Evolution of HighMass Stars

1
Evolution of High-Mass Stars
A 15-solar- mass star can fuse not just hydrogen,
helium and carbon but also oxygen, neon,
magnesium, and even heavier elements as its inner
core continues to contract and its central
temperatures continue to rise. The star achieves
a central temperature of 108 K while still quite
close to the main sequence. As each element is
burned to depletion at the center, the core
contracts, heats up, an fusion starts again. A
new inner core forms, contracts again, heats
again, and so on. The stars evolutionary track
continues smoothly across the H-R diagram,
seemingly unaffected by each new phase of
burning. The stars luminosity stays roughly
constant as its radius increases and its surface
temperature drops. It swells to become a red
supergiant.
2
Heavy Element Fusion
Each stage of thermonuclear reactions in a
high-mass star helps to trigger the succeeding
stage. In each stage, when the star exhausts a
given variety of nuclear fuel in its core,
gravitational contraction takes the core to
ever-higher densities and temperatures, thereby
igniting the ash of the previous fusion
stage-and possibly the outlying shell of unburned
fuel as well. Each stage of core fusion in a
high-mass star generates a new shell of material
around the core. After several such stages, the
internal structure of a truly massive star-say 25
solar mass or greater-resembles that of an onion,
as the Fig. shows. Because thermonuclear
reactions can take place simultaneously in
several shells, energy is released at such a
rapid rate that the stars outer layers expand
tremendously. The result is a supergiant star.
Several of the brightest stars in the sky are
supergiants.
3
The Death of a High-Mass Star
Once the inner core begins to change to iron, our
high-mass star is in trouble. Iron is so stable
(i.e. its biding nuclear energy is so high) that
energy cannot be extracted by fusing it into
heavier elements. With the appearance of
substantial
quantities of iron, the central fires cease for
the last time at the core, and the gravity pull
overwhelms the pressure of the hot gas, so the
core compresses and its temperature rises to
nearly 10 billion K. At these temperatures
individual photons are energetic enough to split
iron into lighter nuclei and then break those
lighter nuclei apart until only protons and
neutrons remain. This process is known as
photodisintegration. In less than a second the
collapsing core undoes all the effects of nuclear
fusion that occurred during the previous 10
million years. But to split iron and lighter
nuclei into smaller pieces requires a lot of
energy. After all, this splitting is the opposite
of the fusion reactions that generated the
stars energy during earlier times. The process
thus absorbs some of the cores heat energy,
reducing the pressure and accelerating the
collapse. Now the core consists entirely of e- n,
p and ? at enormously high densities making
possible the nuclear reaction e- p ? n ? .
Most of the neutrinos (?) escape into space
carrying away still more energy.
4
Nuclear Binding Energy
5

• The second movie "Back to the beginning"
  (Origins by Neyl DeGrasse Tyson, NOVA) will be on
  Tue. Dec. 6, 2005. Last chance for the written
  assignment!

• Homework 4 From Dec. 6 To Dec. 15 (Sec. 2)
  or Dec. 13 (Sec. 3).
• Individual Exam 4
• Sec. 2 Dec. 15, 800 AM
• Sec. 2 Dec. 13, 300 PM

6
Type II Supernovae Explosion
Animation
There is nothing to prevent the collapse from
continuing all the way to the point at which the
neutrons themselves come into contact which each
other, at the incredible density of about 1015
kg/m3. At this density, the neutrons in the
shrinking core play a role similar in many ways
to that of the electrons in the white dwarf. When
this point is reached then the cores contraction
comes to a sudden halt, and the innermost part of
the core actually bounces back and expands
somewhat. This core bounce sends a powerful wave
of pressure, like an unimaginably intense sound
wave, outward into the outer core. During this
critical stage, the cooling of the core has
caused the pressure to decrease profoundly in the
regions surrounding the core. Without pressure to
hold it up against gravity, the material from
these regions plunges inward at very high speeds.
When this inward-moving material encounters the
outward-moving pressure wave it rebounds back
toward the stars surface. Although the details
of how this shock wave reaches the surface and
destroys the star are still uncertain, the end
result is not The star explodes in one of the
most energetic events known in the universe. This
spectacular death rattle of a high-mass star is
known as a core-collapse supernova.
7
The Formation of Heavy Elements
As known fusion stops at iron. The fact that iron
nuclei will not fuse to release energy and create
more massive nuclei is he basic underlying cause
of Type II supernovae. How then were even heavier
elements, such as copper, lead, gold, and
uranium, formed? Some of these elements can be
formed during the late red-giant stage of
low-mass stars via reactions involving neutrons.
But an important contribution can be made by
supernovae. Before the outer layers of the
supernovae are ejected into space, it is
compressed so much by the passage of the shock
wave through the stars outer layers that a new
wave of thermonuclear reactions sets in. These
reactions produce many more chemical elements,
including all the elements heavier than iron.
These reactions require a tremendous input of
energy, and thus cannot take place during the
stars pre-supernova lifetime. The energy-rich
environment of a supernova chock wave is almost
the only place in the universe where such heavy
elements as zinc, silver, tin, gold, mercury,
lead, and uranium can be produced. Remarkably,
all of these elements are found on the Earth.
This means that our solar system, our Earth, and
our bodies include material that long ago was
part of a star that lived, evolved, and died as a
supernova .
8
Neutron Stars
What remains after a supernova explosion? For a
Type II (core-collapse) supernova, theoretical
calculations indicate that part of the star might
survive the explosion. Recall that in a Type II
supernova, the iron core of a massive star
collapses until its neutrons
effectively come into contact with one another.
At that point the central portion of the central
core rebounds, creating a powerful shock wave
that races outward through the star, violently
expelling matter into space. The key point here
is that the shock wave does not start at the very
center of the collapsing core. The innermost part
of the core-the region that rebounds-remains
intact as the shock wave it produces destroys the
rest of the star. After the violence of the
supernova has subsided, this ultracompressed ball
of neutrons is all that is left. Researchers
colloquially call this core remnant a neutron
star. Neutron stars are extremely small and very
massive, they are not much larger than many of
Earths major cities (see the Fig.), or than a
small asteroid. With so much mass squeezed into
such a small volume, neutron stars are incredibly
dense (1017 or even 1018 kg/m3), nearly a billion
times denser than a white dwarf. A single
thimbleful of neutron-star material will weight
100 million tons-about as much as a good-sized
terrestrial mountain. Gravity there is so high
that a 70-kg human standing on it would weight
the Earth equivalent of about one million tons.
So the person will be flattened as a paper.
9
Supernova Remnants
After the explosion there is a remnant core (a
neutron star) and a nebula, the envelope of the
high-mass star that can expand into space at
several thousand kilometers per second (See in
each Fig. the remnant nebula corresponding to two
supernovae).
10
Neutron Stars Properties
It was not until 1932 that the English physicist
James Chadwick discovered the subatomic particle
called the neutron. Within a year of Chadwicks
discovery, two astronomers, Fritz Zwicky, from
the California Inst. of Technology, and Walter
Baade, from Mount Wilson Observatory, were
inspired to predict the existence of neutron
stars. Nevertheless, most scientists politely
ignored it for years. Only after astronomers
discovered pulsating radio sources in the late
1960s neutron stars were considered
seriously. In addition to large mass and small
size, newly formed neutron stars have two other
very important properties. First, they rotate
extremely rapidly, with periods measured in
fraction of a second. This is a direct result of
the law of conservation of angular momentum,
which tells us that any rotating body must spin
faster as it shrinks, and the core of the parent
star almost certainly had some rotation before it
began to collapse. Second, they have very strong
magnetic fields. The original field of the parent
star is amplified as the collapsing core squeezes
the magnetic field lines closer together,
creating a magnetic field trillions of time
stronger than Earths field. These two
properties make possible neutron stars
observations. The fact that we now have strong
observational evidence not just for their
existence but for their vitally important roles
they play in many areas of high-energy
astrophysics is yet another testament to the
fundamental soundness of the theory of stellar
evolution.
11
Neutron Star Observed!
The first observation of a neutron star occurred
in 1967 when Jocelyn Bell, a graduate student at
Cambridge University, observed an astronomical
object emitting radio radiation in the form of
rapid pulses. Each pulse consisted of a roughly
0.01-s burst of radiation, separated by precisely
1.34 s from the next. Cambridge team at first
suspected that those pulses might not be of
natural origin. Instead, they might be signals
from an advanced alien civilization. That
possibility had to be discarded within a few
months after several more of these pulsating
radio sources, which came to be called pulsars,
were discovered across the sky. Another
possibility-that pulsars were rapidly rotating
white dwarf stars-was also finally ruled out
after the discovery of a pulsar in the middle of
the Crab Nebula, the supernova remnant left
behind by the supernova observed in 1054 by Yang
Wei-Te, the imperial astronomer to the Chinese
court. When Bell made her discovery in 1967,
she did not know what she was looking at. Indeed,
no one at the time knew what a pulsar was. The
explanation won Bells thesis advisor, Anthony
Hewish, the 1974 Nobel Prize in Physics.
12
Pulsars
A pulsar should be a compact, spinning neutron
star that periodically flashes radiation toward
Earth. The combination of powerful magnetic
fields and rapid rotation act like a giant
electric generator, creating very strong electric
fields. These fields are so intense that part of
their energy is used to create electrons and
positrons in a process called pair production.
The created charged particles can be accelerated
to extremely high energies by the stars rotating
magnetic field. As the charged particles are
accelerated they emit energy in the form of
electromagnetic radiation. The result is that two
narrow beams of radiation pour out of the neutron
stars north and south magnetic polar regions.
The name pulsar may lead to think that the source
of radio waves is actually pulsating. Instead, in
pulsars beams of radiation are emitted
continuously from the magnetic poles of the
neutron star. The detected pulses on the Earth
are a result of the rapid rotation of the star,
which brings one of the beams periodically into
our line of sight.
13
Gamma Ray Pulsars
Most pulsars emit pulses in the form of radio
radiation. However, some also pulse in the
visible, X-ray, and gamma-ray part of the
spectrum. The Fig. shows the Crab and nearby
Geminga pulsars in gamma rays. In Fig. (a) the
Crab and Geminga pulsars, which happen to lie
very close to one another in the sky, are shown.
Unlike the Crab, Geminga is barely visible at
optical wavelengths, and undetectable in the
radio. In Fig. (b) it is shown a sequence of
images of Gemingas 0.24-s pulse (The Crabs 33
millisecond period is too rapid to be resolved by
the detector).
14
Crab Pulsar

• Even though most pulsars are probably not
  visible from our vantage point, current pulsars
  observations are consistent with the ideas that
• Every high-mass star dies in a supernova
  explosion,
• Most supernovae leave a neutron

star behind (although a few result in black
holes, as we will see), c) All young neutron
stars emit beams of radiation, like the pulsars
we actually see. In the Fig. the pulsar (arrow)
in the core of the Crab Nebula blinks on and off
about 30 times each second. In the Fig we can see
optical (b) and X-ray (c) images. In (d) it is
shown the central pulsar (arrow) and its
accretion disk.
15
Black Holes
Neutron stars are supported by the resistance of
tightly packed neutrons to further compression.
Squeezed together, these particles form a hard
ball of ultradense matter that is difficult to
compress further. But, there is a limit that once
reached not even tightly packed neutrons can
withstand the star gravitational pull. Most
researchers concur
that the limit mass for a neutron star is 3
times the mass of the Sun. Thus, if enough
material is left behind after a supernova
explosion that the central core exceeds this
limit, or if enough matter falls back onto the
neutron star after the supernova has occurred,
gravity wins the battle with pressure once and
for all, and the central core collapses forever.
Stellar evolution theory indicates that this is
the fate of any star whose main-sequence mass
exceeds about 25 times the mass of the Sun. As
the core shrinks, the gravitational pull in its
vicinity eventually becomes so great that
nothing-not even light-can escape. The resultant
object therefore emits no light, no other form of
radiation, no information whatsoever. Astronomers
call this bizarre endpoint of stellar evolution,
in which the core of a very high-mass star
collapses in on itself and vanishes forever, a
black hole.
16
Observational Evidence of Black Holes
Do Black Holes Exist?
The answer is probably yes. There exists strong
reasoning that supports the case for black holes.
Can we guar-
antee that future modifications of the theory of
compact stars will not invalidate our arguments?
No, but similar statements could be made in other
many other areas of science. We conclude that,
strange as they are, black holes have been
detected in our galaxy and beyond. Perhaps
someday future generations of space travelers
will visit Cygnus X-1 or the center of our Galaxy
and test these conclusions first hand.
17
How We Detect Black Holes
Black holes don't give off light, so we can't
just look for them. However, astronomers can find
black holes by observing the gravitational
effects on other objects nearby.X-rays Astronom
ers can discover some black holes because they
are sources of x-rays. The intense gravity from a
black hole will pull in particles from a
surrounding cloud of dust or a nearby star. As
the particles speed up and heat up, they emit
x-rays. So the x-rays don't come directly from
the black hole, but from its effect on the dust
around it. Although x-rays don't penetrate our
atmosphere, astronomers use satellites to observe
x-ray sources in the sky.
18

• The Universe
• Cosmology The Big Bang and the Fate of the
  Universe

19
The Expanding Universe Olberss Paradox
Why is the sky dark at night?
Newton thought that the universe has to be filled
with an infinite number of stars scattered more
or less randomly throughout infinite space, since
the gravitational forces between any finite
number of stars would in time cause them all to
fall together, and the universe would soon be a
compact blob.
Obviously, this has not happened. Thus, Newton
concluded that we must be living amid a static,
infinite expanse of stars. In this model, the
universe is infinitely old, and it will exist
forever without major changes in its
structure. In XIX Century the German amateur
astronomer Heinrich Olbers pointed out that if
the space goes on forever, with stars scattered
throughout it, then any line of sight must
eventually hit a star. In this case, no matter
where you look in the night sky, you should
ultimately see a star. Thus at night the sky
should blaze like the surface of the Sun.
Olberss paradox is that the night sky is
actually dark. To solve this paradox we should
accept that at least one of the Newtons
assumptions is not correct-Either the universe is
finite in extent, or it evolves in time.
20
Cosmic Expansion The Hubbles Law
The American physicist Edwin Hubble is usually
credited with discovering that our universe is
expanding. He found a simple linear relationship
between the distances to remote galaxies and the
corresponding galaxy recessional velocity. The
Hubble Law states that remote galaxies are moving
away from us with speeds proportional to their
distances.
Specifically, the recessional velocity v of a
galaxy is related to its distance d from the
Earth by the equation
Where H0 is the Hubble constant. Because clusters
of galaxies are getting farther and farther apart
as time goes on, astronomers say that the
universe is expanding. The value of the Hubble
constant H0 is the slop of the straight
line-recessional velocity divided by distance.
Reading the numbers of the experimental graph,
this comes to roughly 70,000 km/s divided by 1000
Mpc, or 70 km/sxMpc. The Hubble constant is one
of the most fundamental quantities of nature-it
specifies the rate of expansion of the entire
cosmos.
21
What does it actually mean to say that the
universe is expanding?
The expansion of the universe is the expansion of
space. The expanding universe remains homogeneous
at all times. There is not empty space beyond
the galaxies into which they rush. To illustrate
this ideas, imagine an ordinary balloon with
coins taped to its surface, as shown in the Fig.
The coins represent galaxies and the
two-dimensional surface of the balloon represents
the fabric of our three-dimensional universe.
Imagine yourself as a resident of one of the coin
galaxies on the leftmost balloon, and note your
position relative to your neighbors. As the
balloon inflates (that is, as the universe
expands), the other galaxies recede from you and
more distant galaxies recede more rapidly.
Regardless of which galaxy you chose to consider,
you would see all the other galaxies receding
from you, but nothing is special or peculiar
about this fact. There is not center to the
expansion and no position that can be identified
as the location from which the universal
expansion began.
22
The Birth of the Universe
We have seen that all the galaxies in the
universe are rushing away from us as described by
Hubble Law
Assuming that all velocities have remained
constant in time, we can ask how long it has
taken for any given galaxy to reach its present
distance from us. The answer follows from Hubble
law. The time taken is simply the distance
traveled divided by the velocity
For H070km/sxMpc, this time is about 14 billion
years. Notice that it is independent of the
distance-galaxies twice as far away are moving
twice as fast, so the time they took to cross the
intervening distance is the same. The value of H0
has an uncertainty of about 5, so our simple
estimate of the age of the universe is likewise
uncertain by at least 5. It is now known that
the expansion rate of the universe has changed
over its history. When these factors are taken
into account , we find that the age of the
universe is 13.7 billion years, with an
uncertainty of 0.2 billion years. This is
remarkably close to our simple estimate.
23
The Big Bang
The term Big Bang is used both in a narrow sense
to refer to a point in time when the observed
expansion of the universe (Hubble's law)
begancalculated to be 13.7 billion years agoand
in a more general sense to refer to the
prevailing cosmological paradigm explaining the
origin and expansion of the universe, as well as
the composition of primordial matter through
nucleosynthesis. One consequence of the Big Bang
is that the conditions of today's universe are
different from the conditions in the past or in
the future. From this model, George Gamow in 1948
was able to predict, at least qualitatively, the
existence of cosmic microwave background
radiation (CMB). The CMB was discovered in the
1960s and served as a confirmation of the Big
Bang theory over its chief rival, the steady
state theory.
24
(No Transcript)
25
The Fate of the Cosmos
Lets begin by assuming that gravity is the only
force affecting cosmic expansion. Until
relatively recently this was the conventional
wisdom among cosmologists. However, as we will
see in a moment, new observations have recently
changed this view of the universe. Lets start by
considering as an analogy a rocket ship launched
from the surface of a planet. There are basically
two possibilities, depending on the launch speed
of the ship. If the speed is high enough, it will
exceed the planets escape speed and the ship
will never return to the surface. The spacecraft
leaves the planet on an unbound trajectory, as
illustrated in Fig. (a). Alternatively, if the
launch speed is lower than the escape speed, the
ship will reach a maximum distance from the
planet, then fall back to the surface. Its bound
trajectory is shown in Fig. (b).
26
Will the universe expand forever?
The same possibilities as for the rocket ship
exist for two galaxies at some known distance
from one another with relative velocity given by
the Hubble law-the distance between them can
increase forever, or it can increase for a while
and then start to decrease. Whats more, the
cosmological principle says that whatever the
outcome, it must be the same for any two
galaxies-in other words, the same statement
applies to the universe as a whole. Thus, as
illustrated in the Fig., the universe can
continue to expand forever-an unbound universe-or
the present expansion will someday stop and turn
around into a contraction-a bound universe. What
determines which of these possibilities will
actually occur? In the case of a rocket ship of
fixed launch speed, the mass of the planet
determines whether or not escape will occur. For
the universe, the corresponding factor is the
density of the cosmos.
27
The Density of the Universe
A high-density universe contains enough mass to
stop the expansion and eventually cause a
recollapse. Such a bound (closed) universe is
destined ultimately to shrink toward a
superdense, superhot singularity much like the
one from which it originated (The Big Crunch). A
low-density universe, conversely, is unbound
(open) and will expand forever.
Where is the critical density of
the universe (with G the universal constant of
gravitation. is the density
parameter. is the combined average mass
density of all forms of matter and energy.
28
The Cosmic Microwave background and the Curvature
of Space
By calculating what conditions were like in the
primordial fireball, astrophysicists find that in
a flat universe, the dominant hot spots in the
cosmic background radiation should have an
angular size of about 1 . This is just what the
BOOMERANG and MAXIMA experiments observed, and
what the WMAP observations have confirmed as
indicated in the Fig. where the entire microwave
sky, as seen by the WMAP spacecraft (the bright
blobs are slightly overdense regions of the
universe at an age of roughly 400,000 yrs. they
will eventually collapse to form clusters of
galaxies. Once we know the curvature of the
universe we can determine the density parameter
Oo and hence the combined average density ?0. By
analyzing the data shown in the Fig.
Astrophysicists find that Oo1.0 with an
uncertainty of about 2. In other words, ?0 is
within 2 of the critical density ?c. Thus the
Universe will be expanding forever.
29
Dark Energy
The flatness of the universe poses a major
dilemma. Considering the mass density of matter
and radiation, and even including the necessary
contribution of the dark matter (Galaxies may
contain as much as 10 times more dark matter than
luminous material) we have a total density
parameter of 0.27 (it is, almost ¼ of the
observed one Oo1.0 ). In other word, radiation
and matter, including dark matter, together
account for only 27 of the total density of the
universe! The dilemma is this What could account
for the rest?
The source of the missing density must be some
form of energy that we cannot detect from its
gravitational effects (the technique astronomers
use to detect dark matter). It must also not emit
detectable radiation of any kind. We refer to
this mysterious energy as dark energy

Thus, whatever dark energy is, it accounts for
73 of the contents of the universe! The concept
of dark energy is actually due to Einstein. When
he proposed the existence of a cosmological
constant ?, he was suggesting that the universe
is filled with a form of energy that is dark.
Shortly after formulating his general theory of
relativity, Einstein tried to understand how his
theory affected the structure of the universe as
a whole. At that time , the prevailing view was
that the universe was static, just as Newton had
thought. According to the general theory, the
universe must be either expanding or contracting.
In a desperate move to force his theory to
predict a static universe he added by hand the
cosmological constant. When the universe
expansion for discovered, Einstein has been
quoted as saying that the cosmological constant
was the greatest blunder of my life. If dark
energy is in fact due to ?, the value of this
constant must be far larger than Einstein
suggested. This is needed if we are to explain
why O?0.73. If Einstein felt he erred by
introducing the idea of a cosmological constant ,
his error was giving it too a small a value!
30
Cosmic Acceleration
We have seen that the universe is expanding. But
does the rate of expansion stay the same? Because
there is matter in the universe, and because
gravity tends to pull the bits of matter in the
universe toward one another, we would expect that
the expansion should slow down with time (In the
same way, a cannonball shot upward from the
surface of the Earth will slow down as it ascends
because of the Earths gravitational pull) . If
there is a cosmological constant, however, its
associated dark energy will exert an outward
pressure that tends to accelerate the expansion.
To determine which of these effects is the more
important, astronomers study the relationship
between redshift
(redshift is just velocity divided by the speed
of light) and distance for extremely remote
galaxies, since we see these galaxies as they
were billion of years ago. If the rate of
expansion was the same in the distance past as it
is now, the same Hubble law should apply to
distant galaxies as to nearby ones. But if the
rate of expansion has either increased or
decreased, we will find important deviations from
the Hubble law. In the late 1990s two groups of
astronomers announced the results of independent,
systematic surveys of distant supernovae. These
finding seem to indicate (seethe Fig.) that the
expansion of the universe is not slowing, but
actually accelerating! According to the supernova
data, galaxies at large distances are receding
less rapidly than Hubbles law would predict.
These observations are inconsistent with the
standard gravity only Big Bang theory, and
necessitate a major revision of our view of the
cosmos. All observations indicate that we are
living in a dark-energy-dominated universe.