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The history of the Galaxy got a little muddled, for a number of
reasons: partly because those who are trying to keep track of
it have got a little muddled, but also because some very
muddling things have been happening anyway.
From “Mostly Harmless”, by Douglas Adams
HW – for substitute grade only. Due
Dec 9
Ch. 22 RQ 6, P 1
Ch. 20 DQ 6
What is creationism (in 1-4
sentences)?
What is the theory of evolution (in 1-4
sentences)?
What is the evidence for evolution?
X-ray image of Cygnus
X-1 from NASA’s
Marshall Flight Center.
Escape Velocity
Let’s re-think Newton’s
experiment.
If you launch something
from Earth with a high
enough velocity, it goes into
orbit.
If the velocity is increased
further it can escape. The
escape velocity depends of
the mass and radius of
Earth.
Schwartzchild Radius
What if vesc = c?
(c=300,000 km/s)
This happens at a distance from mass M:
R = 2GM/c2,
known as the Schwartzschild radius.
Both matter and light within this distance to
a black hole (inside the Schwartzschild
radius) can not escape.
Black Holes in Binary Systems
The most straightforward way to
search for a black hole is to Kepler’s
third law. The best place to apply this
technique is an x-ray binary. In these
systems one of the stars is seen in
visible light and the other is a copious
source of x-rays. The x-rays show the
position of the (possible) black hole.
How do x-rays escape from a black
hole? They don’t. The x-rays are
emitted by matter from the visible star
that falls into the black hole
accelerating to velocities near the
speed of light as it falls.
If we can determine the orbital period
of the binary system, we can then use
Kepler’s 3rd law to calculate the mass.
If the mass of the unseen companion is large, this and the presence of x-rays
suggest that it is a black hole. Currently, the best candidate is Cygnus X-1.
Super Massive Black Holes
An x-ray image of the center of the Milky Way
The center of our galaxy is also a copious source of x-rays and appears to be
extremely massive. Stars in the Milky Way orbit around an unseen central object.
Analysis of the orbital velocities of the stars about the center of the galaxy (using
Kepler’s 3rd law) imply a mass of 2.6106 solar masses inside a volume 0.03 light
years in diameter. It is impossible to pack stars together that tightly – they would
collide, destroying each other very quickly. It is likely that the object at the center of
our galaxy is a super massive black hole. The same is believed to be true of many
other galaxies.
Summary
• Stars die by expelling catastrophically the outer layers.
The inner layers contract to a very dense amber.
• Massive stars (8 x Ms) explode into supernova, while
solar-type stars explode as less energetic planetary
nebula.
• The remnant of the Sun will be a white dwarf,
supported by electron degeneracy.
• The remnant of a massive star is a neutron star,
supported by neutron degeneracy.
• A stellar core more than 3 Ms has enough gravity to
overwhelm the neutron degeneracy pressure. No
known force can support gravity and collapse
continues. The result: a black hole.
• The Schwarzschild radius is the distance from a black
hole within which even light can not escape.
Cosmology
There is a theory which states that if ever anybody discovers exactly what the
Universe is for and why it is here, it will instantly disappear and be replaced by
something even more bizarre and inexplicable. There is another theory which states
that this has already happened.
Douglas Adams
Only two things are infinite, the universe and human stupidity, and I'm not sure
about the former.
Albert Einstein
The Large-Scale Structure of the
Universe is Dominated by Galaxies
A Spiral Galaxy
An Elliptical Galaxy
The Milky Way
•
•
•
•
The Milky Way is a spiral galaxy.
The Milky Way is flat and appears
as a bright band in the night sky.
It contains ~100 billion (1011)
stars.
There are ~100 billion galaxies in
the universe.
Olbers’s Paradox
• Why is the sky dark at night?
• If the universe were infinite and the density
of galaxies was the same everywhere in
the universe, then there should be a star in
every single direction: the sky should be
bright at night, but it is not.
Resolution
• Distant stars are dim, so we receive little light
from them.
– Wrong. Though we receive less light from distant
stars there are also more distant stars and this makes
up for the dimming with distance.
• There’s invisible dust between us and the distant
stars
– Wrong. The dust would eventually heat up and emit
its own radiation. We don’t see this.
• The universe has a finite age and size.
– Right. These two concepts are closely related. When
we look to great distances in the universe we are
looking far back in time. If we look to distances so
great that we are seeing times before the formation of
stars, then the sky in that direction is dark.
The force of gravity is always
attractive. Why then doesn’t the
universe collapse under it’s own
gravitational attraction?
Is the Universe Stable?
• The force of gravity is always attractive. Why then
doesn’t the universe collapse under it’s own gravitational
attraction?
• Newton said it was an act of God (Give him credit
though, no one else realized that there was a problem).
• Einstein realized the same problem occurred with his
theory of gravity and, in what he called the biggest
blunder of his life, fudged the equations so that the
universe would be stable.
• Despite these deep-seated prejudices from mankind’s
two greatest luminaries, the answer is simple: the
universe is not stable, it is expanding.
Doppler Shifts
A stationary (with respect to an
observer) light source emits radiation
with a wavelength  (the green
wave). If that same source is moving
towards the observer, the waves get
compressed and the wavelength is
less than  (the blue wave). If the
source is moving away from the
observer, the waves are stretched
out and the wavelength is greater
than  (the red wave).
Light from approaching objects is
called blue shifted and light from
receding objects is called red shifted,
because blue and are on the shortwavelength and long-wavelength
ends of the visible spectrum. The
light is not necessarily blue or red.
We can tell if light is blue or red shifted by examining the characteristic
spectral lines of the elements. In the spectrum shown above the entire
pattern of spectral lines is shifted to either the blue or red. Because we
can recognize the pattern, we can identify the spectral lines and
therefore know what the un-shifted wavelengths should be.
Hubble, Galaxies
and Red Shifts
In 1925, Hubble accumulated radial
velocities for 40 galaxies.
Shown to the right are images of
galaxies and their spectra. The
distance of the galaxies from Earth in
millions of light years (Mly) is listed
under the images and the velocity
deduced from the Doppler shift is
listed under the spectra. The bright
bands above and below the spectra
are used for calibration. The two dark
bands in central stripe are used to
measure the Doppler shift. The size of
the Doppler shift is indicated by the
red arrow.
The more distant galaxies show larger
Doppler shifts.
63 Mly
990 Mly
1440 Mly
2740 Mly
3960 Mly
The Relationship Between
Distance and Velocity
Mpc stands for Mega Parsec. 1 Mpc = 3.3 million light years
Hubble’s Law Suggests that Galaxies
were once much closer together
Imagine that all the galaxies were
once much closer together and
had a spread of velocities –
some were moving fast and
some slow. After a period of time
(billions of years) the fast moving
galaxies would be very far away,
but the slow moving galaxies not
so far away. The faster a galaxy
was moving, the further away it
would be. This is just what
Hubble measured. Thus, it
seems that galaxies were once
much closer together.
Notice that, no matter where you
are, everything seems to be
moving away.
Balloons and Raisin Bread
As the bread rises the distance
between raisins increases in a more or
less uniform manner. It would look the
same no matter what raisin you were
sitting on. Think of the raisins as
galaxies.
A better analogy is the expanding
balloon. As the balloon expands the
dots on the balloon get further apart;
however, there is no center to the
surface of the balloon and the
expansion would look the same no
matter where you were.
General Relativity & the Big Bang
The most beautiful thing that we can experience is the
mysterious. It is the source of all true art and science.
- Albert Einstein
Space is curved
Forget forces. Planets travel in orbits, following the curvature of space.
Cosmic Microwave Background
Arno Penzias and Robert Wilson were
trying to make observations of radio
emissions from a distant supernova
and then hoped to make a map of radio
emissions from the Milky Way. They
adapted a radio dish previously used
for communication satellites. They
were startled to find that no matter
where they pointed the antenna, they
measured the same low-level radio
signal. After great efforts to determine
that there was nothing wrong with the
antenna (e.g. cleaning the antenna of
a “thin white dielectric film” left by
pigeons), they concluded that the
signals were real and a property of the
universe.
Unknown to Penzias and Wilson, a
Russian Astrophysicist, George
Gamov, had predicted the existence of
these radio signals as a consequence
of something we now call the Big Bang.
Chance favors only the mind that is
prepared
- Louis Pasteur
COBE Measurements
Data & prediction of emission from a 2.726 K body.
Why microwave radiation?
Why isotropic?
Why 2.726 K?
Explanation 1
Gas gets hot when it is compressed and to cool when it
expands. The same is true for the Universe. The early
Universe was a mixture of matter and radiation. Shortly
after the Big Bang, the universe was tightly compressed,
and thus extremely hot. Its radiation was typical of that for
warm bodies (i.e. it obeyed Wein’s law). As the universe
expanded both the matter and radiation cooled. In fact,
the radiation cooled from unimaginably high temperatures
to 2.726 K, the temperature of the universe today.
Explanation 2
We get the same answer by considering that when we
look very far away we are seeing diffuse radiation from
the hot big bang. However, this radiation comes from
great distances (10-20 billion light years) and has been
Doppler shifted to very long wavelengths. In fact, it has
been Doppler shifted all the way from gamma rays to
radio waves; the radio waves discovered by Penzias and
Wilson.
Evidence for the Big Bang
• The universe is expanding at a rapid rate and
seems to have been doing so since its creation.
• The Cosmic Background Radiation can be
explained as the afterglow of the Big Bang.
• The cosmic abundances of hydrogen,
deuterium, and helium are consistent with
expectations based on synthesis by nuclear
reactions in the Big Bang.
The Big Bang solves the problem of the stability of the universe:
the force of gravity does, in fact, pull the universe together;
however, it is not collapsing because it is still flying apart at high
speeds.
This raises the question, will the universe continue to fly apart or
is gravity strong enough to stop the expansion and pull all the
pieces back together in a Big Crunch?
Put another way, is the escape velocity of the universe greater or
lesser than the rate at which it is expanding?
The collision of 2 galaxies.
Fate of the Universe
1) The amount of luminous matter in the universe appears to be
too small to stop its expansion. This might imply that the
universe would continue expanding forever, except that there
seems to be a “dark matter” in the Universe. If there is
enough dark matter, the universe could be “closed,” i.e. it
may someday stop expanding and start contracting.
2) Very careful studies of the velocities of galaxies should be
able to reveal if the rate of expansion is slowing down.
However, the latest studies found a big surprise: the rate of
expansion is increasing! If true, the universe will probably
expand forever, but the results are only a couple of years old
and deserve more careful scrutiny before firm conclusions are
adopted.
Evidence for Dark Matter
Stars far from the galactic center obey Kepler’s 3rd law, but they indicate
the presence of far more mass in the galaxy than expected based on
visible light, hence the term “dark matter.”
Is the Expansion Speeding Up?
Summary
• The solution to Olbers’s paradox is that the night sky is dark
because the universe is a finite age.
• The universe is expanding from a primordial creation event
10-20 billion years ago.
• The universe is filled with thermal radiation at a temperature
of 3 K that is the modern residue of the primordial fireball.
• There is far more mass in the universe than can be seen. The
nature of this “dark matter” is unknown.
• We’re not sure if the universe will keep expanding forever or if
gravity will cause it to contract to a Big Crunch. The latest
evidence favors expansion forever.