Download E5 stellar processes and stellar evolution (HL only)

E5 stellar processes and stellar
evolution (HL only)
Star formation
Star formation
Interstellar space consists of gas (74% H, 25% He
by mass) and dust at a density of about 10-21
kg.m-3. This is about one hydrogen atom to every
cm3 of space.
Star formation
When the gravitational energy of a given mass of
gas exceeds the average kinetic energy of the
molescules the gas cloud becomes unstable and
starts to collapse.
GM2/R > (3/2)NkT
“Jeans criterion”
Star formation
As the cloud collapses, the particles get faster
and eventually clumps form that are hot enough
to emit light. Protostars are formed.
Star formation
If the star is big enough the collapse will
continue until the star is hot enough for nuclear
fusion to occur. The radiation pressure produced
by the fusion balances the pull of gravity and
equilibrium is reached. The star is a main
sequence star (like our sun).
Main sequence
41H → 4He + 2e+ + 2γ + 2νe (26.7 MeV)
Mass v luminosity relation
Lα
α
M
where 3 < α > 4
Mass v luminosity relation
Since the luminosity could be the total energy
given out by the star (E) divided by the
lifetime of the star T we get
E/T α Mα
Since E = Mc2 from Einstein’s formula
Mc2/T α Mα
T α M1-α
Taking α = 4 we get T α M-3
Lifetime of a star
Tα
-3
M
The bigger the mass of a star, the shorter
its life (it “burns” out quicker)
A star with a mass 10x greater than the sun will
have a life time a factor 10-3 (1/1000) less than
the sun
When the hydrogen runs out?
Schönberg – Chandrasekhar limit
• After the star has used up about 12% of its
hydrogen, its core will contract but the outer
layers will expand substantially ()fusion
continues there). The star leaves the main
sequence and moves over to the Red Giant
branch
Mstar < 0.25Msun
• No further nuclear reactions
• Core stays as Helium
• After a Red giant it becomes a White Dwarf
0.25Msun < Mstar < 4Msun
• Core temperature reaches 108 K enabling
Helium fusion (higher temperature is needed
because Helium nuclei have 2 positive
charges)
• Helium fuses to form oxygen and carbon
• After a Red gaint a White Dwarf with a
carbon/oxygen core is formed
4Msun < Mstar < 8Msun
• Core temperature rises further enabling the
fusion of carbon and oxygen to take place
producing a core of oxygen, neon and
magnesium
• After a Red giant a White Dwarf with an
oxygen/neon/magnesium core is formed
8Msun < Mstar
• Core temperature rises further so heavier
elements fuse. Helium in the outer layers
continues to fuse too. Eventually iron is
produced (which does not fuse – see topic 7)
• This is a RED SUPERGIANT
• Will eventually become a NEUTRON STAR
Anatomy of a RED SUPERGIANT
and neon
Evolution of stars < 8Msun
• Core contracts under its own weight
• It stops when electrons have to be forced into
the same quantum state. This is not allowed
so this “electron degeneracy pressure” stops
the star collapsing further
• The outer layers are released to form a
planetary nebula
• The resultant White dwarf has no energy
source so is doomed to cool down to become
a Black dwarf.
Evolution of stars > 8Msun
• If the core is above 1.4 solar masses (the
Chandrasekhar limit) Electrons are forced into
protons producing neutrons.
• The core is only made of neutrons and
contracting rapidly.
Evolution of stars > 8Msun
• The neutrons get too close to each other (this
time it is “neutron degeneracy pressure”
caused by neutrons not being allowed to
occupy the same quantum state) and the
entire core rebounds to a larger equilibrium
size.
• The causes a catastophic shock wave which
explodes the star in a SUPERNOVA
Evolution of stars > 8Msun
• The neutron star left over after the supernova
remains stable provided its has a mass of no
more than 3 solar masses (the OppenheimerVolkoff limit)
Evolution of stars > 8Msun
• Neutron stars with masses substantially more
than the Oppenheimer-Volkoff limit continue
to collapse as the neutron pressure is
insufficient. They become Black holes
• At the centre of the black hole is a singularity
• The boundary around the singularity where
even light does not have sufficient escape
velocity to escape is called the event horizon
or gravitational radius.
Stellar evolution
Evolution of stars on the HR diagram
Evolution of stars on the HR diagram
Pulsars
• Another very important property of neutron star is
its strong magnetic field. When electrons move in
spirals around magnetic lines of force, radio waves
are produced and radiated out along the two
magnetic poles of the star.
Pulsars
• Usually, the rotational axis of the neutron star does
not align with the magnetic axis. The radiation
beams will sweep around and create the light house
effect. What we observe on Earth will be pulses of
radio wave with very stable period. This is a pulsar.