Download Astronomy Library wk 8.cwk (WP)

8–1
Evolution of massive stars
(> few solar masses)
Unlike medium mass stars, the cores of massive
stars eventually become hot enough (around 1 billion
K) to fuse carbon into oxygen and neon.
Massive stars follow a similar trend to medium mass
stars through the hydrogen and helium burning
phases.
Star then has a carbon burning core surrounded by a
helium and then a hydrogen burning shell.
Like their lighter brethren, the massive stars expand
after hydrogen (and later helium) in their cores is
exhausted.
In this phase they occupy the supergiant region of
the H-R Diagram.
When the carbon is used up the core collapses, heats
up and carbon burning continues in a shell.
The process then continues burning the heavier and
heavier elements. The oxygen, neon, and magnesium
are burned to create silicon and sulfur.
Finally, the silicon is burned to make iron
However, the evolution of massive stars diverge from
that of medium mass stars after this point.
Because each stage of fusion releases less and less
energy, each stage lasts less and less time.
During these death throes the star wanders around
the supergiant region in the HR diagram.
Supernova Explosion
Because iron is the most tightly bound nucleus, no
more energy can be produced by fusion (or fission)
of iron.
Iron is the end of the line.
Thus, once iron is produced in the core, fusion
declines and the core starts to collapse.
With lighter elements, this contraction led to fusion
of the spent fuel, but this is impossible with iron.
Any nuclear reaction involving iron takes energy.
The core becomes hot enough that energetic photons
become numerous and start breaking nuclei apart
removing energy.
Also, as nuclei are broken apart, the protons start to
absorb electrons to become neutrons.
The removal of energy and loss of electrons that
were exerting some degeneracy pressure leads to an
even more rapid collapse of the core.
This all happens very quickly (within a second).
As the core collapses material around the core
collapses with it.
Except it all can’t collapse together at once leading to
a rebound and an outward moving shock wave.
This outbound shock wave eventually bursts through
the surface and blows the star apart in what we see as
a supernova explosion.
8–2
Heavy Elements
Fusion cannot release energy by fusion of elements
heavier the iron.
However, such fusion can still occur if there is
enough energy around to drive the fusion.
In a supernova explosion copious amounts of energy
are produced.
This energy drives fusion of heavier elements
(beyond iron).
All elements heavier than iron (gold, silver, lead,
uranium...) in the universe were produced in a
supernova.
Supernova explosions also blow many of the heavy
elements produced earlier by the nuclear fusion in
the star into space.
When we look at the site of previous supernovae we
see the gas shell moving away from the supernova
explosion as a nebula of excited material called a
supernova remnant.
The capture of electrons by the protons in the core of
a star about to go supernova is predicted to produce
large numbers of neutrinos.
The interstellar medium is constantly being enriched
in heavier elements.
Indeed, the “big bang” at the beginning of the
universe only created hydrogen, helium, and small
amounts of lithium and deuterium.
All of the heavier elements we see on Earth were
created previously in stars.
Evidence of Theory of
Supernovae:
Supernovae are rare. A few have been noted
throughout history:
Chinese astronomers noted one in 1054.
Tycho Brahe’s supernova occurred in 1572.
Kepler’s supernova occurred in 1604.
Neutron Stars
What happens to the core of the supernova?
After the protons capture electrons to become
neutrons, the core consists primarily of neutrons.
The core continues to collapse.
Most of these flow straight out through the star and
should actually be seen before the shock wave bursts
Just like electrons, neutrons obey the Pauli exclusion
through the star’s surface and any visible
principle.
brightening occurs.
Such a burst of neutrinos was actually seen from the
supernova 1987A supporting our theory of
supernovae.
Eventually, the neutrons become degenerate and a
neutron degeneracy pressure is created.
If not too massive (more than 2-3 solar masses or
so) this neutron degeneracy pressure can hold the
star up against gravity and we have a neutron star.
8–3
Neutron stars are much denser the white dwarfs.
Pulsars
While white dwarfs are typically about the size of the
Earth, neutron stars are the size of a city (tens of
kilometers).
In 1967, Jocylen Bell found a series of regular radio
pulses coming from a celestial location.
The density of a neutron star is similar to that of an
atomic nucleus.
Shortly thereafter, more such objects were found
around the sky.
In fact, a neutron star can in some sense be thought
of as a giant atomic nucleus.
The pulses had very regular periods of a few
thousandths to a few seconds.
Neutron stars were predicted in the 1930s.
What could they be?
However, being so small they should be very faint.
The periods of the pulses places an upper limit on
the size of the objects: the time it would take a light
signal to cross the object.
Thus, it was not surprising at the time that none were
seen.
Evidence for neutron stars came from an unexpected
quarter.
Nor could it be a normal rotating star. Such a star
would fly apart if it rotated that fast.
For a 0.001sec period, the limit is about 300km,
much smaller than normal stars (even white dwarfs).
Black Holes
==> They couldn’t be regular stars.
Neutron degeneracy pressure can hold up a neutron
star if it is not more massive than 2-3 solar masses
However, neutron stars are small enough to pulse
that fast.
What if the star is more massive?
Also, being very dense, they can rotate that fast and
not fly apart.
Neutron degeneracy pressure will not hold the star
up and it will continue to collapse.
Could they be neutron stars?
What holds the star up at this point?
In 1968, a pulsar was discovered at the center of the
crab nebula, a supernova remnant: right where theory
predicts a neutron star should be created!
Nothing: the star will collapse down into a black
hole. Gravity finally wins.
8–4
Escape Velocity
Gravity will keep objects from escaping.
“What goes up must come down”
Spacecraft must obtain at least this much speed to
escape from the Earth.
From the equation, we see that the escape velocity
would be larger if the Earth (or other object) were
made smaller.
Well, not quite. If moving fast enough gravity will
never stop an obect’s motion and it will escape.
If we continued to shrink the Earth, the escape
velocity would increase until it reached the speed of
light.
The velocity needed to escape from a body is called
the escape velocity.
Nothing can move faster than the speed of light.
The escape velocity from a spherical body is given
by:
Thus, nothing could escape from the Earth, not even
light.
v=
2GM
R
At this point the Earth would be a black hole—a
body from which nothing can escape.
Escape velocity from the Earth is about 24,000 miles
per hour.
If we substitute in the speed of light for the escape
velocity, we can find the Schwarzchild radius of a
black hole:
R = 2GM
c2
Anything within this radius cannot escape.
For the Earth, the Scharzchild radius is about 1cm.
For 1 solar mass the radius is about 3 km.
Black holes with masses typical of stars are very
dense.
The boundary between the region from which an
object can escape and from which it is trapped in the
black hole is called the event horizon.
The event horizon is not a physical boundary.
In fact, one would not notice anything special at the
event horizon.
One could move right through it and not notice it.
8–5
Search for Black Holes
Material flowing onto a neutron star would also be
heated up and emit x-rays.
If nothing can escape from a black hole, how might
we find evidence for their existence?
How can one tell the difference between x-rays from
accretion onto a neutron star or black hole?
Clearly not by any light emitted by the black hole
itself.
It would look the same.
However, their presence can be inferred.
However, if one can determine the mass of the
compact object and it is more massive than 2-3 solar
masses we know it can’t be a neutron star.
If material flows into a compact object, it will heat up
as gravitational energy is released.
Several candidate black holes have been found.
For a compact object, the material will be heated up
to very high temperatures: temperatures at which it
emits x-rays.
Thus one can look for x-ray sources.
Galaxies
Galaxy Morphology:
–Elliptical
–Spiral
Galaxy Evolution
Can we explain how galaxies evolve?
Why do some become ellipticals, some spirals, some
irregular?
–Irregular
Might galaxies evolve from one type into others?
Galaxy Properties
For example, might ellipticals evolve into spirals and
then irregulars or vice versa?
Evolution of Galaxies
Unfortunately, this idea does not seem to work.
Ellipticals contain little dust and gas, thus they
cannot later evolve into spiral galaxies.
Irregular galaxies contain some old stars, thus they
cannot be young compared to the other two classes
8–6
Galaxy Interactions
In fact, we do see evidence for galaxy interactions.
Elliptical and S0 galaxies appear in larger numbers
in rich clusters than in poor clusters.
This suggests that a galaxy’s environment is an
important factor in its evolution.
Many galaxies appear to be connected by bridges of
gas.
Tidal forces between galaxies can distort galaxies
creating tails, bridges and other peculiar forms.
Interactions between galaxies may be important.
Do we see evidence that galaxies indeed do interact
significantly?
Stars (except in binary systems) are much smaller
than the average distance between them, thus
collisions are very rare.
However, galaxies are on average only about 20
times their diameters apart.
Collisions between galaxies should be common.
What happens in a collision?
It is unlikely that any stars in the two galaxies will
collide.
However, their orbits can be distorted by
gravitational interactions.
Our own Milky Way appears to have collided
recently with the Large and Small Magellanic
Clouds.
Bridges of gas appear to connect the Milky Way to
these galaxies.
The LMC and SMC have been severely distorted by
their interactions with the Milky Way.
Ring galaxies apparently are formed during high
speed head on collisions between galaxies.
How might galaxy interactions help us explain the
evolution of galaxies? Why do some become
ellipticals others spirals and irregulars?
Galaxies in rich clusters are expected to have more
interactions.
Such collisions lead to bursts of star formation.
Even if the stars don’t collide any dust and gas in the
two galaxies will definitely interact.
Sudden interaction can cause compression of the
gasses leading to a burst of star formation.
It is also possible for one galaxy to strip the other of
gas and dust.
If galaxies lose enough momentum in the encounter
they may become gravitationally bound and
eventually merge together.
Galaxies which suffer such collisions would be
expected to use up their dust and gas more quickly.
Also, galaxies in clusters might get stripped of their
dust and gas.
Thus ellipticals may be galaxies which have
undergone a number of collisions and used up their
fuel of dust and gas quickly.
8–7
Elliptical and particularly the giant elliptical galaxies
may also be the products of mergers between
galaxies.
In contrast, spiral galaxies have used their dust and
gas more slowly, probably because they have not had
many collisions.
Irregular galaxies appear to have been torn apart by a
recent collision.
If ellipticals are the products of collisions, would you
predict that ellipticals would be more common today
or earlier in the history of the universe?
Nice prediction, but how can we check it?
The Hubble Deep Field
HST took a ten day exposure of a small
region of space in Ursa Major.
Today we will be classifying galaxies in
the Hubble Deep Field.
Why take the Hubble Deep Field?
Wow!
However, there are scientific rationale for
the HDF as well:
With such a deep exposure one can see
very distant galaxies.
Examining such galaxies allows us to, in
essence, look backwards in time.
In fact, they are so far away we can see
back a significant fraction of the way to
the beginning of the Universe.
Theories predict that we should see
some evolution in galaxies over this
time.
Are galaxies the same today as in the
past?
Or are they different and, if so, how?
Need to classify galaxies in the HDF to
find out!