Chapter 22 Neutron stars, gamma ray bursters, black holes

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Chapter 22 Neutron stars, gamma ray bursters,
black holes Result of Supernova explosion of
massive star (Type II) is a neutron star, if the
remnant core is less than about 3 solar masses.
Neutron star held up by degenerate neutron
pressure density 1018 kg/m3
Pulsars synchrotron radiation by spiralling
electrons around magnetic field lines make
beacons. Pulse milliseconds to seconds
rotation axis
magnetic axis
Gamma ray bursts of fraction of second duration.
May be associated with rotating black hole whose
magnetic field makes an electic generator.
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Special relativity laws of physics the same in
all inertial frames speed of light the same in
all frames. General relativity mass and energy
warp space and time. Objects move near large
masses on curved geodesics. In General
relativity, can form black holes objects so
massive that nothing can escape, not even light.
Radius within which nothing can escape is
Schwarzchild radius at the Event
Horizon. Supernova remnants with mass above
about 3 solar masses will form black holes. See
evidence for black holes that are part of binary
systems. We see the visible star orbitting
around a very massive invisible companion. See X
ray emission when material from the companion
streams into the black hole. We believe a very
massive black hole exists (3 million solar
masses) at the center of our galaxy see stars
orbiting rapidly around a very massive core that
is dark.
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Chapter 23 Milky Way Our Milky Way galaxy is
disk shaped with a central bulge, and a low
density spherical halo. Disk and halo are
about 30kpc across. The central bulge is a few
kpc across. There are about 1011 (100 billion)
solar masses in the galaxy, so about that many
stars. Measuring distances in the galaxy uses
variable stars RR lyra and Cepheid variables,
whose atmospheres are unstable and oscillate up
and down, producing temperature, hence
luminosity, variations. The period of
oscillation in luminosity is related
observationally to the absolute peak luminosity,
so they are standard candles. The sun lies in
one of the spiral arms, about 8kpc from the
center (2/3 of the way out). The halo consists
mainly of globular clusters which orbit in
elongated ellipses around the center of the
galaxy. A globular cluster has Population II
stars, all formed at the same early time in the
Milky Ways history. Observations of Cepheids
in globular clusters enabled the early size
measurements of the galaxy. There is little gas
in the halo. The disks are rich in dust and gas
and young (Population I) stars. See 21 cm radio
emission characteristic of neutral hydrogen gas.
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Formation of the Milky Way came through
gravitational forces pulling gas into regions of
slightly higher density, creating the seeds for
stars. The disk was formed by the rotation of
the initial cloud. Spiral arms do not rotate like
a rigid body. (Rigid body would have all parts
of disk rotate in same time same angular
speed). The sun takes about 200 million years to
make one circuit (much less than the life of the
galaxy).
The differential rotation would tend to wind up
the spiral. So the arms must be constantly
forming and reforming. We believe that they are
caused by spiral density waves that travel
outward from the center. Where the density is
higher, stars form rather quickly and give the
light we see as the spiral arms.
Near the galaxy center there is a bright region
that emits strongly also in radio and X-ray
Sagitarius A. We believe there is a massive
black hole there with about 3 million solar
masses.
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Ch 24 Normal galaxies Spirals, Elliptical and
Irregular galaxies. Spirals have many young
stars and gas clouds. Spirals arms in the disk,
a central core, spherical halo. Milky Way and
Andromeda are typical spirals with 1011 stars
and 30 kpc across Ellipticals have mainly older
stars. Can be huge (1012 stars) or dwarf (107
stars) Size up to 1 Mpc Get distances from
Cepheid variables, Tully Fisher, Supernovae Type
Ia. Standard candles and use Ltrue 4pd2
Iapp Galaxies come in clusters ours is Local
Group of 45 galaxies nearby Virgo cluster has
1000s of galaxies. Cluster sizes are several
Mpc Superclusters of clusters 50 Mpc
across. Superclusters form filaments, patches,
voids Galaxies collide rather frequently can
merge or pass through to change the shape and
size of the galaxies.
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Cosmological red shift (Doppler effect) See
distant galaxies moving away from us, with speed
proportional to distance. Hubble Law v H0
d
Measure mass of galaxies by looking at rotational
velocity of stars at some distance from
center. P2 a3/(M m) ? P2 ? a3/M Find
that there is mass well outside the visible
galaxy Dark Matter
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Ch. 25 Active galaxies and quasars Seyfort
galaxies and Radio Galaxies 10 to 100 times
radiation from normal galaxy. Non-thermal
radiation (radio and infrared). Often have jets.
Vary output over short times (days/months) so
radiation region is small. Quasars huge red
shifts (up to z 6) and very energetic 1000 to
100,000 times Milky Way luminosity rapid time
variations much radio emission (non-thermal)
seem to have existed between 2 and 3 billion
years after Big Bang, then quit. Jets are
common.
Energy believed to be generated by accretion of
material (stars, clouds) on to supermassive black
holes at the centers of young galaxies (BH of
billions of solar masses). Infalling material is
ionized (broken into pieces) with electrons being
squirted out along the magnetic field lines and
we see the synchrotron radiation in IR and radio.
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When supermassive BH eats all the material in its
vicinity, the period of intense radiation stops
and quasar/active galaxy becomes a normal
galaxy. Light from distant quasars can be used
to probe intermediate space Lyman alpha
absorption by intervening dust clouds maps the
clouds. Gravitational lensing of qausar light
maps dark matter.
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Chapter 26 Cosmology and the expanding
universe Cosmological Principle assumes that on
the largest scale, the universe is homogeneous
and isotropic. The cosmological red shift lets
us map the distant universe Doppler red shift
tells us velocity of recession Hubble Law then
gives a distance Converting the distance to a
time using dct gives the time at which the light
was emitted. When we calculate when any distant
galaxy was on top of us, all give a common answer
around 13-14 billion years ago. The time when
everything in the universe was at the same point
is called the BIG BANG. All galaxies see all
others receding with the Hubble expansion. We
attribute this to the expansion of the universe
itself the space coordinates are growing with
time. The cosmological red shift is attributed
to the universe expanding the wavelength of the
light.
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What will happen in future (or what happened in
past) depends on the amount of matter and energy
in the universe. If the matter has a density
equal to the critical density W0 8x10-27 kg/m3
at present, then we would expect the universe to
coast to zero expansion velocity in future. If
less, universe expands forever if more,
universe recollapses to a big crunch. Previous
history of universe also depends on W0 less
matter means it is slowing down less and is older
(Big Bang earlier)
Actual age of universe is less in bound than
unbound case
Counting up the visible matter (atoms) and dark
matter gives about the critical density.
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Good evidence for the big bang comes from the
cosmic microwave background. This is photons
emitted when radiation decoupled from matter at
the time neutral atoms formed. At that time,
the temperature of the universe was about 3000K
and the photons had a blackbody spectrum
appropriate to that temperature. Since then the
expansion has cooled the universe and stretched
the original visible light to microwaves. Observa
tions show an excellent blackbody spectrum now
with 2.73K. This is the earliest snapshot
available of the universe at 380,000 years
after the big bang.
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A new wrinkle. Looking at distant supernovae
whose distance AND velocity are measured shows us
that the universe is actually decelerating, not
accelerating as it would if there were only
matter.
Velocity of expansion was smaller in the past
accelerating universe
distance
velocity
The deceleration forces us to conclude there is a
new ingredient in the universe that we call Dark
Energy something that tries to repel the
galaxies, or expand the universe.
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Another measure of the density of matter and
energy in the universe comes from measuring the
curvature of space. Flat universe has critical
density, W01 . Closed or positive curvature
universe has higher than critical density and
Open or negative curvature universe has lower
than critical density. The cosmic microwave
background lets us measure the curvature. CMB
has tiny fluctuations in temperature due to
density fluctuations in early universe. The
spacing of these tells us that space is flat so
the total amount of matter plus energy is the
critical density.
The supernovae tell us about the difference of
matter and dark energy (one tries to contract,
the other tries to expand). Two measurements
together give 27 of critical density of matter
and 73 of dark energy. The matter is about 4
atoms and 23 dark matter.
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Chapter 27 Evolution of the early universe
Two problems with the observations Why is the
CMB have so nearly the same temperature
everywhere? The different parts of the universe
were not in contact when the radiation left the
atoms. Horizon problem Why is the density of
matter and energy so nearly equal to one? This
seems too much an accident. Flatness
problem. Both of these are solved by Inflation
postulate that the universe underwent a very
rapid increase in size in its early moments due
to supercooling ( phase transition ). The
blowing up makes the universe much flatter than
it previously was. And before inflation,
different places were in contact, so it is
sensible that all points had the same temperature.
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• 8 epochs of the history of the universe
• Planck epoch when all forces unified and all
  particles present
• GUT epoch when gravity decoupled inflation
  occurred when strong force decoupled (phase
  transition leading to large energy deposit)
• Hadron epoch when ordinary particles in
  equilibrium with photon bath. At end of hadron
  epoch almost all protons disappear
• Lepton epoch when only electrons in equilibrium
  with photon bath. At end of lepton epoch, most
  electrons disappear.
• Nuclear epoch cool enough that protons and
  neutrons fuse into deuterons and these rapidly
  form helium. Origin of deuterium and helium.
• Atomic epoch cool enough for electrons to be
  trapped on nuclei cosmic microwave background is
  launched.
• Galactic epoch clumpings of matter develops
  enabling first stars and galaxies to form. Dark
  matter needed to make galaxies form so soon as 1
  billion years.
• Stellar epoch star formation in existing
  galaxies our present era.

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• Matter dominates in universe today (and since the
  atomic epoch) Gravity is the dominant force
  shaping the structure of the universe in this
  era.
• Radiation dominates in the early universe (before
  a few 1000 years). Particles like electrons,
  protons, quarks are in equilibrium with the
  radiation and the fundamental particle forces are
  dominant in shaping the universe.
• Fundamental forces
• Strong nuclear force (makes nuclear reactions
  go)
• Electromagnetic force (makes chemistry work
  force between charges)
• Weak nuclear force (radioactive decay, inportant
  in stellar burning)
• Gravity (between all masses very weak compared
  to the others)