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The H-R Diagram
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A star’s location on the HR diagram is
given by its temperature (x-axis) and
luminosity (y-axis)
We see that many stars are located on a
diagonal line running from cool, dim
stars to hot bright stars
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Other stars are cooler and more
luminous than main sequence stars
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The Main Sequence
Must have large diameters
(Red and Blue) Giant stars
Some stars are hotter, yet less luminous
than main sequence stars
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Must have small diameters
White Dwarf stars
The Family of Stars
Stars come in all sizes…
The Mass-Luminosity Relation
• If we look for trends in
stellar masses, we notice
something interesting
– Low mass main
sequence stars tend to be
cooler and dimmer
– High mass main
sequence stars tend to be
hotter and brighter
• The Mass-Luminosity
Relation:
L » M 3.5
Massive stars burn brighter!
Massive stars burn brighter
L~M3.5
Luminosity Classes
Stellar Evolution –
Models and Observation
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Stars change very little over a human lifespan, so it is impossible to
follow a single star from birth to death.
We observe stars at various stages of evolution, and can piece together
a description of the evolution of stars in general
Computer models provide a “fast-forward” look at the evolution of
stars.
Stars begin as clouds of gas and dust, which collapse to form a stellar
disk. This disk eventually becomes a star.
The star eventually runs out of nuclear fuel and dies. The manner of
its death depends on its mass.
Evolution of low-mass stars
Evolution of high-mass stars
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?