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The Star Cycle
Birth
• Stars begin in a DARK
NEBULA (cloud of gas and
dust)… aka the STELLAR
NURSERY
• The nebula begins to
contract due to gravity in
response to some
interstellar disturbance
• One or more PROTOSTARS
are formed
Birth
• The protostar
continues to
collapse until its
centre reaches a
few million
degrees K
• Nuclear fusion
begins. Hydrogen
is converted to
Helium.
Brown Dwarf
• Brown Dwarf - a
protostar that does
not have enough
mass to reach the
required
temperature and
pressure for nuclear
fusion
Main Sequence
• Pressure, from
energy released
during fusion,
balances gravitational
contraction
• Low mass stars take
about 2 million years
to reach main
sequence
• High mass stars only
take 10, 000 years
Main Sequence
• Lowest mass main
sequence stars are
Red Dwarfs. Surface
temp: 3000K
• High mass main
sequence stars are
Blue Giants. Surface
temp: 30,000K +
Aging: Low Mass
• When hydrogen in
the core is all used
up:
– The core of helium
collapses
– The temp in the core
increases
– The area around the
core expands
• Becomes a Red
Giant
Aging: Low Mass
• Eventually, the
helium core ignites
• Carbon and Oxygen
are produced from
“Helium burning”
Aging: Low Mass
• Late in the life of a
Red Giant, oxygen
and carbon can be
“dredged up” by
convection currents.
• A star with enough
carbon on its surface
is called a Carbon
Star
Aging: Low Mass
• If the mass is less
than 8 solar masses,
nuclear fusion stops
with the production
of Carbon and
Oxygen
• This is the fate of our
Sun
Death: Low Mass
• Low mass stars eject
their outer layers in
the form of Planetary
Nebula
• The carbon and
oxygen core is left
behind as a White
Dwarf
Death: Low Mass
• The White Dwarf is
very hot and very
small
• Small white dwarfs
are more massive
than large white
dwarfs
Death: Low Mass
• The White Dwarf
eventually consumes
the last of its fuel
• Becomes a cold,
dense, non-luminous
Black Dwarf
Aging: High Mass
• If a star is greater
than 8 solar masses,
fusion does not stop
with Helium burning
• The carbon and
oxygen core
contracts and temp
increases
Aging: High Mass
• Surface expands
further. Becomes a
Red Supergiant.
• Surface as large as
the orbit of Jupiter
(1.5 billion km in
diameter).
• Core the size of
Earth
Aging: High Mass
• Carbon and oxygen
burning produce
Neon, Magnesium,
Silicon, and Sulphur
• If it is massive
enough, these will
later form nickel, iron
and other elements
of similar atomic
weight
Aging: High Mass
• When the core is full
of iron, no more
fusion is possible.
Death: High Mass
• Supernova
– Core reaches a density of
4x1017 kg/m3.
– Regions surrounding the
core rush inward at
unbelievable speeds
– Increased pressure and
temperature force the
material back out
– When shock waves reach
the surface of the star, the
outer layers explode
Death: High Mass
• Depending on the
Mass, the supernova
explosion will leave
behind one of three
things:
• 1. White Dwarf
– Glowing core similar
to the remains of a
low mass star. Its
high mass will mean
it is very small.
Death: High Mass
• 2. Neutron Star
– Incredibly compact stellar
corpse
– Escape velocity is about
half the speed of light
– All protons and electrons
are converted to neutrons
Fast spinning neutron stars
send out radio waves and
are called PULSARS
Death: High Mass
• 3. Black Hole
– The burned out core
presses inward to
create a density even
greater than the
neutron star
– Escape velocity is
greater than the
speed of light
– The gravity of the
mass alters the fabric
of spacetime
Death: High Mass
• 3. Black Hole
– It forms a rapidly
rotating disk around
itself, called the
ACCRETION DISK
– X-rays are given off
and matter is ejected
in jets
– Video: http://dsc.discovery.com/tvshows/other-shows/videos/how-theuniverse-works-birth-of-a-blackhole.htm