Transcript Astrophysics 12 - Stellar Evolution

Stellar Evolution
Astrophysics Lesson 12
Learning Objectives
To know: How stars form from clouds of dust and gas.
 How main sequence stars evolve as they run
out of hydrogen.
 How this evolution appears on the HR diagram.
Homework
 Remember to bring the completed open book
exam this Friday.
Some terms…
 Radiation pressure - the pressure exerted
upon any surface exposed to electromagnetic
radiation.
 Hydrogen “burning” – actually refers to the
fusion of hydrogen into helium not reacting
with oxygen.
Pillars of Creation
Pillars of Creation
Star Formation
 Stars are formed from great
clouds of gas and dust, most of
which is the remnants from
previous supernovae.
 The denser clumps collapse
slowly contract under the force
of gravity.
Protostars  Hydrogen Fusion
 When the clumps get dense enough, the cloud
fragments into regions called protostars that
continue to contract and heat up.
 Eventually the temperature at the centre of the
protostar reaches a few million degrees and
hydrogen nuclei start to fuse together to form
helium.
Protostars in Orion I
Protostars in Orion II
Protostars in Orion III
Observe this with naked eye…
On the Main Sequence
 The fusion of hydrogen
releases enough energy to
create enough radiation
pressure to stop the
gravitational collapse.
 The star has now reached the
main sequence and will
remain there while it fuses
hydrogen to helium.
 Core hydrogen “burning”
Leaving the Main Sequence
Stars spend most of their lives
as main sequence stars.
As the star ages more and
more helium builds up in the
core.
Eventually all the hydrogen is
gone and you are left with a
core of only helium.
Shell Hydrogen Burning
 When the hydrogen in its core runs
out, the outward radiation pressure
stops, gravity wins and the core
starts to contract.
 As the core contracts it heats up.
This raises the temperature of
hydrogen surrounding the core
enough for it to fuse.
 This is shell hydrogen burning –
very low mass stars stop here.
Red Giant
 The core continues to
contract until it is hot and
dense enough for helium to
fuse into carbon and oxygen.
  Core helium “burning”.
 This releases a huge amount
of energy which pushes the
outer layers of the star
outwards which then cool.
  Red Giant.
Shell Helium “Burning”.
When the helium runs out, the carbon-oxygen
core contracts again  shell helium “burning”.
For stars with mass similar to the of the Sun, the
carbon-oxygen core isn’t hot enough for fusion.
The core continues to contract until electrons
exert enough pressure to stop it collapsing
further.
Ejecting the Outer Layers
 The helium shell becomes more
and more unstable as the core
collapses.
 This causes the star to pulsate and
eject its outer layers into space
  planetary nebula.
 A hot dense solid core is left
behind
  white dwarf.
White Dwarf  Black Dwarf
Within a million years the nebula fades and the
core will simply continue to cool and finally the
star is said to be dead.
This is the fate that awaits our Sun in about 5
billion years.
Animations…
Planetary Nebula I
Planetary Nebula II
Planetary Nebula III
The Whole Story
The HR Diagram Evolutionary
Track for a 1 Mo star