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Chapter 12
Stellar Evolution
Guidepost
This chapter is the heart of any discussion of
astronomy. Previous chapters showed how astronomers
make observations with telescopes and how they
analyze their observations to find the luminosity,
diameter, and mass of stars. All of that aims at
understanding what stars are.
This is the middle of three chapters that tell the story of
stars. The preceding chapter told us how stars form,
and the next chapter tells us how stars die. This chapter
is the heart of the story—how stars live.
As always, we accept nothing at face value. We expect
theory to be supported by evidence. We expect carefully
constructed models to help us understand the structure
inside stars. In short, we exercise our critical faculties
Guidepost (continued)
and analyze the story of stellar evolution rather than
merely accepting it.
After this chapter, we will know how stars work, and we
will be ready to study the rest of the universe, from
galaxies that contain billions of stars to the planets that
form around individual stars.
Main Sequence Stars
The structure and evolution of a star is
determined by the laws of
• Hydrostatic equilibrium
• Conservation of mass
• Energy transport
• Conservation of energy
A star’s mass (and chemical composition)
completely determines its properties.
That’s why stars initially all line up along the main sequence.
Maximum Masses of Main-Sequence Stars
Mmax = 100 solar masses
a) More massive clouds fragment into
smaller pieces during star formation.
b) Very massive stars lose mass in
strong stellar winds
h Carinae
(Eta Carinae)
Example: η Carinae: Binary system of a 60 Msun and 70 Msun star.
Dramatic mass loss - major eruption in 1843 created double lobes.
Minimum Mass of Main-Sequence Stars
Mmin = 0.08 Msun
Gliese 229B
At masses below
0.08 Msun, gas
doesn’t get hot
enough to ignite
thermonuclear fusion.
These are called
Brown Dwarfs
Brown Dwarfs
Hard to find because they are very faint
and cool; emit mostly in the infrared.
Many have been detected in star forming
regions like the Orion Nebula.
Evolution on the Main Sequence (1)
Main-Sequence
stars live by fusing
hydrogen (H) into
helium (He).
Zero-Age
Main
Sequence
(ZAMS)
MS evolution
A finite supply of
hydrogen means
a finite life time.
Evolution on the Main Sequence (2)
A star’s life time T = energy reservoir / luminosity
Energy
reservoir = M
Luminosity
L = M3.5
T = M/L = 1/M2.5
Massive stars
have short
lives!
Evolution off the Main Sequence:
Expansion into a Red Giant
When hydrogen (H) in
the core is completely
converted to helium (He),
fusion stops.
H burning continues in a
shell around the core.
He core and H burning
shell produce more
energy than needed for
pressure support
Expansion and cooling
of the outer layers of
the star produces a
Red Giant
Expansion onto the Giant Branch
Expansion and
surface cooling during
the phase of an
inactive He core and
a H burning shell
Sun will expand
beyond Earth’s orbit!
Red Giant Evolution
Hydrogen burning
shell keeps dumping
helium onto the core.
4 H → He
He
He-core gets denser
and hotter until the
next stage of nuclear
burning can begin in
the core.
Helium Fusion
He nuclei can fuse to
build heavier elements
like carbon and oxygen
When pressure and
temperature in the He core
become high enough,
Fusion Into Heavier Elements
Fusion into heavier elements (than carbon
and oxygen) requires very high
temperatures and occurs only in massive
stars (more than 8 solar masses).
These stars fuse:
He  C and O then C  Ne, Na,
Mg, O then Ne  O, Mg then O
 Si, S, P then Si  Fe, Co,
Ni….all in the final 0.00008 of it’s
life!
Evidence for Stellar Evolution: Star
Clusters
Stars in a star cluster all have
approximately the same age!
More massive stars evolve more quickly
than less massive ones.
If you put all the stars of a star cluster on a
HR diagram, the most massive stars
(upper left) will be missing!
HR Diagram of a Star Cluster
Example: HR diagram of the star cluster M 55
High-mass stars
evolved onto the
giant branch
Turn-off point
Low-mass stars
still on the main
sequence
Estimating the Age of a Cluster
The lower
on the MS
the turn-off
point, the
older the
cluster.
Evidence for Stellar Evolution:
Variable Stars
Some stars show brightness
variations not caused by eclipsing
in binary systems.
Most important example:
d Cephei
Light curve of d Cephei
Cepheid Variables:
The Period-Luminosity Relation
The variability period of
a Cepheid variable is
correlated with its
luminosity.
The more luminous it
is, the more slowly it
pulsates.
=> Measuring a
Cepheid’s period, we
can determine its
absolute magnitude!
Cepheid Distance Measurements
Comparing absolute and apparent magnitudes of Cepheids,
we can measure their distances (using the 1/d2 law)!
The Cepheid distance
measurements were
the first distance
determinations that
worked out to
distances beyond our
Milky Way!
Cepheids are up to
~ 40,000 times more
luminous than our sun
=> can be identified in
other galaxies.
Pulsating Variables: The Valve Mechanism
Partial He ionization zone is opaque and
absorbs more energy than necessary to
balance the weight from higher layers.
=> Expansion
Upon expansion,
partial He ionization
zone becomes more
transparent, absorbs
less energy => weight
from higher layers
pushes it back inward.
=> Contraction.
Upon compression, partial He ionization zone
becomes more opaque again, absorbs more
energy than needed for equilibrium => Expansion
Period Changes in Variable Stars
Periods of some Variables are not constant over time
because of stellar evolution.
 Another piece of evidence for stellar evolution.
New Terms
conservation of mass law
conservation of energy
law
stellar model
brown dwarf
zero-age main sequence
(ZAMS)
degenerate matter
triple alpha process
helium flash
open cluster
globular cluster
turnoff point
horizontal branch
variable star
intrinsic variable
Cepheid variable star
RR Lyrae variable star
period–luminosity relation
instability strip