Requiem for a Star

1
Requiem for a Star

• Stellar Collapse

2
Gravity

• Gravity is an inexorable force always trying to
  cause further collapse
• Nebulae ? Protostars
• Protostars ? Main Sequence Stars
• Main Sequence Stars ? ?
• At each stage stability is only achieved if there
  is a balancing force.

3
Main Sequence Stars

• Force of gravity is balanced by internal pressure
  of star
• Pressure increases with depth
• Internal pressure is maintained by heat produced
  by the fusion of light elements into heavier
  elements
• If fuel is exhausted then internal pressure is
  not maintained, and gravity causes collapse of
  star

4
Main Sequence Stars

• Question
• Will anything other than hydrostatic equilbrium
  stop inevitable collapse?

It all depends on the mass
5
Options

• Fusion of elements other than hydrogen
• Electrons
• Neutrons
• Nothing

6
Fusion Reactions

• 4 H ? He
• 3 He ? C
• 2 C ? Mg
• C He ? O
• O He ? Ne
• O O ? Si
• O He ? Ne
• Si 7 He ? Ni
• Ni ? Co (radioactive decay)
• Co ? Fe (radioactive decay)
• Cannot have fusion reactions for elements above Fe

7
Fusion Reactions

• Fusion of heavier elements becomes increasingly
  difficult
• Impossible for elements above iron
• Requires increasingly higher temperatures and
  pressures
• Requires more gravitational collapse more mass
• All stars must eventually run out of fuel

8
Stars Comparable to Sun

• M up to about 3 or 4 solar masses
• As a Main Sequence star can only use hydrogen as
  a fuel
• When hydrogen is exhausted collapse of interior
  is inevitable
• Increase in temperature caused by collapse
  suddenly ignites unprocessed hydrogen, causing
  star to expand to become red giant

9
Stars Comparable to Sun

• Red Giant star pushes its outer atmosphere into
  space
• Planetary nebula
• Unused hydrogen
• Helium residue in core ignites and rapidly
  turns to carbon
• Carbon core cannot be used. Remains as a hot
  carbon remnant, slowly cooling in space
• White dwarf

Ring Nebula
10
White Dwarf Stars

• What causes further collapse?

• Electrons
• Electron degeneracy
• Can only be packed so far
• Prevents further collapse when white dwarf is
  approximately the size of the Earth
• Only possible if the mass is less than about 1.4
  solar masses
• Chandrasekhar limit
• Above 1.4 solar masses electrons are not strong
  enough to balance out gravity

11
Heavier stars

• If mass gt 3-4 solar masses the core left behind
  would be larger than Chandrasekhar limit
• Cannot use electrons to stabilize collapse of
  star
• Cannot form white dwarfs
• Can use elements heavier than hydrogen as fuel
  during main sequence life
• Onion like structure

12
Cores of Heavier Stars

• He

C
O
Si
Fe
13
Final products fromDeath of Heavier Stars

• M lt 3-4 solar masses
• Core less than 1.4 solar masses
• Collapse stabilized by electrons
• White dwarf
• 3 lt M lt 6 solar masses
• Core between 1.4 and 3 solar masses
• Elements disappear, only neutrons remain
• Collapse stabilized by neutrons
• Neutron star Pulsar
• M gt 6 solar masses
• Core greater than 3 solar masses
• Collapse cannot be stabilized
• Continues indefinitely
• Black hole

14

• ? Black hole

15
Main sequence Star
Red Giant
Red SuperGiant
Red SuperGiant
Black Hole
Planetary Nebula
Planetary Nebula
White Dwarf
Neutron Star
Planetary Nebula
M 3
M 6