Download Evolution of High

• Star Formation
• Evolution of Low-Mass Stars
• Evolution of High-Mass Star
Evolution of High-Mass Stars – I
M > 8 – 10 M⊙
The early stages of a high-mass star’s life are similar to the early
stages of the life of low-mass stars, except they proceed much more
rapidly. This is because of the high temperature and high density
condition in the core of the high-mass stars.
• During the main-sequence phase of the star’s life, it allows for a
more efficient process (the CNO cycle) to fuse hydrogen into
helium at a much higher rate.
• The high temperature and high density conditions also allow
fusion of increasingly heavy elements to happen.
– The core fuses heavier and heavier elements
– A multiple-shell-burning is developed
 Supergiants!
• In the final stages of life, the highest-mass stars exhausted all
possible fusion sources. Without an energy source to push
against gravity, the core of the stars implodes suddenly, and the
star explodes into a supernova.
• The left-over core becomes a neutron star!
The CNO Cycle
In the high temperature condition in
the core of the high-mass stars,
another fusion process (the CNO
cycle) can fuses hydrogen into
helium at a much faster rate than the
proton-proton cycle.
• The heavier elements (carbon,
nitrogen, and oxygen) act as
catalysis to speed up the
hydrogen fusion process
• The net result is the same as the
proton-proton chain – the creation
of a helium atom and release of
energy from fusion of four
hydrogen nuclei (protons).
• The numbers of carbon, nitrogen,
and oxygen remain the same
before and after the reaction.
If the stars were born from the primordial interstellar medium of only
hydrogen and helium, then, where are carbon, nitrogen, and
oxygen coming from for CNO cycle to work?
• First generation high-mass main-sequence stars would not have
carbon, nitrogen, and oxygen for CNO cycle to work efficiently
despite of their high core temperature.
Fusion Reactions in Stars to Make Heavy
Elements
Fusion of carbon into heavier elements requires
very high temperature, around 600 million
degrees. There are many fusion reactions
happening in the core of the stars. These
reactions are responsible for producing the
heavy elements. The simplest form is helium
capture by heavier elements. Fusion
between heavy elements are also possible.
• Helium Capture:
capture of helium by heavier elements
such as Carbon, Oxygen, Neon, etc…
• Heavy element fusion…
• And a whole lot more reactions…
Why is it so hard to fuse heavy
elements?
Nuclear fusion of heavier and heavier elements requires higher
and higher temperature.
– The nuclei of heavy elements have larger electric charges.
To fuse them, it is necessary to push them very close
together to overcome the Coulomb barrier between the
nuclei. The high speed necessary to achieve this is
attained at high temperature.
– Recall that the repulsive force between two charged
ee
particles is
F
 1 2
Coulomb
r2
– Therefore, the repulsive force between two carbon nuclei (e
= 6) is 36 times stronger than that between two hydrogen
nuclei.
Evolutionary High-Mass Stars – II
Tracks in the H-R Diagram
For high-mass stars, fusion of successively
heavier and heavier elements (helium,
carbon, nitrogen, oxygen, etc) can take
place in the core.
• For medium-mass stars:
As the star goes through several
stages of core contraction, shell
burning, and core re-ignition, the star
expands into a supergiant and then
contracts accordingly. The star
expands during the shell burning
stage, and contract when the core
fusion is ignited.
• For very-high-mass stars
The contraction and expansion cycle
in the core region proceeds too fast
for the shell to respond. It just grow
steadily into a red supergiant.
Evolution of High-Mass Stars – III
Supergiants
Multiple-Shell Burning in supergiants:
Structure of red supergiant with an iron
core and multiple burning shells.
• Similar to the process that leads
to an inert carbon core and
double-shell fusion of helium
and hydrogen for low-mass
stars, high-mass stars will
develop into a heavy element
fusing core, and multiple-shell
burning outer envelop, releasing
large amount of energy.
• The outer layer of the star is
heated by the multiple-shell
fusion and expands into a
supergiant.
• The core fusion ends when irons
produced by fusion of lighter
elements accumulate in the core.
The Iron Limit
According to quantum mechanics, iron has
the lowest energy per nuclear particle:
• Fusing atoms lighter than iron create
a heavier elements and release
energy. This energy keeps the core of
the stars hot and resists gravitational
collapse…
• However, fusing iron or atoms
heavier than iron into even heavier
elements does not generate energy,
but absorbs energy.
• Once an iron core is formed, the star
runs out of fusion fuel to keep the
core hot and generate thermal
pressure to resist gravitational
contraction.
 The core collapses
 Supernova!
Evolution of High-Mass Stars – IV
Supernova
The degenerate pressure of electrons in the inert iron core cannot
support the star against the pull of gravity only briefly, due to the
high mass of the star. In an instant, electrons are force to combine
with the protons in the iron nuclei to form neutrons, releasing
neutrinos in the process.
• The collapse of the iron core can be stopped by the neutron
degenerate pressure of the newly formed neutron core, if the star
is not too heavy.
• The core becomes a neutron star with a mass of about 1 Msun,
with a size of just a few kilometer! (Chapter 13)
• If the mass of the star is high enough to overcome the neutron
degenerate pressure, then the core collapse into a black hole!
(Chapter 13)
• In either cases, the energy released can be as high as the total
energy released by the Sun through its entire life time. A
supernova can out-shine an entire galaxy!
• The released energy pushes the outer envelop of the star into
surround space.
Supernova of 4th of July, 1054 –
Crab Nebula
Chinese (and Arab and perhaps
Native American) astronomers
recorded the appearance of a
bright new star that can be seen
during the day in 1054…
• Crab nebula was discovered
near the reported location of
the new star in 1731.
• A pulsar (rapid rotating radio
source) was discovered in the
center of the nebula in 1968,
determined to be a rapidly
rotating neutron star!
SN1572 – Tycho Brahe’s
Supernova
On November 11, 1572, Tycho Brahe
observed a very bright star which
unexpectedly appeared in the
constellation Cassiopeia.
• The supernova remnant was
discovered in the 1960s.
• No neutron star has been found
in the supernova remnant!
• Type Ia supernova (Chapter 13)?
X-ray image of SN1572 from
Chandra X-ray Observatory.
http://chandra.harvard.edu/ph
oto/2002/0005/index.html
Supernova SN1987A
Supernova 1987A was observed in
February 23, 1987. It is located in the
Large Magellanic Cloud, about
160,000 lightyears away from us  It
exploded 160,000 years ago.
• Neutrinos burst (total of 24)
were observed by Kamiokande,
IMB (in Ohio), and Baskan
Neutrino Observatory about three
hours before the visible
brightenning…
• No Neutron Star has been
found!
– Progenitor of SN1987A is a
Blue Supergiant (?)
http://chandra.harvard.edu/photo/2005/sn87a/
During supernova explosion,
the electrons can be pushed
into the nuclei to combine
with the proton, producing
neutrons and neutrinos…
Summary: Evolutionary History of Stars
High Mass Star
Low Mass Star
The lifetime of highmass stars are quite
short. For example, it
takes only about 7.5
million years for a 25
M⊙ to complete its
life cycle…
How good is our theory for stellar evolution?
• Stellar Nuclear Synthesis and
Elemental Abundance of the Universe
• Observations of Supernovae
Nuclear Synthesis and Abundance
of Heavy Elements in Stars
In our theory of the stellar evolution, heavy elements (elements
heavier than Helium) are made inside high-mass stars. Therefore,
we expect that
1. first generation stars should not contain heavy elements, and
2. only recently formed stars should have appreciable heavy
elements content, because the can incorporate heavy elements
produced in previous generation of stars during their formation.
Observations of the metallicity of stars have show that:
– Young stars (Population I, formed recently) have metallicity of 2
to 3 %.
– The Sun (age of ~ 5 billion years, formed when the universe was
about 9 billion-year-old) has metallicity of about 1.6%.
– Old stars (Population II) are low in metal. Very old stars in
globular cluster have metallicity less than 0.1%
Nuclear Synthesis and Abundance of
Heavy Elements in the Universe
Our theory of the evolution of stars and the
nuclear fusion processes predict that
1. elements with even-number protons
should out number elements with oddnumber protons, because helium has
two protons in its nuclear. Helium
capture that fuses helium into heavier
elements produces elements with
even-number protons…and
2. Elements heavier than iron should be
very rare, because they are formed
only shortly before and during
supernova expolsion.
Measurement of the abundance of heavy
elements of confirmed these predictions!
Observed relative abundance of
elements in galaxy…
How good is our theory for stellar evolution?
• Stellar Nuclear Synthesis and Elemental
Abundance of the Universe
• Observations of Supernovae
• The Algol Paradox
Supernova SN1987A
Supernova 1987A was observed in
February 23, 1987. It is located in the
Large Magellanic Cloud, about
160,000 lightyears away from us  It
exploded 160,000 years ago.
• Neutrinos burst (total of 24)
were observed by Kamiokande,
IMB, and Baskan Neutrino
Observatory about three hours
before the visible brightenning…
• No Neutron Star has been
found!
– Progenitor of SN1987A is a
Blue Supergiant (?)
http://chandra.harvard.edu/photo/2005/sn87a/
SN1987A – Blue Supergiant
Supernova?
The progenitor of SN1987A was a blue giant
with a mass of about 18 Msun.
– Probably, the high-mass progenitor of
SN1987A lost most of its outer layer by
a slow stellar wind long before the
supernova explosion.
– Right before the supernova explosion,
a fast wind pushes the envelop to
make a cavity around the star. Making
the outer layer of the star unusually
thin and warm  Blue Supergiant.
– The outer gas cloud forms a ring.
– The shockwave from the supernova
explosion was expected to hit the edge
of the ring around 1999.
– Chandra X-ray images from 1999 to
2005 shows brightening of the ring.
SN1987A – Where is the Neutron
Star?
With a mass of 18 Msun, SN1987A was expected to create a neutron
star…However, none has been found so far.
– The neutron star is there, but it is not pulling in materials. Without
materials around it, no X-ray emission can be detected.
– Maybe a black hole (Chapter 13), instead of a neutron star, was
formed?
How good is our theory for stellar evolution?
• Stellar Nuclear Synthesis and Elemental
Abundance of the Universe
• Observations of Supernovae
• The Algol Paradox
The Algol Paradox
Algol is a binary system with a 3.7 M⊙
main-sequence star and a 0.8 M⊙
subgiant. The stars in binary system are
usually formed at the same time. So, why
is the more massive star remains in
main sequence, while the less massive
star has evolved into a giant?
As usual, the real world is often more
complicated than our simplified theory
describe. In the case of the Algol, the
explanation can be found in terms of the
interaction between the two stars…