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Evolution from the Main
Sequence
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Chapter 18
Chapter 18
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During its stay on
the main
sequence, any
fluctuations in a
star’s condition
are quickly
restored; the star
is in equilibrium.
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Evolution from the Main Sequence
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Projected Lifetime of Stars
Normal to Giant
Normal to Giant Size Comparison
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Again to follow the post-main-sequence evolution of a
star we will resort to the stage method. Not every star
adheres to this sequence but it serves to describe the
steps that many stars take
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Evolution from the Main Sequence
Even while on the main sequence,
Stage 7, the composition of a star’s
core is changing.
Eventually, as hydrogen in the core
is consumed, the Star leaves the
main sequence, Stage 8.
Its evolution from then on depends
very much on the mass of the star:
Low-mass stars go quietly.Mediummass stars struggle.High-mass stars
go out with a bang!
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When the fuel in the
core is used up the
fusion ceases. The
result is a contraction
of the Star and the
formation of a new
fusion furnace in a
shell around the
helium core. This is
Stage 8
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Evolution from the Main Sequence
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Stage 9: The Red-Giant
Branch. The now much larger
surface of the furnace causes
outer layers of the star to
expand and cool. It is now a
red giant, extending out
beyond the orbit of Mercury.
Despite its cooler surface
temperature, its luminosity
increases enormously due to
its large size.
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Evolution from the Main Sequence
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Stage 10: Helium fusion. Once the core
temperature has risen to 100,000,000 K, the
helium in the core can fuse, through a three-alpha
process:
The 8Be nucleus is highly unstable, and will decay in
about 10–12 sec unless an alpha particle fuses with
it first. This is why high temperatures and densities
are necessary.
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Evolution from the Main Sequence
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Evolution from the Main Sequence
UVU Survey of Astronomy
The pressure within the helium core is almost totally due to
“electron degeneracy” – two electrons cannot be in the same
quantum state, so the core cannot contract beyond a certain
point. This pressure is almost independent of temperature so
when the helium starts fusing, the pressure cannot adjust and
the core explodes completely disrupting the surrounding shell
furnace.
Helium begins to fuse extremely rapidly; within hours to days
the enormous energy output is over, but the star is now on its
way to White Dwarf, Stage 13
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The helium flash:
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Stage 10. Disruption of the
hydrogen furnace throws the
star out of equilibrium and it
starts to shrink, but it has much
heat to dissipate from the
Helium Flash. The result is the
surface gets smaller as the
surface temperature gets higher,
causing movement across the
graph toward the blue while
maintaining nearly the same
brightness.
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Evolution from the Main Sequence
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Stages 11 and 12 depend very much on the mass of
the star.
From .5 to 1.4 solar masses the transition from the
horizontal branch White Dwarf goes smoothly.
From 1.4 to about 5.5 solar masses they must shed the
extras mass to get down to the Chandrasakar limit of
1.4 solar masses, then they can transition to White
Dwarf.
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Horizontal Branch, Stage 10.
The Helium Flash has disrupted
the nuclear shell and the star is
dying. There is much heat to be
shed before the star can
become a White Dwarf. The star
shrinks by gravity but remains
bright by the surface getting
hotter. Stars over 1.4 Solar
masses also must rid
themselves of the extra mass.
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Evolution from the Main Sequence
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The differences in the H–R diagrams of
open and globular clusters is that the
globular clusters are very old, while the open
clusters are much younger.
The absence of massive main sequence
stars in the globular cluster is due to its
extreme age – those stars have already
used up their fuel and have moved off the
main sequence.
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This is a young star cluster called the Pleiades. The H–
R diagram of its stars is on the right. This is an example
of an open cluster.
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This is a globular cluster – note the absence of massive
main-sequence stars, and the heavily populated Red
Giant region.
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The Instability Strip, still
Stage 10 .. As the dying star
moves along the horizontal
branch it encounters a
region, discovered by
Hertsprung, called the
Instability Strip. The star
becomes a variable star
changing brightness slightly
in a very few days.
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.
Born July 4, 1868
Lancaster, Massachusetts
Died December 12, 1921 (aged 53)
Cambridge, Massachusetts
Henrietta Levitt
Institution: Harvard University
Alma mater: Radcliffe College, Oberlin
College
Known for period-luminosity relationship
of Cepheid stars
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Henrietta Levitt discovered a
direct relationship of Period
to Luminosity of the Cephied
Variables and the RR Lyra
Variables
Absolute Magnitude
It was later shown that
there are two families
of Cephieds
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Evolution from the Main Sequence
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Some stars with more than about
5.5 solar masses have a different
problem. The Helium flash
becomes a permanent nuclear
furnace. The Helium core fuses
helium to carbon and the shield
furnace continues to fuse Hydrogen
to Helium and the star is now in a
some what stable state. Many stars
go into a new Red Giant condition
for a period. This is the Asymptotic
Giant Branch, Stage 11
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