars
• Their position along the line of the Main
  Sequence depends on their mass.
• Almost the entire lifetime of a star is spent on
  the Main Sequence.

The H-R diagram showing the Main Sequence line
(in purple). More massive stars are to the upper
left, less massive stars to the lower right.
4
Differences Between High Mass and Low Mass Stars

• Stars that are more massive than the Sun have
  stronger gravitational forces.
• These forces need to be balanced by higher
  internal pressures.
• These higher pressures result in higher
  temperatures which drive a higher rate of fusion
  reactions.
• The Hydrogen within the core of a high mass star
  therefore gets used up much faster than in the
  Sun and ages faster.
• Low mass stars age slower.

A star like our Sun will remain on the Main
Sequence for about 10 billion years. A very
massive star may only be on the Main Sequence for
a few million years.
5
When the Sun Leaves the Main Sequence

• When a star uses up the Hydrogen in its core it
  can no longer support itself against gravity.
• The core compresses and temperatures begin to
  rise.
• Temperatures may get high enough outside the core
  to begin Hydrogen fusion there instead.
• The pressure from this shell around the core
  pushes the outer layers of the star out.
• These outer layers cool and get redder.

The life cycle of a star like the Sun
6
The Last Years of the Sun

• During this Red Giant stage the core of the Sun
  will continue to contract and heat up.
• Eventually temperatures will be high enough for
  the fusion of Helium in the core. The Sun then
  converts Helium into Carbon Oxygen. The surface
  temperature of the Sun increases and it becomes a
  Yellow Giant.
• This stage lasts as long as there is Helium
  available in the core.

The motion of the Sun through the H-R diagram as
the Sun ages. Notice that the Sun spends most of
its life on the Main Sequence.
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The Suns Planetary Nebula

• As the core exhausts its Helium fuel it begins to
  contract and heat causing the Helium to get used
  even faster. The Sun increases its luminosity.
  The outer layers of the Sun expand, cool and
  redden again.
• The outer layers of the Sun start streaming away
  from the core. This material forms a nebula
  surrounding the Sun.

Except for the core, the rest of the Sun will
eventually be dispersed into space forming a
planetary nebula like this one.
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White Dwarf Stars

• The core of the Sun eventually stops all nuclear
  fusion but remains extremely hot.
• The core will form a White Dwarf star, a very
  dense, small object about the size of the Earth.
  
• Over time the White Dwarf will cool and dim.
• By measuring the temperature of white dwarfs you
  can estimate how long ago they formed.

White Dwarf stars are very hot but also very
small. They appear in the lower left corner of
the H-R Diagram.
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What Happens When High Mass Stars Die?

• For stars greater than 10 times the Suns mass
  after Helium fuel is exhausted the core of the
  star contracts, heats up and starts to fuse
  Carbon Oxygen into Neon and Silicon.
• Helium and Hydrogen fusion continue in shells
  around the core.
• As long as the star can raise its core
  temperature high enough it can continue to fuse
  new elements. Until iron is created.

10
Supernova

• The formation of iron actually absorbs rather
  than releases energy.
• Nuclear fusion at the core stops and it begins to
  collapse.
• The pressures of the surrounding layers are so
  high that the atoms of the iron core are crushed,
  smashing the electrons into the protons forming
  neutrons.
• Once neutrons are formed the collapse stops, the
  surrounding gas is heated and explodes off the
  core. This is a supernova explosion.
• The explosion is so energetic that it can
  outshine the combined light of a galaxy!
• Heavy elements are formed in the material blown
  off the star. These elements are dispersed into
  space where they can be used to form planets and
  new stars.
• Depending on its mass the core may become a
  neutron star or collapse further to a black hole.