Download Stellar Evolution – Life of a Star

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.
• What? Red is cool? Why?
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 its enormous
mass ., 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.
Stellar Evolution - Summary
• This is a Hertzsprung-Russel
Diagram…a very powerful too.
• It shows the relationship between
luminosity (Y-axis) and surface
temperature (X-axis). It shows all
the possible types of stars.
• The long band is the Main
Sequence. Our Sun is in the
middle. Why?
• The star Gliese is also in the Main
Sequence. On the “habitability
scale,” it is more habitable than
Earth
Stellar Evolution - Summary
• This diagram illustrates all the possible “star lives.” Our
Sun is in the fourth line of stellar evolution.
Just for fun
Go to this URL of a Hertzsrung-Russel diagram
http://astro.unl.edu/naap/hr/animations/hr.html
• Look at the diagram. Note that you can change the x-axis and the yaxis. Set the x-axis to TEMPERATURE and the y-axis to
LUMINOSTITY.
• Make sure the “Luminosity Classes” is turned ON. Our sun should
have a temperature of about 5800oK and a luminosity of 1.0 (This
number is used to compare a star’s brightness to our Sun. Thus, we
are 1.0)
• Below this information is the radius (size) of the star. This number is
also compared to our Sun. Again, we are 1.0.
• Note that you can click and drag the red “X” around on the HR
diagram. The star type will be shown compared to our Sun. The
temperature, luminosity and size will change. Also note the reset at
the top right.
Fun continued
Questions
• For all questions, draw a thumbnail sketch of the HR diagram.
• Move the “x” around the region of the Red Giant stars. What is the
range (lowest to highest) of temperature? …of luminosity?
• Move the “x” around the region of the Supergiant stars. How and
why do the colours oft hese stars change? Discuss the range of
colour, luminosity and size.
• Most of the stars in the Universe are Red Dwarfs. How would you
describe their characteristics (i.e.,size, colour, temperature,
luminosity)?
• What are the characteristics of White Dwarf stars? What
characteristic do they share (other than colour)? Where are they on
the diagram?
• Move the “x” along the Main Sequence. Moving from bottom right to
upper left, describe the ways that the stars differ.
• Change the x-axis to “Spectral Type.” What is the spectral class (O,
B, A) or type of our Sun?. What is Polaris’ class?
• Betelgeuse (in Orion) has a temperature of 3500oK and a luminosity
140,000 X greater than our Sun. Where is it on the diagram? What
is its radius compared to our Sun? What is its group and spectral
type?
Another Bust-a-Gut Production