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TitleStellar Evolution and Nucleosynthesis, Ryan, Norton
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Page 120

Chapter 5 Helium-burning stars

1 × 104

3M!

1

L/L!

102

2.5 × 103

104

He

2M!

ZA
M
S

5 × 103

15M!

4 × 104

9M!

5M!

106

2 × 104

1M!

Teff/K

Figure 5.4 Helium-burning
evolutionary tracks, following
on from the hydrogen-burning
phase shown in Figure 4.5.
The red lines show the
hydrogen-burning phase, and
the blue lines show the
helium-burning phase. In stars
with M ≤ 2.5M&, helium is
ignited in a degenerate core, and
the star moves back down from
the top of its red-giant track to
the red-giant clump halfway
up the track. In stars with
2.5M& < M ≤ 9M&, the core
is not degenerate when helium
ignites, and these stars move
blueward (i.e. to the left) in the
H–R diagram, but move redward
again (i.e. to the right) once
core-helium burning is replaced
by shell-helium burning. This
path is called a blue loop. In
stars with M > 9M&, helium
ignites when the star is still
close to the main sequence and
does not interrupt its redward
evolution.

hydrogen-burning shell still burns. The hydrogen-burning shell now burns closer
towards the surface, which causes the hydrogen envelope to expand and cool once
more. See Figures 5.5 and 5.6.

Low-mass stars (M ≤ 3.5M&) increase in luminosity again as convection
again becomes important in energy transport, and make a slow return up the
giant branch, the return leg being slightly more luminous than for lower-mass
first-ascent red giants, thus delineating a separate asymptotic giant branch
(AGB), as shown in Figure 5.5a. The AGB is so named because it approaches the
earlier red-giant branch, but does not overlay it.

Intermediate-mass stars (3.5M& < M ≤ 8M&) reverse their blue loops. Broadly
speaking, stars move back towards their helium-ignition points in the H–R
diagram, as shown in Figure 5.5b. The envelope expansion causes the envelope to
become more convective, just as it did when the star moved from core-hydrogen
burning on the main sequence to shell-hydrogen burning on the giant branch.

To distinguish the two phases of giant-branch evolution, the first being from the
main-sequence turnoff to the helium ignition point, and the second being the
AGB, the former is sometimes referred to as the first-ascent giant branch. In
AGB stars with M > (3.5–4) M& the convection reaches deep enough to effect
another dredge-up episode, called the second dredge-up in recognition of its
similarity to the event that occurs on the first ascent of the giant branch. Energy
generation during the first portion of the AGB – the early-AGB or E-AGB – is
dominated by the helium-burning shell. (The hydrogen-burning shell may have
extinguished temporarily.) Later in this phase, the hydrogen shell dominates, and
the fresh production of helium causes the helium-burning shell periodically to
burst into life again. Thermal instabilities in the helium shell drive thermal

120

Page 121

5.8 Helium-burning in a shell

RGB

first thermal pulse

core H exhaustion

first dredge-up

(a) (b)

TP-AGB

core He ignition

Z
A
M

S

post-AGB

He-burning

core
He flash

log10 Teff

core He exhaustion
second dredge-up

E-AGB
post-AGB

E-AGB TP-AGB

core He exhaustion

core H exhaustion
first dredge-up

RGB

ZAM
S

log10 Teff

magnitude

5 M!1 M!

magnitude

Figure 5.5 Schematic evolutionary tracks, including the AGB phases, for (a) 1M& and (b) 5M& stars. The extent
of the blueward evolution a star during helium burning depends on its metallicity and the extent of mass loss. The
AGB phases are shown divided into the early AGB (E-AGB) and the later thermally pulsing AGB (TP-AGB). The
superwind develops as the star reaches the highest luminosities, sending the star into the post-AGB phase.

1.81.4

B − V

22

0.6

47 Tuc

16

12

red giants

14

1.0

subgiants

asymptotic giant
branch (AGB)

mV

24

main sequence

0.2

18

20

horizontal
branch

Figure 5.6 Observational
H–R diagram of the old
globular cluster 47 Tuc,
showing a clear asymptotic
giant branch (AGB) occupied
by shell-helium burning stars
making their way from the
horizontal branch toward the
top of the giant branch.

pulses; this later phase is called the thermally pulsing TP-AGB. Several tens
of pulses, each lasting only ∼ 102 years, may occur for a star at intervals of
∼ 104 years, and it is during these pulses that conditions appear suitable for
s-process nucleosynthesis. This process, which you will study in Chapter 6, is
responsible for the production of many of the elements heavier than iron.

121

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