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The April Brooks Observatory sessions
The one remaining session is this Thursday, April 24.
You may also attend regular Friday public observing at
Brooks, weather permitting, this Friday, April 25. Bring a
Or Friday, May 2. In
blue ticket.
that case, report
due 12 noon May 7.
Your report will be due at the final exam, Wednesday,
April 30. Not required if you have already been to Brooks
this semester & written a report.
As before, take elevator to 5th floor of this building, walk
up to 6th floor. Bring your blue ticket with your name
and my name (Nancy Morrison) written on it. Extra blue
tickets are available.
The last available planetarium shows are:
Fridays, April 25 and May 2, 7:30 PM
Saturdays, April 26 and May 2, 1:00 PM. Report due as noted above.
Homework 6 is graded and available for
pickup in back
Planetarium and observing reports are also checked and
available for pickup.
This Wednesday
Course evaluation questionnaire, 15 minutes at beginning
of class
About the final exam
Wednesday, 30 April 2008, 7:30–9:30 PM, MH 1005
Comprehensive
Between 75 and 100 multiple-choice questions similar to
tests
Counts 37.5% of grade (75 points out of 200)
Study suggestions
• Old tests: make sure you understand missed questions
• Homework, especially frequently missed questions
• Look for connections across segments of course
Frequently missed questions, Homework 6
1. Which of these stars has the hottest core?
(a) red giant
(b) red main-sequence star
(c) blue main-sequence star
Red giants are on their way to fusing helium into carbon,
which is possible because the core has shrunk and heated
up.
2. What would you be most likely to find if you returned
to the solar system ten billion years from now?
(a) white dwarf
(b) neutron star
(c) black hole
The Sun will turn into a white dwarf, not a black hole or
a neutron star.
3. What would happen if the Sun suddenly became a
black hole without changing its mass?
(a) The black hole would quickly suck in the
Earth.
(b) Earth would gradually spiral into the black
hole.
(c) Earth’s orbit would not change.
The Earth’s orbit depends only on the mass of the object
at the center of the solar system.
This question does not state that the Sun will become a
black hole. It is only asking what would happen if,
somehow, a black hole were substituted for the Sun.
5. Where in the Galaxy would you least expect to find an
ionization nebula?
(a) halo
(b) disk
(c) spiral arm
Ionization nebulae are found near hot, young stars, which
are concentrated in spiral arms. The spiral arms are
within the disk. So they are least likely to be found in the
halo.
Importance and implications of Hubble’s Law
• The galaxies move away from each other—not just from
our galaxy.
• Distances between galaxies enlarge; space expands; the
universe is expanding.
• Actually, clusters of galaxies do not expand and, of course,
galaxies themselves do not expand. Therefore, it is more
accurate to say that the clusters of galaxies move apart.
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• Estimate distances to very distant galaxies
– Measure the radial velocity of a galaxy from its
spectrum, then divide the radial velocity by the Hubble
Constant to calculate the galaxy’s distance.
– In this way, estimate distance to a galaxy even when
you know nothing about it other than its radial
velocity.
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Active galaxies and quasars
An active galaxy is a galaxy that has an unusually bright point
source of light at its center.
Often, a jet of material is being expelled from the center of
the galaxy.
The bright, point light source cannot be just stars.
• It is too bright and too small.
• Its spectrum is unlike the spectrum of starlight.
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The small size of the central light source
• The fact that this light source is a point is not the issue;
even a quite large object would appear as a point of light
at the great distance of a typical galaxy.
• The changes that occur in the brightness of the source put
a limit on how big the source can be.
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• The most rapid change possible would be for the whole
object to brighten simultaneously and instantaneously.
• But an external observer will not see an instantaneous
change.
• Instead, light from the edge of the source nearest the
observer reaches him/her first.
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• The observer will see a gradual brightening.
• The time required for the brightening will be (in our case)
equal to the time needed for light to cross half of the
source.
• Again: this reasoning sets an upper limit on the size of the
source.
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• The brightness of an active galactic nucleus can vary in
the course of times as short as 1 week. Conclude: the
source of the energy is at most 1 light week across.
• But it emits the energy equivalent of millions of stars!
• Obviously, a highly efficient process for energy conversion
is at work.
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Currently favored model for the energy source at the center of
an active galaxy
• Hidden inside is a massive black hole—perhaps 1 million
Suns.
or much
more!
• Surrounding it is an accretion disk of material spiralling
into the hole.
• The accretion disk radiates the energy that we observe.
• Stars and interstellar material from the surrounding galaxy
supply material to the accretion disk.
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Quasars
“Quasar” stands for “quasi-stellar radio source.”
• Discovered as bright, powerful sources of radio waves
• In visible-light images, they look like stars (points of light).
• But spectra are not starlike: emission-line spectra with
very large redshifts.
In recent years, many quasars have been discovered from the
large redshift alone.
In fact, most are not radio sources.
Therefore, the more neutral term, “quasi-stellar object
(QSO)” is sometimes used instead of “quasar.”
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Similarities to active galactic nuclei
• The spectra are very similar.
• The luminosities are similar, but quasars are generally
more luminous.
• Similar variations in brightness
• Many quasars are located in the centers of galaxies.
These similarities suggest that quasars are the same type of
object as active galactic nuclei, just farther away.
So the same model is applied to both.
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Properties of the universe
The universe: everything about which we can get information
through light
(1) Expansion of space
• Hubble Relation: clusters of galaxies move apart with
speeds proportional to their mutual separations
• We imagine space itself as a stretchable fabric with
clusters of galaxies pasted onto it.
• Mutual recession of clusters of galaxies is caused by
expansion of space itself. Hence, “expansion of the
universe”
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Imagine you are living on the surface of a balloon (Fig. 15.18).
Light waves are “stretched” as they travel through the
expanding fabric (Fig. 15.19).
The farther & longer they travel, the more they are
“stretched:” an explanation for Hubble’s Law of redshifts.
(2) Large-scale structure
Clusters of galaxies are the largest gravitationally bound
structures.
They are grouped into superclusters.
Superclusters line up in filaments, which are separated by
empty spaces or voids.
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(3) Looking back in time
• We see galaxies as they were when the light left them.
• The farther away a galaxy is, the farther back in time we
are looking.
• For example: in the Hubble Ultra Deep Field, very faint,
distant galaxies are seen as they were fairly soon after
they formed.
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(4) Age of the universe
• Current estimates
– Based on observed radial velocity of any galaxy and its
estimated distance; assumes this rate has been
constant over time
– About 14 billion years. Uncertainty 2 billion years.
• Compare with age of oldest known objects: globular star
clusters, about 12 to 15 billion years.
But has the expansion rate always been the same as it is now?
The mutual gravitational pull of all the galaxies in the
universe could be slowing it down.
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(5) Average density of the universe
Estimate the average density in a large volume: reckon up the
total mass in the volume, divide by the volume
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To determine the mass of a galaxy
• Study Doppler shifts in outer disk to learn orbital speeds
of stars (rotation of galaxies)
• Study galaxies in clusters to learn their motion in relation
to each other
Learn: in most galaxies, this method indicates more mass
than the detected stars in the galaxy can account for.
This fact argues that much of the matter in galaxies is dark.
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The critical density
• The average density that would cause the cosmic
expansion to slow down continually but never quite stop
• There are good theoretical reasons for thinking that the
average density of the universe is equal to the critical
density.
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(6) Acceleration
A new standard candle for great distances: white dwarf
supernovae
• A different kind of stellar explosion
• In a binary system, a white dwarf gathers material from its
companion until its mass exceeds the maximum for a
white dwarf.
• Then it explodes, producing a great burst of light but
leaving no collapsed remnant.
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• Because the mass and composition of the exploding star
are always the same, the luminosity of the explosion is
uniform.
• These explosions have been calibrated in nearby galaxies.
• Allow distance estimates to greater distances than ever
before.
• The farthest ones are closer than their redshifts would
indicate.
This means that the universe expanded more slowly in the
past than it does now: the expansion is accelerating.
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No one knows for sure what is causing the acceleration, but
we give it a name anyway: dark energy.
Dark energy is a property of space itself, even in vacuum.
If we assume the dark energy has certain properties, then it
has the equivalent of enough mass to bring the average
density of the universe up to the critical value. (Remember
E = mc2!)
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(7) The cosmic background radiation
• Definition & properties
– An infrared & microwave “glow” from empty sky
– Thermal radiation at about 3o Kelvin
– Almost perfectly uniform in brightness
• Interpretation
– This radiation fills the universe as if we were inside a
room with glowing walls.
– In the past, the waves were less “stretched.”
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– Observers then would have measured the temperature
of the “walls of the room” to be higher than we do
now.
– The farther back in time, the hotter and denser the
universe
– Suggests that the universe originated in an extremely
hot, dense state: a primordial cosmic fireball
– The brightness of the background is very slightly
nonuniform, so the primordial fireball was too.
– Denser regions in the fireball eventually gave rise to
superclusters in the universe today, through
gravitational contraction.
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– Theory predicts the angular size of those denser,
brighter regions today, depending on the geometry of
space.
Geometry
• In flat space (like a table top), the angular size of an
object decreases in proportion to its distance.
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• But in curved space (like the surface of a sphere), the
angular size of an object still decreases with increasing
distance from an observer, but not as much.
• So if the universe were curved, the brighter spots in the
cosmic background would be larger (in angular size) than
they actually are.
• A spacecraft called WMAP has found the angular sizes of
the bright spots to match the prediction of the theory of a
flat universe.
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(8) Flatness of space
The geometry of empty space is determined by the average
density, and vice versa.
If space is flat, rather than curved like the surface of a sphere,
the density equals the critical value.
The consequence is that the universe will never stop
expanding, but the expansion will continually slow down.
But it’s remarkable that the density is just equal to the
critical density, since it could have been much larger or much
smaller. This is a very special situation.
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