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Stellar Evolution – Life of a Star
Stellar evolution is the process in
which the forces of pressure (gravity)
alter the star.
Stellar evolution is inevitable; stars
deplete their own fuel source.
Stellar evolution is very important. It is
responsible for the production of most
of the elements (all natural elements
after H and He). As well, it aids in the
formation of galaxies, new stars and
planetary systems.
Stellar Evolution – Life of a Star
•
The rate of stellar evolution is
primarily due to mass. More
mass requires greater fuel
consumption and faster
evolution.
•
Recall from the HertzsprungRussel Diagram, the
measurements of star
luminosity and star
temperature highlight a link
to mass.
Stellar Evolution – Life of a Star
• New stars are found in the
Main Sequence of the
Hertzsprung-Russel
Diagram. Based on size
and surface temperatures,
the hot-bright-blue stars
occur in the upper left
corner while the cool-dimreddish stars occur in the
lower right.
Stellar Evolution – Life of a Star
• Stars often begin as a nebulae. A
nebulae is a cloud of gas and dust
in space. Some nebulaes are
regions where new stars are being
formed, while others are the remains
of dead or dying stars. The word
nebulae comes from the Latin word
for cloud.
• In the nebulae, gravity pulls the
materials together. The
concentration increases internal
temperatures. When the temperature
reaches 10 000 000 OC, the nuclear
reaction FUSION begins and the star
“turns on”.
Stellar Evolution – Life of a Star
• The fundamental property shared by all
Main Sequence stars is THERMAL
EQUILIBRIUM. The liberation of
energy from the interior of the star is
balanced by the energy released at the
surface of the star. The energy is
produced by hydrogen burning in the
core of star (conversion by fusion of H
to He).
• A second property is HYDROSTATIC
EQUILIBRIUM. There is sufficient
pressure produced by burning H in the
core to support the pressure (i.e.,
weight of the outer plasma layers).
Stellar Evolution – Life of a Star
• This is the state of our
Sun…a MAIN SEQUENCE
STAR. It has been burning
for about 4.5 billion
years…and will continue to
burn as such for another 4.5
billion years.
• Most stars are Main
Sequence Stars (recall the
Hertizsprung-Russell
Diagram)
Stellar Evolution – Life of a Star
• Low mass stars are Red Dwarfs.
• A Red Dwarf consumes its
hydrogen slowly, losing its mass
very slowly. In the end, all that
remains is the very hot core of
the star: a White Dwarf.
• A typical White Dwarf is half as
massive as the Sun, yet only
slightly bigger than the Earth.
This makes White Dwarfs one of
the densest forms of matter,
surpassed only by Neutron Stars.
Stellar Evolution – Life of a Star
• As the supply of H in the core is
depleted, thermal imbalance occurs
and the pressure in the core lessens.
With the change, the star beings to
collapse inward because the fewer
particles inside cannot maintain the
pressure needed to support the
outer layers.
• Under the outer weight, the core
begins to collapses. This event
increases the internal pressure,
raises the core temperature and
increases surface luminosity. The
escaping energy increases the
surface temperature so that the outer
layers of H begin fusion. This is
called SHELL HYDROGEN BURNING.
Stellar Evolution – Life of a Star
• At this point, there is no H fuel and
the burning in the core ceases.
The core collapses again. Without
H, the star converts its
gravitational energy into thermal
energy to maintain thermal
equilibrium.
Stellar Evolution – Life of a Star
• He is now the fuel source, but the
temperature and energy needed to
ignite He fusion is greater than H.
Thus, the energy released by He
fusion in the core is greater than
needed to support the weight of the
outer layer. The excess energy
expands the outer layers beyond its
previous radius and star’s volume and
mass increases. This expansion is a
Red Giant.
• The added mass and pressure
increases the star’s core temperature
to start helium fusion. The burning
reestablishes thermal equilibrium,
stops gravitational contraction and
restarts outer layer expansion.
Stellar Evolution – Life of a Star
• A Red Giant can be a Red Giant
or a Red Supergiant. The only
difference is radius. Red Giant
is up to 100X larger than our
Sun while the Red Supergiant
is over 1000X larger.
• Due to its enormous size, the
Red Giant’s outer layers are
very cool relative to the core.
This creates the colour and
brightness of the star.
Stellar Evolution – Life of a Star
• After a long period, SHELL HELIUM
BURNING begins, but the surface is very
thin and the star becomes unstable.
Temperature and energy increase, and the
outer layer ignites into fusion by
thermonuclear reactions.
• This stage is called THERMAL PULSE.
Luminosity increases by a factor of ten.
The outer layer expands, and it is ejected
emitting ultraviolet radiation. The radiation
ionizes any ejected gases creating a glow
called planetary nebula. The death is a
LOW-MASS STAR SUPERNOVA.
Stellar Evolution – Life of a Star
• Due to the enormous mass of the
core, the core begins to, once
again, collapse and fuse. The
collapse is not temperature
dependent, and as such
gravitational collapse stops. The
contracted stellar core is a WHITE
DWARF.
Stellar Evolution – Life of a Star
• White Dwarfs are approximately
the size of Earth, but their mass
and density is much greater. A
teaspoon of matter from a white
dwarf would weigh 5.5 metric
tons on the Earth.
• A White Dwarf glows for billions
of year from the energy released
from cooling thermal radiation
until it reaches the temperature
of surrounding space which is a
few degrees above absolute
zero.
Stellar Evolution – Life of a Star
• If the core remnant of a
low-mass star is too
dense to form a white
dwarf, the core collapse
may form a NEUTRON
STAR.
• Neutron stars are
incredibly small AND
extremely dense.
Everything is crushed
such that one teaspoon of
material would weigh a
billion tons.
Stellar Evolution – Life of a Star
• White Dwarfs are also burning, but these
stars burn heavier elements (e.g. C).
Each burning creates and burns a
heavier element (e.g, Ne, O, Si, S and Fe).
The burning raises the core temperature
and each elemental burning period is
shorter than the previous. Fusion cannot
continue after Fe, internal mass
increases, thermal imbalance occurs and
the star collapses. The pressure causes
the protons and electrons to become
neutrons. The additional neutrons are
released as neutrinos by electromagnetic
forces. The shock wave caused by this
force is a HIGH-MASS STAR
SUPERNOVA.
Stellar Evolution – Life of a Star
• If the gravitational
force of a high-mass
star supernova
prevents the escape of
matter, a BLACK HOLE
forms.
• A Black Hole is very
dense…and the
extreme conclusion of
gravity and stellar
evolution.
Stellar Evolution – Life of a Star
• A Black Hole is a region
where matter collapses
to infinite density, and
where, as a result, the
curvature of space-time
is extreme. As well, the
intense gravitational field
of the black hole
prevents any light or
other electromagnetic
radiation from escaping.
Stellar Evolution – Life of a Star
• To summarize, a Black Hole is a
region where matter and energy
disappear from the visible
universe.
• A Black Hole grows by pulling in
the mass (…and the associated
gravitational energy…) around it.
• Theoretically, a Black Hole can
emit particles. A big Black Hole
would emit a particle very slowly;
whereas, a small Black Hole
would have explosions.
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