Transcript Slide 1

The Sun in the Red Giant Phase
(view from the Earth!)
Evolution Low-Mass Stars
Beyond the Main Sequence
• M < 4M_Sun
• Once the star reaches the MS, it spends most of
its lifetime in the H  He nuclear burning phase
• When the hydrogen in the center is exhausted,
the star forms a He-core and the H-burning shell
moves outward; the star expands and cools, and
becomes a Red Giant moving up from the MS
• Helium in the center of core remains inert until
the density, pressure, and temperature increase
to 108 K needed to ignite it  Helium Flash
Helium Burning: Triple-a Reaction
• Intermediate step: Beryllium formation
4He + 4He  8Be + energy (photons)
• Fusion to Carbon
8Be + 4He  12C + energy g (photons)
• Helium core is highly dense and electrons are
packed together in a degenerate state
• Electrons as close together as possible and
therefore exerting degeneracy pressure against
further gravitational contraction
• But temperature rises  explosive He burning
He-Burning: He C
Triple-Alpha (He-nuclei) Reaction
At temperatures T >
Oxygen:
108
K
Notation:
4He
2
2 protons
+
2 neutrons
# Protons:
Atomic
Number
in Periodic
Table
Solar-type star
Main Sequence Lifetime of Solar-type Star
Evolution beyond the Red Giant
• L does not increase at the onset of the He-flash itself
since the central region of the core is quite opaque
• The H-burning shell is slowly extinguished and L
decreases, even as the star shrinks and temperature
rises; the star moves leftward along a nearly
Horizontal Branch on the H-R diagram
• Luminosity rises again as the energy from the Heburning core of the RG rises to the surface
• The star then resumes its climb up the H-R diagram
along a second vertical branch – the
Asymptotic Giant Branch (AGB)
Evolution Beyond the AGB Phase
• He-burning via the triple-alpha fusion is highly
temperature sensitive
• The AGB star is unstable; radiation pressure from
the interior push away the envelope – hot core
separates from the envelope
• Hot core is mainly C-O (products of triple-alpha)
• Hot core is very luminous initially, but rapidly cools
through a Planetary Nebula (PN) phase (NO relation
to planets!)
• The PN C-O core surrounded by the brightly lit
ejected envelope appears as a ‘ring’
• The PN core cools and collapses to White Dwarf
Central Star and Spherical Ejected Shell
Cat’s Eye Planetary Nebula
Planetary Nebulae and White Dwarfs
• The ring shaped PN is ionized and heated by the hot
central core; takes about 10,000 years
• Hot PNe have C-O stellar core at about 100,000 K
• Moves left on the H-R diagram as it is exposed
• Moves BELOW the MS as it cools, shrinks, and
becomes less luminous
• Matter in the cold core is ‘degenerate electron gas’,
not an ideal gas; Pressure is independent of
temperature; contraction of the core stops when the
pressure equals gravity; star becomes White Dwarf
• R (WD) ~ 0.01 R (Sun) ~ R (Earth)
• WD cools away into a ‘stellar corpse’ ! BUT, may turn
into a huge DIAMOND (Carbon crystal) !!
Pne  WD Tracks
Post-MS Evolution of Low-Mass Stars
1.
2.
3.
4.
5.
6.
7.
End of H  He burning in the core of MS star
Red Giant phase with inert He-core and outer H-burning
shell; star expands and cools, but is brighter
Climbs up the RG branch until He-flash in the core
Core expands and cools; H-burning decreases; outer
layers contract; luminosity decreases but temperature
increases; star moves LEFT on the H-R diagram along
the Horizontal Branch
He-burning shell eventually moves outward and the star
becomes more luminous and climbs up the AGB, with
He- and H-burning outer shells but inert C-O core
The envelope of the AGB star is radiatively pushed
away, separates from the core, and the star becomes a
Planetary Nebula
The C-O core eventually becomes a White Dwarf
Stellar Lifetimes
• Lifetimes depend on Mass M and Luminosity L
• L determines the rate of energy production, and
is proportional to M3.5
• A fraction of M is converted to energy E = fMc2
• If t is the lifetime of the star then
L t = fMc2
OR
lifetime t is proportional to M / L
e.g. If M = 2 M(Sun), then L = 12 times L (Sun), and
has a lifetime about 6 times shorter
Ages of Stellar Clusters
• H-R diagram yields information on L, M, T, R, and
color of stars; most characteristics except age
• But may determine the age of a stellar cluster,
formed at the same time and composition, from the
evolution of stars in the cluster with different
masses  isochrones
• High mass stars evolve off the MS (“turn off”)
before low mass stars
Evolution and nucleosynthesis of
High Mass Stars
• Very different structure and evolution from low mass
star
• Mass more than about 4 times M(Sun), but luminosity
up to 10,000 times L(Sun) or more
• Burn brightly, evolve rapidly, die relatively quickly
• CNO cycle is more efficient in H  He fusion than the
p-p chain; requires higher temperatures prevalent in
cores of high-mass stars
• At over 600 million K elements heavier than CNO are
fused, e.g.
12C + 12C  24Mg + energy
H  He Nuclear Fusion Via the C-N-O Cycle in Massive Stars
e+ positron
Positive
electron
annihilates
negative
electron
(matterantimatter)
e - + e+ = g
energy
Ordinary Isotopes:
12C, 14N, 16O
act as catalysts
Evolution of Supergiants: Constant Luminosity
Evolution of Supergiants Beyound He-buring
Evolution of High-Mass Stars Beyond the MS
• M > 4 M (Sun) – O and B stars
• Burn H  He via the more efficient CNO cycle
• After H-core exhaustion the He-core contracts and
heats up, but the H-burning continues around the
He-core and the star puffs up
• The star expands and cools, but the luminosity
remains constant since the huge outer layers are
opaque
• It moves right on the H-R diagram as a Red
Supergiant
• Takes about a million years to cross the H-R
diagram
Blue Supergiant Phase
• Core temperature reaches T > 100 million K; the
He-flash ignites He-burning to C and O via the
Triple-alpha nuclear fusion reaction
• With a H-burning shell, a He-burning core, the star
builds up a C-O core and becomes a Blue
Supergiant, moving leftward on the H-R diagram,
following the He-flash
• After He-core exhaustion, the C-O core collapses
and heats up, with H and He burning outer shells,
and the star expands and becomes a Red SG again,
moving right on the H-R diagram
• Carbon ignites when core T > 600 MK, density >
150,000 g/cc
Crisscrossing the HR Diagram
Intermediate and High Mass Stars
A dichotomy emerges:
1. Intermediate mass star: 4 M(Sun) < M < 8 M(Sun)
- Carbon burning reactions produce O, Ne, Mg
- no further burning, inert O-Ne-Mg core WD,
after about 1000 years
2. High mass stars: M > 8-10 M(sun)
- evolve rapidly with strong stellar winds
(radiation driven)
- O-Ne-Mg core heats up to T ~ 1.5 billion K,
density ~ 10 million g/cc, and ignites Neon burning
to Mg and Si; lasts only a few years
- Oxygen shell burns up to Si, S, P…(Si-core)
SUPERNOVA
Fiery Explosive Death of Massive Stars
• In M > 8 M(Sun) stars the Si-core ignites and burns
up to Fe-Ni
• No further fusion possible since fusion beyound
iron requires energy rather than produce it
• Once an iron-core has been formed, the star no
longer has any fuel source
• When M (Fe-core) > 1.4 – 2 M(Sun), the Fe core
contracts, heats up, and explodes….SUPERNOVA
• The envelope is ejected and the iron core collapses
into
Neutron Star or BLACK HOLE