Transcript Requiem for a Star

Requiem for a Star
Stellar Collapse
•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.
Nebula
Nebula
Star
Protostar
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
Main Sequence Stars
Question
Will anything other than hydrostatic equilbrium
stop inevitable collapse?
It all depends on the mass
Options
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Fusion of elements other than hydrogen
Electrons
Neutrons
Nothing
Fusion Reactions
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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
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
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
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
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
Heavier stars
• If mass > 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
Cores of Heavier Stars
He
C
O
Si
Fe
Final products from
Death of Heavier Stars
• M < 3-4 solar masses
– Core less than 1.4 solar masses
– Collapse stabilized by electrons
– White dwarf
• 3 < M < 6 solar masses
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Core between 1.4 and 3 solar masses
Elements disappear, only neutrons remain
Collapse stabilized by neutrons
Neutron star = Pulsar
• M > 6 solar masses
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Core greater than 3 solar masses
Collapse cannot be stabilized
Continues indefinitely
Black hole
← Black hole
Main sequence
Star
Red
Giant
Planetary
Nebula
Red
Super
Giant
White
Dwarf
Planetary
Nebula
M=3
Red
Super
Giant
Planetary
Nebula
Neutron
Star
M=6
Black
Hole