Download Unit 1

Units to cover: 76, 78, 79, 80
Tasks for those who want to get extra points (not
obligatory):
1. If you could go shopping on the Moon to buy a pound of
chocolate, would you get more chocolate than if you
bought a pound of chocolate on Earth?
2. Two kids of the same height are throwing balls. One
kid throws the ball horizontally with an initial speed of
5 mph. At the exact same instant, the other kid lets the
ball fall to Earth by dropping it — with no horizontal
velocity. Which ball strikes the ground first and why?
Additional tasks
3. a. Is there gravity between Earth and the Space Shuttle as it orbits
Earth?
b. Is the satellite accelerated as it goes around the Earth? If yes,
what is causing the acceleration?
c. Does the satellite need to fire its engines constantly in order to
maintain its speed?
4. In the constellation Orion, the star Betelgeuse is orange-red
in color, and another star in the constellation, Bellatrix, is
blue. What does that tell you about the temperature of each
of these stars?
5. Why does the 2.4 m Hubble Space Telescope give images with
better resolution than a 2.4 m telescope on the ground?
Tasks (continuation)
Energy L evel Diagrams
i
n
t
e
n
s
i
t
y
1

2


n=5
n=4
n=3
n=5
n=4
n=3
n=2
n=2
n=1
n=1
tr ansition A
tr ansiti on B
6. Absorption lines of two different transitions are shown above
on the left. Energy level diagrams of these elements are shown on
the right. Which energy level diagram corresponds to absorption
line 1 located at 1?
7. The Sun has been fusing hydrogen into helium for about 4.6
billion years now. How much has the amount of helium on the
surface (i.e., in the photosphere) increased over this time?
continued
8. The Sun is a low mass star and so will never have the core
fusion reactions of massive stars. Yet, the Sun has iron in its
atmosphere, as well as gold, silver and other elements much
heavier than iron. Why?
9. If iron is the heaviest element that a massive star can make in its
core, how and where are metals like gold, silver, tin and uranium
made
10. If the universe were contracting instead of expanding, how
would we know (what would the observations be)?
11. The Andromeda galaxy and the Milky Way are rushing
toward each other at a velocity of 130 km/s (or, 300,000 mph!).
We will collide in about 60 billion years. Andromeda is about
one and a half times larger in diameter and in mass than our
Milky Way. Describe what might happen during and after the
collision.
12. The Hubble Deep Field image shows (in true color) that
some galaxies are blue. Why is this?
Additional tasks
13. The volume of the nucleus of an atom, which contains
99.98% of the mass, is only one hundred millionth the size of
the whole atom. Explain, using the electron cloud model of the
atom, why we cannot walk through walls if the wall and we are
mostly empty space.
14. As we look around the sky at night, we can
distinguish about 6,000 individual stars with our eyes.
Many of these stars are close together in the sky and
cultures all over the world have connected the dots to
draw figures from them, much like seeing figures in
passing clouds. We call these traditional groupings of
stars "constellations." Are these stars physically
related?
Additional tasks
15. The extremely elliptical orbital path of an
imaginary planet is shown in the picture above.
According to Kepler's Laws, where would the
star have to be located?
A neutron star can be detected from Earth as a
pulsar by its regular radio pulses only if
• A. Earth lies within the path of the narrow beam of
radiation generated by the neutron star
• B. Earth lies in the neutron star’s “equator”, the plane
perpendicular to its spin axis
• C. Earth lies directly above the rotation axis of the
rotating neutron star
• D. none of the above
The main reason for the observed slow down of
pulsars is
• A. the slowing of rotation caused by slow expansion
of the neutron star
• B. friction between the stellar surface and interstellar
medium
• C. loss of rotational energy to provide energy for the
emission of radiation
• D. a build-up of the magnetic field with rotational
energy being transferred to magnetic energy
Black holes are named because
• A. all of their electromagnetic radiation is in the X-ray
range
• B. they emit a perfect blackbody spectrum
• C. no light or any other electromagnetic radiation can
escape from inside them
• D. their most prominent spectral lines are in the radio
Energy Source for Active
Galactic Nuclei
•
•
•
•
•
Active galactic nuclei emit a
tremendous amount of radiation
over a broad range of
wavelengths
A black hole can be both very
small, and have an accretion disk
that can emit enough radiation
Likely that at the centers of these
galactic nuclei, there are
supermassive black holes
Intense magnetic fields in the
accretion disk pump superheated
gas out into jets that leave the
nucleus
There are still many questions to
be answered…
Model of AGN
Quasars are
• A. the remnant cores of exploding stars
• B. the focused image of a distant galaxy by the
gravitational lens effect of a closer galaxy
• C. the central nuclei of very distant, very active
galaxies
Figure 78.06
The Redshift and Expansion of the Universe
•
•
•
Early 20th century astronomers
noted that the spectra from
most galaxies was shifted
towards red wavelengths
Edwin Hubble (and others)
discovered that galaxies that
were farther away (dimmer)
had even more pronounced
redshifts!
This redshift was interpreted as
a measure of radial velocity,
and it became clear that the
more distant a galaxy is, the
faster it is receding!
The Hubble Law
• In 1920, Edwin Hubble
developed a simple
expression relating the
distance of a galaxy to its
recessional speed.
• V=Hd
– V is the recessional
velocity
– D is the distance to the
galaxy
– H is the Hubble Constant
(70 km/sec per Mpc)
• This was our first clue that
the universe is expanding!
Which two quantities are shown to be related to one
another in Hubble Law?
•
•
•
•
A. distance and brightness
B. distance and recession velocity
C. brightness and recession velocity
D. brightness and dust content
Large Scale Structure in the Universe
• Using modern technology,
astronomers have mapped the
location of galaxies and
clusters of galaxies in three
dimensions
• Redshift is used to determine
distance to these galaxies
• Galaxies tend to form long
chains or shells in space,
surrounded by voids
containing small or dim
galaxies
• This is as far as we can see!
How are galaxies spread through the Universe?
• A. They are grouped into clusters that in turn are
grouped into clusters of clusters (superclusters)
• B. Galaxies are spread more or less evenly throughout
the Universe
• C. They are grouped around our galaxy
• D. none of the above
Seyfert Galaxies
• Seyfert galaxies are
spiral galaxies with
extremely luminous
central bulges
• Light output of the
bulge is equal to the
light output of the
whole Milky Way!
• Radiation from
Seyfert galaxies
fluctuates rapidly in
intensity
Radio Galaxies
• Radio Galaxies emit large
amounts of energy in the
radio part of the spectrum
• Energy is generated in two
regions
– Galactic nucleus
– Radio lobes on either side
of the galaxy
• Energy generated by
energetic electrons
– Synchrotron radiation
– Electrons are part of the gas
shooting out of the core in
narrow jets
What type of object does explain activity at the very
center of our galaxy?
•
•
•
•
A. a rapidly rotating neutron star
B. a supermassive black hole
C. a giant molecular cloud
D. a supernova explosion
Why do the spiral arms show up so clearly in spiral
galaxies?
• A.The brightest stars occur mostly in the spiral arm,
making arms stand out
• B. Stars occur only in the spiral arms
• C. The number of stars in spiral arms is orders of
magnitude larger than in the regions between
• D. The dust forms a spiral pattern
What is an elliptical galaxy?
• A. A galaxy with an elliptical outline and a smooth
distribution of brightness
• B. A spiral galaxy seen from an angle
• C. Any galaxy with an extended radio halo
• D. A spiral galaxy with an elliptical bulge
Missing Mass
•
•
•
•
•
In Unit 73, we calculated the mass of
the Milky Way by measuring the orbital
velocities of dwarf galaxies in orbit
around our galaxy
We can also count the number of stars
in the galaxy, and estimate the galactic
mass. The two numbers do not agree!
Rotation curves do not show the
expected decrease in stars’ orbital
velocities with distance from the
galactic center, so there must be much
more mass present in our galaxy
Astronomers cannot find a large
majority of this mass!
Astronomers call the missing mass dark
matter
Many galaxies have flat rotation curves!
Dark matter is not unique to the Milky Way!
Figure 78.03
•
•
99 percent of the stars in a galaxy are
within 20 kpc of the center
Gas extends far out into the disk, but is not
very massive!
•
•
Galaxies are now thought to be embedded
in a dark matter halo that surrounds the
entire galaxy
Unfortunately, dark matter cannot be
detected directly.
Dark Matter in Clusters of Galaxies
• Missing mass is also a problem
in clusters of galaxies!
– Not enough visible mass to hold
the clusters together by
gravitation, and to keep hot gas
in their vicinity
– Cluster mass must be 100 times
greater than the visible mass!
– Once again, dark matter seems
to be the solution
Gravitational Lenses
•
•
•
•
Dark matter warps space just like ordinary matter
does
The path of light rays bends in the presence of
mass
A galaxy or other massive object can bend and
distort the light from objects located behind it,
producing multiple images
This is called gravitational lensing
Gravitational lensing of light by clusters of galaxies
A. Indicates the existence of dark matter
B. Proves that the Universe has large positive curvature
C. Proves the expansion of the Universe
D. Indicates the existence of dark energy in the
Universe
An Expanding Universe
•
•
•
The expansion of the Universe is not like
the explosion of a bomb sending fragments
in all directions
Space itself is expanding!
We can detect photons that appear to have
moved at different speeds through space
•
•
Rather, the speed of light is constant, and it
is space that was moving relative to the
photon
If each galaxy is like a button attached to
a rubber band, an ant walking along the
band as it is stretched will appear to have a
velocity slower than it really does. The
buttons (galaxies) are fixed relative to
space, but space itself is moving.
One More Analogy
• The expansion of the universe and the
increasing distance between galaxies is
similar to the increase in distance
between raisins in a rising loaf of raisin
bread.
• The raisins are fixed relative to the
dough, but the dough expands,
increasing the space between them.
• Problem with these analogies – loaves
and rubber bands have edges!
– We have seen no ‘edge’ to the Universe;
there are an equal number of galaxies in
every direction!
– Also, galaxies can move relative to space,
as sometimes gravity can accelerate one
galaxy toward another faster than space
expands!
The Meaning of Redshift
•
•
•
As light waves travel through
space, they are stretched by
expansion
This increases the wave’s
wavelength, making it appear
more red!
An objects redshift, z, is
z


Here,  is the change in
wavelength, and  is the original
wavelength of the photon
• This is equivalent to:
z
Change in average distance between galaxies
Average distance between galaxies
The Age of the Universe
• Thanks to the Hubble Law, we can
estimate the age of the universe
• At some point in the distant past,
matter in the universe must have
been densely packed.
• From this point, the universe would
have expanded at some high speed
to become today’s universe
• Assuming a constant expansion
over time, we find that the age of
the universe is around 14 billion
years.