Transcript Life cycle of Stars Notes

Stellar Evolution
Chapters 16, 17 & 18
Stage 1: Protostars
• Protostars form in
cold, dark nebulae.
• Interstellar gas and
dust are the raw
materials from
which stars form.
Example
of a star
forming
nebula
Star Cluster N81
•Gas condenses due to
gravity, temperature
increases.
•Gives off heat (infrared,
but no light)
•Planets may form.
Growth of a Protostar
• They continue
to accrete matter
until
temperature and
pressure in core
are high enough
for fusion.
Gravity versus Pressure
Stage 2: Main Sequence
• It takes small red stars
over 50,000,000 years
to reach stage 2.
• Large blue stars take
only 60,000 years to
reach stage 2.
• Nuclear fusion begins
when hydrogen starts
to burn.
Main Sequence Stars
•These stars generate energy by
hydrogen fusion.
• 4 Hydrogen molecules smash
together to form Helium and
energy.
•Star begins to shine.
Main Sequence Stars
Blue Giants
Sun Class
20,000 K +
Red Dwarfs
3,000 K
•The type of Main Sequence star depends on initial mass.
Bigger = Higher Temperature = bluer color.
Lower Limit for stars
• Brown
dwarfs, not
massive
enough to
start fusion
M<0.08MSun
Upper limit for stars
• Very massive stars are
give off so much energy
the pressure of photons
drives their matter into
space
• Observations show the
limit is 100MSun
Stage 3: Red Giant
• Hydrogen fuel is
running out.
• Core shrinks, begins
helium fusion.
• Radiation pressure
pushes atmosphere out
and it expands.
What’s happening:
• As the core contracts,
it gets hotter, heating
the layer of gas around
it.
• Hydrogen fusion starts
in this shell causing
the atmosphere to
expand.
• As it expands, it cools
and becomes redder.
• During stage 3,
dredge ups occur
when the star has
a small mass.
Stage 4: Low Mass Star - White Dwarf
• White dwarfs, are the carbon and
oxygen cores of dead stars.
• WD are about the size of earth.
• The more massive a WD is, the smaller
it is in size.
• Electron degeneracy
pressure supports them
against gravity.
• Eventually WD cool
down, they end up
cold, dense and dark –
a black dwarf.
• WDs are surrounded
by planetary nebula,
the remains of the
star’s atmosphere.
Nova
• Nova- Occurs in
binary system, white
dwarf + other aging
star.
• Gases from
companion fall on
white dwarf surface.
• Outer layer of WD
burns hydrogen.
• Can happen
repeatedly.
Stage 3 High Mass Stars- Red
Supergiant
• A high mass star, can have a diameter of
778 Million km, which is almost the size
of Jupiter’s orbit.
• Once helium in the core
is consumed.
• The core contracts &
heats up. A new element
begins to burn.
• The surrounding layers
heat, they also undergo
fusion.
• The last stage is when
iron is formed in the
core!
High Mass Star Stage 4:
Supernova
• When the most massive
stars run out of fuel
gravity quickly crushes
the core.
• The atmosphere is
ripped apart by shock
waves in a cataclysmic
explosion.
• A supernova explosion
can create a neutron
stars or black holes.
Stage 5: Neutron Stars
• Formed by core
collapse of very
massive star.
• This star is so heavy
due to protons and
electrons being
converted to neutrons
in a very small space.
• Neutron degeneracy
pressure of neutrons
supports star against
gravity.
Most Massive Stars Stage 5:
Black Hole
• Collapsed core of most
massive stars. Infinitely
small & dense.
• Its gravity stops even light.
• The spherical surface is
known as the event horizon.
• Astronomers believe black
holes exist because they
bend the fabric of space.
If we can’t see black holes, how
do we know they are there?
Summary of Stages