Download 2.5.2 development of a star

Like all other bodies
in the universe, stars
are born, they evolve
and then they die.
 This section will give
you the main stages in
stellar evolution.

 Stars
condense from
clouds of hydrogen,
 The gravitational
attraction between the
hydrogen atoms pull
them together and the
cloud collapses under
its own gravitational
forces – a kind of free
fall collapse.
 This heats the dust to
low temperatures (3050K) and emits infrared
radiation.
 At this stage, it is
called a proto-star.
 As
the atoms get closer
the GPE of the
individual atoms
decreases and the KE
increases. KE is
directly proportional to
temperature, and this
rises to around 1000K
in the centre of the
proto-star.
 As the hydrogen atoms
get closer the
temperature increases
and the atoms move
around faster. This
increases the
probability of collision
between atoms.
 The
Main Sequence is
the period during
which a star ‘burns’
hydrogen.
 This is where
hydrogen atoms
collide and fuse
together to form
helium.
 The process is called
the p-p chain and
there are 3 versions
of this. You only need
to know PP1.
1H
+ 1H  2H + e+ + 
(1.44 MeV)
2H
+ 1H  3He + 
(5.49 MeV)
3He
+ 3He  4He + 1H + 1H
(12.9 MeV)
The numbers in brackets are the energies released at each
stage of the chain. It doesn’t seem like a lot but when you
consider how many hydrogen atoms are in a star …)
A
star spends ~80% of its
lifetime on the main
sequence.
 During this time it is
stable as the
gravitational forces that
enable hydrogen
burning balance and
pull the star in, balance
with the gas pressure
pushing out.
 This is much like the gas
pressure inside a
balloon balancing with
the tension in the
plastic of the balloon.
 In the star it is known as
hydrostatic
equilibrium.
Stars with an initial mass
of 0.4 - 8 solar masses
become red giants as part
of their evolution.
 On the main sequence the
star burns H but these
nuclear processes stop
once there is not enough
present for fusion.
 The core still needs to
heat up to fuse helium
and there is an shell of
unburnt H around the
core.

As the H in the core stops
fusing the core contracts
due to the gravitational
forces taking over. This
heats the envelope to
enable H burning to take
place here.
 This then means that
there is enough energy in
the envelope to cause it
to expand which in turn
cools it.
 The diameter of the star
increases by about 100
and this is the Red Giant
phase.

If the star is bigger the core
temperature is higher and
this means that there is
more energy to fuse heavier
elements (Be and C).
 As fusion continues in the
core, the temperature
keeps rising which enables
the creation of even
heavier elements in shells.
 The most massive stars are
capable of forming an iron
core (the top of the fusion
spike, A = 57) These are
called Red Supergiants.

 Due
to the size of
the star, the
gravitational field on
the outside envelope
is weak so large
amounts of matter
are lost. This mass
loss is 10 million
times faster than the
mass loss from the
sun.
 Eventually the
energy from the
collapse of the inner
shells is not great
enough to trigger
the next stage of
fusion.
•The outside layers
continue to fuse
while the inactive
core region grows.
•Eventually the
gravitational forces
in are greater than
the gas pressure
pushing the star
out so the star
collapses in on
itself and shrinks.
 As
a Red Giant
collapses in on itself,
the core starts to heat
up again to ‘white
hot’.
 A White Dwarf is the
end state for a star of
0.4 – 8 solar masses.
 The upper limit for the
final mass of a White
Dwarf is approximately
1.4 solar masses.
 For masses greater
than that, the end
state is much
different.