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• 65, 66, 67
Evolutionary tracks of giant stars
CNO cycle happens
A. In protostars as they are not hot enough
B. In the stars similar to our Sun
C. In high mass stars with very hot core
D. In fully convective low mass stars
When a star leaves the main sequence and expands
towards the red giant region, what is happening
inside the star?
• a. Hydrogen burning is taking place in a spherical
shell just outside the core; the core itself is almost
pure helium.
• b. Helium is being converted into carbon and oxygen
in the core.
• c. Helium burning is taking place in a spherical shell
just outside the core.
• d. hydrogen burning is taking place in a spherical
shell, while the core has not yet started thermonuclear
reactions and still mostly hydrogen.
Helium Fusion
• Normally, the core of a star is not hot
enough to fuse helium
– Electrostatic repulsion of the two
charged nuclei keeps them apart
• The core of a red giant star is very
dense, and can get to very high
temperatures
– If the temperature is high enough,
helium fuses into Beryllium, and then
fuses with another helium nuclei to
form carbon.
A (temporary) new lease on life
• The triple-alpha
process provides a
new energy source
for giant stars
• Their temperatures
increase
temporarily, until
the helium runs out
• The stars cool, and
expand once again
• The end is near…
Light Curves
• To characterize the
variability of a star,
scientists measure the
brightness, and plot it as a
function of time.
– Light Curves
• Different kinds of
variability
– Irregular Variable
• Novae (death)
• T Tauri stars (birth)
– Pulsating Variable
• Periodic changes in
brightness
Yellow Giants and Pulsating Stars
• If you plot the positions of variable stars on the HR diagram,
many of them fall in the “instability strip”
– Most have surface temperatures of ~5000K, so appear yellow
– Most are giants (Yellow Giants)
– Instability comes from partial absorption of radiation in the interior
of the star
• Helium absorbs radiation, and the outer layers of the star get pushed away
from core
• As the star expands, the density decreases, letting photons escape
• Outer layers head back inward toward core
• Repeat
– RR Lyrae and Cepheid variables are useful for finding distances to
the stars, as the star’s period is proportional to its luminosity.
The Valve Mechanism
A Cepheid variable is
• a. a low mass red giant that varies in size and
brightness in an irregular way
• b. a big planet
• c. a high-mass giant or supergiant star that pulsates
regularly in size and brightness
• d. a variable emission nebula near a young star
Cepheid and RR Lyrae Variables
The Period-Luminosity
Relation
Periods of Variable Stars
In terms of nuclear reactions, what is the next stage
of a star's life after the end of hydrogen burning in
the core?
•
•
•
•
a. Hydrogen burning in a thin shell around the core
b. Helium burning in the core
c. carbon burning
d. death
What makes a red giant star so large?
• a. The star has many times more mass than the Sun.
• b. The helium-rich core expanded, pushing the outer
layers of the star outward.
• c. Red giants are rapid rotators, and the centrifugal
forces pushes the surface of the star outwards.
• d. The hydrogen-burning shell is heating the envelope
and making it expand.
A Cepheid variable is
• a. a low mass red giant that varies in size and
brightness in an irregular way
• b. a big planet
• c. a high-mass giant or supergiant star that pulsates
regularly in size and brightness
• d. a variable emission nebula near a young star
The Fate of Sunlike Stars
• The Sun’s Lifetime:
– 10 billion years on the main sequence
– Once the hydrogen is consumed, it
will enter the red giant phase
– Helium burning begins, starting the
yellow giant phase
– Once helium is consumed, core
contracts and outer envelope
expands, beginning the red
supergiant phase
– Core begins to cool and the outer
envelope expands again, forming a
planetary nebula
– The core remains as a white dwarf
The Life-path of the Sun
Formation of Planetary Nebula
• As a red giant expands, it cools
– Outer layers cool enough for carbon
flakes to form
– Flakes are pushed outward by
radiation pressure
– Flakes drag stellar gas outward with
them
– This drag creates a highspeed stellar wind!
– Flakes and gas form a
planetary nebula
The Hourglass Nebula
White Dwarf Stars
• At the center of the
planetary nebula lies the
core of the star, a white
dwarf
– Degenerate material
– Incredibly dense
• Initially the surface
temperature is around
25,000 K
• Cools slowly, until it
fades from sight.
Figure 64.05e
Our Sun will end its life by becoming
•
•
•
•
A. a molecular cloud
B. a pulsar
C. a white dwarf
D. a black hole
Mass Transfer and Novae
• A Roche lobe can
be seen as a sphere
of gravitational
influence around a
star
• Red Giant stars can
fill their Roche
lobes
• In a binary star
system, the Roche
lobes of the two
stars can touch, and
mass can pass
between them.
• If a white dwarf is in orbit around a red giant
companion star, it can pull material off the
companion and into an accretion disk around
itself
• Material in the accretion disk eventually falls to
the surface of the white dwarf
Novae
• If enough material accumulates on the white dwarf’s
surface, fusion can be triggered, causing a massive
explosion
• This explosion is called a nova
• If this process happens repeatedly, we have a recurrent
nova.
A Post-nova expansion
The Chandrasekhar Limit
and Supernovae
•
•
•
If mass is added to a white dwarf, its
gravity increases
If the white dwarf mass exceeds 1.4 solar
masses (the Chandrasekhar Limit), the end
of the white dwarf is near.
The additional gravity squeezes the
degenerate material in the white dwarf,
causing it to compress by a small amount
• This compression causes the
temperature to soar, and this
allows carbon and oxygen to
begin to fuse into silicon
• The energy released by this
fusion blows the star apart in a
Type 1a supernova
Type 1a Supernova – Another
standard candle!
• The light output from a
Type 1a supernova
follows a very
predictable curve
– Initial brightness
increase followed by a
slowly decaying “tail”
• All Type 1a supernova
have similar peak
luminosities, and so can
be used to measure the
distance to the clusters
or galaxies that contain
them!
Formation of Heavy Elements
• Hydrogen and a little helium were formed shortly after the Big Bang
• All other elements were formed inside stars!
• Low-mass stars create carbon and oxygen in their cores at the end of
their lifespan, thanks to the higher temperatures and pressures present in
a red giant star
• High-mass stars produce heavier elements like silicon, magnesium, etc.,
by nuclear fusion in their cores
– Temperatures are much higher
– Pressures are much greater
• Highest-mass elements (heavier than iron) must be created in
supernovae, the death of high-mass stars
The Lifespan of a Massive Star
Layers of Fusion Reactions
• As a massive star burns its hydrogen,
helium is left behind, like ashes in a
fireplace
• Eventually the temperature climbs
enough so that the helium begins to
burn, fusing into Carbon. Hydrogen
continues to burn in a shell around
the helium core
• Carbon is left behind until it too starts
to fuse into heavier elements.
• A nested shell-like structure forms.
• Once iron forms in the core, the end
is near…
Core Collapse of Massive Stars
• Iron cannot be fused into any heavier element, so it
collects at the center of the star
• Gravity pulls the core of the star to a size smaller
than the Earth’s diameter!
• The core compresses so much that protons and
electrons merge into neutrons, taking energy away
from the core
• The core collapses, and the layers above fall
rapidly toward the center, where they collide with
the core material and “bounce”
• The “bounced material collides with the remaining
infalling gas, raising temperatures high enough to
set off a massive fusion reaction. The star then
explodes.
• This is a supernova!
Before and After – a Supernova