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Units to cover
• 60-63
Homework 8
Unit 56 problems 6,7
Unit 59 problems 6, 8, 9
Unit 60 problems 6, 8, 11
Unit 61 problems 4, 7
Unit 62, problem 8
Tracking changes with the
HR Diagram
• As a star evolves, its
temperature and luminosity
change.
• We can follow a stars
evolution on the HR
diagram.
• Lower mass stars move on
to the main sequence, stay
for a while, and eventually
move through giant stages
before becoming white
dwarfs
• Higher mass stars move
rapidly off the main
sequence and into the giant
stages, eventually exploding
in a supernova
Interstellar Gas Clouds
• Stars begin as a cloud of cold gas and
interstellar dust, a molecular cloud
• The cloud begins to collapse in on
itself
– Collapse is triggered by a variety of
phenomena
– Stellar winds, explosions, etc.
– Collapse heats the center of the cloud
– gravitational energy is being
converted to heat.
• Rotation of the cloud forces it into a
disk-shape
• After a million years or so, the center
of the disk develops a hot, dense core
called a protostar
Protostars
• Once a dense core
forms in the disk,
the system has
entered the protostar
stage
• Protostars are
difficult to find –
they are shrouded
by gas and dust
• Infrared telescopes
can detect them.
The Eagle Nebula
Protostars
Bipolar Flows
• Once the protostar
heats to around 1
million K, some
nuclear fusion
begins
• Narrow jets of gas
can form, flinging
stellar material
more than a lightyear away!
• These jets can heat
other clouds of gas
and dust
Jets are launched from young stars
A. Due to nuclear blasts in the star
B. Due to magnetic forces acting on accreting material
C. Due to radiation forces from the hot nuclear burning star core
D. Due to gravitational pull of the star on the jet material
Why is it that the majority of stars in the sky are in
the main sequence phase of their lives?
• a. Because this is the only phase that is common to all
stars
• b. because most stars die at the end of main sequence
phase
• c. because most stars in the sky are created at about
the same time
• d. because this is the longest lasting phase in each star
life
Tracking the birth of stars
The birth tracks of lowand high-mass stars
From Protostar to Star
• Low-mass protostars become stars very slowly
– Weaker gravity causes them to contract slowly, so
they heat up gradually
– Weaker gravity requires low-mass stars to compress
their cores more to get hot enough for fusion
– Low-mass stars have higher density!
• High-mass protostars become stars relatively
quickly
– They contract quickly due to stronger gravity
– Core becomes hot enough for fusion at a lower
density
– High-mass stars are less dense!
The CNO cycle
• Low-mass stars rely on the protonproton cycle for their internal energy
• Higher mass stars have much higher
internal temperatures (20 million K!),
so another fusion process dominates
– An interaction involving Carbon,
Nitrogen and Oxygen absorbs protons
and releases helium nuclei
– Roughly the same energy released per
interaction as in the proton-proton
cycle.
– The C-N-O cycle!
Internal Structure of Stars - Convection
• Convection occurs in the
interiors of stars
whenever energy
transport away from the
core becomes too slow
– Radiation carries away
energy in regions where
the photons are not
readily absorbed by
stellar gas
– Close to the cores of
massive stars, there is
enough material to
impede the flow of
energy through radiation
– In less massive stars like the Sun, cooler upper
layers of the Sun’s interior absorb radiation, so
convection kicks in
– The lowest-mass stars are fully convective, and
are well mixed in the interior.
The Main-Sequence Lifetime of a Star
• The length of time a star spends fusing
hydrogen into helium is called its main
sequence lifetime
– Stars spend most of their lives on the main
sequence
– Lifetime depends on the star’s mass and luminosity
• More luminous stars burn their energy more rapidly
than less luminous stars.
• High-mass stars are more luminous than low-mass
stars
• High mass stars are therefore shorter-lived!
• Cooler, smaller red stars have been around for a
long time
• Hot, blue stars are relatively young.
Two Young Star Clusters
How do we know these clusters are young?
Stellar Evolution on the Main Sequence
A Reminder of a Star’s Internal Processes
• The balance of forces in
the interior of a star is
delicate, though stable for
millions or billions of
years.
– A star acts like it has a
thermostat
– If internal temperature
decreases, internal
pressure decreases, and
the star collapses a little,
raising the temperature
• When hydrogen in the
core is exhausted, the
thermostat breaks…
Evolution to red giant phase
• The star is expanding and cooling, so its
luminosity increases while its temperature
decreases
• Position on the HR diagram shifts up and
to the right…
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