Download Lecture 13 (pdf from the powerpoint)

If we know that a group of stars are at the same distance we
can plot the following two parameters in place of
Luminosity and Temperature on the H-R diagram
a) Period and luminosity
b) Surface gravity and color
c) Brightness and color
d) Diameter and brightness
e) None of the above
Stars come in all sizes…
• L vs T
b=
L
4 "d 2
b=brightness,
d=distance away
!
2.9 #106 K " nm
T=
!
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Magnitude vs color
If two stars are on the main
sequence, and one is more
luminous than the other, we can
be sure that the
a) more luminous star will have
the longer lifetime
b) fainter star is the more
massive
c) more luminous star is the
more massive
d) more luminous star will have
the redder color
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Massive stars burn brighter
Stellar Evolution –
Models and Observation
• 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.
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Preview: Evolution of low-mass stars
Preview: Evolution of high-mass stars
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•
•
•
•
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
Stellar Evolution
Before…..During……and After….
The Main Sequence
It’s all about gravity……
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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
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Tracking the birth of stars
Protostar
•Gravitational contraction
•heats to thousands of K
•Huge! Hundreds of times
the sun
•Thousands of times the
surface area
•Much more luminous than
the sun
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!
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The birth tracks of lowand high-mass stars
Star
•Gravitational contraction
reaches hot enough for
fusion
•0.08 M sun
•88 M Jupiter
• smaller > Brown dwarf
•Upper limit ~ 100Msun
Two Young Star Clusters
How do we know these clusters are young?
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Stellar Evolution on the Main Sequence
Stellar Evolution on the Main Sequence
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The Main-Sequence Lifetime of a Star
High-mass stars 10 mpg
Low-mass stars 60 mpg
• 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.
What determines when a star becomes a main-sequence star?
a) When nuclear fusion generates the energy required to balance
gravity
b) When convection begins in the core.
c) When optical radiation leaves the star
d) When the temperature in the core reaches a higher
temperature than the corona.
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Stellar Evolution on the Main Sequence
High-mass stars
Low-mass stars
Evolution to red giant phase
Fuel runs out
Core pressure drops
Gravity compresses
core
Core temperature rises
Shell burning
Pressure puffs outer layers
Core heats up more
Shell burning grows stronger
Atmosphere expands and
cools further
• 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…
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Evolutionary tracks of giant stars
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…
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