ASTR 364 In-class Questions
Possible answers will always be shown in this order:
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RED
BLUE
GREEN
WHITE
Do you agree that Pluto should have been
demoted to a dwarf planet?
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No!
What’s Pluto?
Yes!
I have no opinion
If Ross-Ade Stadium (0.2 km in diameter) was
built on the moon (distance = 400000 km), its
angular diameter would appear to be roughly
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one degree
1/10th of a degree
1/10th of an arcminute
1/10th of an arcsecond
Which is correctly ordered from smallest to
largest?
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astronomical unit, light year, parsec
parsec, light year, astronomical unit
astronomical unit, parsec, light year
light year, parsec, astronomical unit
Which has a higher intensity viewed from 1 m
away?
• A household 100 W light bulb
• A 100 W fluorescent overhead tube
• They will have the same intensity
Star G has an apparent magnitude of +5.0 and an
absolute magnitude of +4.0. Star H has an
apparent magnitude of +4.0 and an absolute
magnitude of +4.0. Which star appears brighter
in the night sky?
• Star G
• Star H
• They will appear the same
Altair’s apparent magnitude is +0.77 and its
absolute magnitude is +2.2. How far is Altair
from Earth?
• Less than 10 parsecs
• 10 parsecs
• More than 10 parsecs
Which of the following is correct?
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Apparent magnitude is a measure of intensity
Absolute magnitude is a measure of flux
Apparent magnitude is a measure of luminosity
Absolute magnitude is a measure of luminosity
This graph shows the blackbody spectra of
three stars. Which has the highest surface
temperature?
• Antares
• Spica
• The Sun
The surface of Rigel is four times as hot as
Betelgeuse, and its diameter is 16 times smaller
than Betelgeuse. Rigel is a bit further away from
us than Betelgeuse.
• Rigel is 16 times less luminous
than Betelgeuse
• Rigel is 16 times more
luminous than Betelgeuse
• The stars have the same
luminosity
• You need their exact distances
to determine their relative
luminosities
The star Aldebaran is 10 times closer than
Betelgeuse and has roughly the same surface
temperature. Using this picture which shows
their intrinsic sizes, which of the following is
approximately true?
• We receive 9 times more
flux from Aldebaran
• We receive 9 times less flux
from Aldebaran
• The stars have about the
same flux
Betelgeuse
40 Rsun
1200 Rsun
Alpha Centauri has about the same radius and
surface temperature as our Sun, and is about
1.33 pc from Earth. It has apparent magnitude
zero, while our sun has apparent magnitude -27.
• The luminosity of α Cen is
larger than our Sun
• The luminosity of α Cen is
much less than our Sun
• The flux from α Cen is about
1010 times less than our Sun
• The absolute magnitude of α
Cen is much smaller than our
Sun
A thin, cold gas cloud lies between you and a bright
blackbody. Which of these spectra would you expect to
see when you look toward the blackbody?
• Spectrum A
• Spectrum B
• Spectrum C
• None of these
Astronomers determine the “color” of a star by
calculating the
• ratio of fluxes as measured
with two different filters
• difference between the fluxes
as measured with two
different filters
• ratio of the absolute and the
apparent brightness
• difference of the absolute and
the apparent brightness
Does Betelgeuse look reddish to
you? Go outside tonight and see!
The main reason that flux measurements
with filters gives temperatures of stars is
• Hotter stars lie further away from us
• More luminous stars lie further away from us
• If you compare any two stars, the colder one is
always less luminous
• The light from colder stars always peaks at
lower frequency
Two stars are in a binary system and are
separated by 200 A.U. Star 1 is 5 solar masses.
Star 2 is 2 solar masses. The period of star 1
around the center of mass is ______ the period
for star 2 around the center of mass.
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larger than
smaller than
the same as
not enough info
given to answer this
The spectrum of Sirius A peaks at 5.8 x 1014 Hz.
The spectrum of Sirius B peaks at 1.5 x 1015 Hz.
From this information alone, you can deduce that
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Sirius A is smaller than Sirius B
Sirius A is less luminous than Sirius B
Both of the above are true
You can’t deduce any of the above
In a binary star system, star A is twice as massive
as star B. The orbital plane is at an inclination of
45°. Which of the following is always true?
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Star A will experience larger blue shifts than B
Star A will experience smaller blue shifts than B
The orbital period of star A is larger than star B
The orbital period of star A is smaller than star B
For stars larger than the Sun, luminosity goes as
(mass)4 , while radius goes as (mass)0.5. If every
star uses up the same fraction of its mass over its
lifetime, then you would expect that:
• High mass stars live longer
• Low mass stars live longer
• High and low mass stars live to about the same
age
• More luminous stars live longer
In space you come across some neutral 3H.
Your ship’s sensors tell you that it has
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1 neutron, 1 proton and 1 electron
1 neutron, 1 proton, and 2 electrons
1 neutron, 2 protons, and 2 electrons
2 neutrons, 1 proton, and 1 electron
An interstellar cloud is made up of a neutral gas
that is 25% Hydrogen by mass and 75% Helium by
mass. Its mean molecular mass is:
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13/4
7/16
16/7
4/13
If you fully ionize an initially pure Helium gas,
its mean molecular mass will:
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Decrease by a factor of 3
Increase by a factor of 3
Increase by a factor of 4
Decrease by a factor of 4
If the gas pressure at the surface of our Sun was
much higher than the gas pressure at its center
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Its radius would slowly expand over time
Its central regions would start to collapse
It would start to pulsate
This describes our current Sun
Given what you know about surface gravity and
mass-radius relations for main sequence stars at
least 1.66 times as massive as our Sun:
• The surface gravity varies very little with mass
• More massive stars have much higher surface
gravity
• Less massive stars have much higher surface
gravity
The sequence ‘OBAFGKMLT’ for main sequence
stars is ordered by decreasing ___________,
whereas the letters are ordered alphabetically by
decreasing _________
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luminosity, temperature
temperature, luminosity
temperature, hydrogen absorption line depth
luminosity, hydrogen absorption line depth
The spectral class of a star depends solely on
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temperature
mass
luminosity
radius
Which class of main sequence star has the
longest lifetime?
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Class A
Class M
Class O
These all have about the same lifetime
The luminosity classes of main sequence
stars:
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depend on both radius and temperature
depend solely on mass
depend solely on luminosity
are all about the same as our Sun’s
Using the spectroscopic parallax method,
you measure a star to be 76 pc away. You
now find out that the star is actually
luminosity class I instead of class V. That
makes your original distance measurement
• Too large
• Too small
• Correct – no change to your estimate is needed
(Optical) Deep Thoughts:
If the Sun is a giant ball of gas whose pressure
drops off as -dP/dr, and photons are produced
only in a small region in its center, then why
does it look like a circular disk?
Which of the following has an optical
depth much smaller than 1?
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Your textbook in visible light
Our atmosphere in gamma-rays
The walls of this classroom in radio waves
A mirror in visible light
The mean free path can be described as:
• Inversely proportional to opacity and density
• The average distance travelled by a particle
between collisions
• The distance over which the optical depth
grows from zero to one
• All of the above
You might expect the mean free path of a
photon that starts out deep inside the Sun to
be
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Much longer than a solar radius
Much smaller than a solar radius
About equal to a solar radius
About equal to one astronomical unit
If you were to suddenly crank up the
energy generation rate at the center of the
Sun, the inner regions of the Sun would
• Be more likely to transfer heat via
convection
• Remain unchanged
• Be more likely to transfer heat via radiation
• Be more likely to transfer heat via
conduction
Increasing the opacity ̅ of a gas layer
within a star will tend to
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make the layer less likely to be convective
flatten the temperature gradient
steepen the temperature gradient
have no other effect on the layer
Consider the following mass comparisons (in atomic mass units):
Proton mass = 1
Neutron mass = 1.0014
Hydrogen atom mass = 1.0007
Helium atom mass = 3.9737
Why does helium (2 protons + 2 neutrons) have a mass less than 4?
• Some helium atoms have less than 2 protons
• The mass is lost when two protons are converted to
two neutrons
• It takes energy to pull a helium nucleus apart
• Helium atoms move so fast that they have a lower
apparent mass
In general, fusion is most likely to occur
between two particles with
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High relative velocity
High electric charge with the same sign
High masses
High magnetic charge
The Sun’s corona has temperatures roughly
as hot as the Sun’s core. So why doesn’t
fusion occur in the corona?
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The gas density is too low
The gas is fully ionized
The gas is only 70% hydrogen
All of the above
About 25% of the mass of a newborn star
is helium. Why doesn’t it fuse while the
star is on the main sequence?
• Helium fusion requires three nuclei to interact
within a very short amount of time
• For a given gas temperature, helium nuclei
move slower than hydrogen nuclei
• Helium nuclei repel each other more strongly
than hydrogen nuclei
• All of the above
The energy from fusion reactions that goes
into neutrinos doesn’t heat up the Sun
because neutrinos
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are massless and thus carry no energy
have no electric charge
have a very large mean free path
rapidly decay
A star remains at constant size and
temperature for a long period of time.
Which of the following is mostly likely to be
true? The star generates
• about as much energy as it radiates
• more energy than it radiates into space
• less energy than it radiates into space
When starlight passes through interstellar
dust
• The wavelengths all get longer (redder)
• It gets fainter
• The red light tends to scatter sideways
while the blue continues to us
• All of the above
An interstellar dust cloud drifts in front of a
star. In terms of numerical values for color
index and apparent magnitude, its observed
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(B – V) is larger, and mV is smaller
(B – V) is larger, and mV is larger
(B – V) is smaller, and mV is smaller
(B – V) is smaller, and mV is larger
Hot emission nebulae are somewhat red, and cold
reflection nebulae are blue. Why are these colors
different from what Wien's law tells us about the
radiation emitted by a blackbody?
• The regions producing light in emission
nebulae are optically thick
• The dust grains in reflection nebulae scatter
longer wavelengths of visible light more
strongly than shorter wavelengths.
• The light we see from emission nebulae has
been strongly affected by dust
• None of the above
Which of these would have the largest
diameter H II region?
• O3 V star surrounded by a hydrogen cloud with a
low free electron density
• G2 V star surrounded a hydrogen cloud with a low
free electron density
• G2 V star surrounded a hydrogen cloud with a
high free electron density
• O3 V star surrounded by a hydrogen cloud with a
high free electron density
If a free electron encounters a magnetic
field it will
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Accelerate away from it and emit nothing
Ignore it
Accelerate toward it and emit radiation
Annihilate and produce a gamma-ray
As you increase the eccentricity of an orbit,
it becomes more elliptical, and the focus
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Moves towards the center of the ellipse
Stays put
Moves towards the edge of the ellipse
Moves to a point outside the ellipse
If you perturb the edge of a molecular
cloud, the perturbation will travel through
the cloud
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on the free-fall timescale
at the speed of light
at the speed of sound
on the thermal timescale
If the material in the primordial Solar
System retained its angular momentum as
it collapsed to form the Sun, the Sun’s
current rotation rate should be
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Slow (more than a month period)
Fast (less than a week period)
Moderate (week to a month period)
Zero (non-rotating)
According to the Virial theorem, if you
compress a ball of gas in hydrostatic
equilibrium:
• The temperature of the gas will remain the
same
• The temperature of the gas will increase
• The temperature of the gas will decrease
For our Sun, which is longest – the thermal
timescale (GM2/LR) , the free-fall timescale
(R3/GM) , or the nuclear timescale (2 Myr x
M/L) ? ------ note: 1 year ≈ π x 107 sec.
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nuclear
thermal
free-fall
all three are roughly the same
If the charge on the proton were increased,
then the masses of the lowest mass stars
on the main sequence would be
• unchanged
• smaller
• larger
The main source of pressure in a degenerate gas is
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The low number density of the particles
The high kinetic energies of the particles
The filling up of available energy states
Degenerate gasses do not supply pressure
In a fully degenerate gas, the
pressure is dependent on:
• The density and temperature
• The temperature only
• The density only
Which is the
oldest star
cluster here?
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M67
NGC 2362
h + χ Persei
These are all
about the
same age
Our Sun doesn’t pulsate because
• Its oscillations aren’t driven at the right
period
• There is no companion star to perturb it
• It is no longer on the main sequence
• Its outermost layer is convective
White dwarfs support themselves against
gravitational collapse by
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Coulomb pressure
Helium gas pressure
Carbon gas pressure
Electron degeneracy pressure
Which is ranked correctly from least dense
to most dense?
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Black hole, white dwarf, neutron star
Neutron star, black hole, white dwarf
White dwarf, neutron star, black hole
All of three have about the same density
Our Sun will end up as
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A white dwarf
A black hole
A neutron star
A supernova remnant
The mechanism that makes a white dwarf
shine is
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Fusion of material dumped on its surface
Gravitational contraction
Nuclear fusion in the core region
Escape of heat left over from its formation
In order for a white dwarf to become a
nova it must
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Have had a main sequence mass > 7 Msun
Be heavier than 1.44 solar masses
Have a companion star
Continue fusing elements up to iron
If you were standing on the surface of a 1
solar mass neutron star and dropped your
smartphone, it will hit the surface at about
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1 m/s
10 m/s
104 m/s
106 m/s
It is much more difficult to escape the
surface of a neutron star compared to a
normal star because
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It is much hotter
It is much heavier
It is much denser
It has much lower surface radiation pressure
If a pulsar rotates 1000 times per second,
special relativity and classical mechanics
imply that compared to a normal star it
must be
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Much larger and less dense
Much larger and denser
Much smaller and denser
Much smaller and less dense
A supernova is
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Something our Sun will eventually do
The eventual fate of a very high mass star
An atypically bright classical nova event
The creation of a very high mass star
The event horizon of a 1.0 Msun black hole
is 3 km. Thus, the event horizon for a 2
solar mass black hole must be
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0.75 km
1.5 km
3.0 km
6 km
If our Sun were to suddenly replaced by a 1
Msun black hole, the Earth would
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Start to fly outward at a tangent to its orbit
Start to spiral inward toward the black hole
Stay in its current orbit
Start to fly outward in a direction opposite to
the black hole
An astronaut falling into a black hole would
be ‘spaghettified’ by
• Magnetic forces: the black hole’s powerful
magnetic field pulls her apart
• Tidal forces: gravity pulls her feet much
stronger than her head
• Centripetal forces: the rapid rotation of the
event horizon stretches her out
Black holes are black because
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They are extremely cold
They are blackbodies
Their escape velocity exceeds light speed
They are theoretical objects that have never
been seen
In a supernova or gamma-ray burst, what
type of energy is converted to radiation?
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Electromagnetic
Thermonuclear fusion
Neutron degeneracy
Gravitational
If Alpha Centauri (distance = 1 pc) were to
suddenly explode in a 1045 W gamma-ray burst,
it would appear: (hint: 1 pc is roughly 200,000
AU, Sun’s luminosity is about 1026 W)
• Considerably fainter than the Sun
• Considerably brighter than the Sun
• About the same brightness as the Sun
The radius of a star’s orbit and its velocity – the
data implied by a star’s location on a rotation
curve – can be used to determine
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The mass interior to the star’s orbit
The mass of the star
The mass of the entire galaxy
The mass at the center of the galaxy
Which curve represents the trend of velocity
versus radius for a solid rotating disk?
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Red
Blue
Green
None of
these
Young stars are predominantly found in the ___
of our galaxy since _____ there causes stars to
form.
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Disk , a density wave
Disk , low metallicity gas
Halo , high density gas
Bulge , supernovae
Which of these galaxies most resembles the Milky Way?
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(A)
(B)
(C)
(D)
Edwin Hubble’s ‘tuning fork’ diagram describes
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How astronomers classify galaxies
Different possible models for our Milky Way
How galaxies evolve, from left to right
How Ed Hubble liked his lattes prepared
Optical telescopes can’t give us a good view of
the galactic center because
• Optical telescopes currently aren’t large
enough
• There is too much dust in the way
• The galactic center is too far away
• Our atmosphere distorts the images too
much
Which observation would provide the best
evidence that the disk of the Milky Way does
NOT rotate like a solid wheel?
• Disk stars twice as far from the galactic center
rotate twice as fast around it
• Disk stars have Doppler shifts
• The brightest disk stars form spiral arms
• The rotation of the disk stars near the Sun
decreases with distance according to Kepler’s laws
Early type galaxies are all
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Relatively young
Found at large distances
Elliptical in shape
Spiral in shape
Consider these widely separated galaxies in an
expanding universe. Imagine you are located in
galaxy A and have observed that both galaxies B
and C are moving away from you. What would
someone in galaxy C say about the motion of B?
A
B
• It is moving away from them
• It is not moving
• It is moving toward them
C
The currently accepted value for the Hubble
constant is about 70 km/s/Mpc. If we were to
suddenly discover it was actually 200 km/s/Mpc,
the age of the universe would then be
• About the same age as we previously thought
• Much older than we previously thought
• Much younger than we previously thought
Say you want to measure the distance to the
closest galaxy (Andromeda). The best method to
use is
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Doppler shift of its spectral lines
Period-luminosity relation for Cepheid stars
The Hubble law of recession for galaxies
Trigonometric parallax using Earth’s orbit
In the radio galaxy M87, you can only see one
jet because
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Light from the other jet is beamed away from us
Dust is obscuring the other jet
Only one jet is currently being produced
The other jet is so far away its light hasn’t reached us yet
When Andromeda and our Milky Way merge,
• Very few of the stars in either galaxy will survive
• The dark matter halos will annihilate each other in a
giant explosion
• A new elliptical galaxy will eventually form
• The two galaxies will annihilate each other, since
Andromeda is made of antimatter
The large Doppler velocity widths of broad emission
lines in active galaxies could NOT be created by hot
emitting clouds that are
• Being ejected along a narrow-angled jet
• Swirling at high velocities around a black hole
• Falling into the neighborhood of a black hole
The most distant quasar at z = 7.1 has an
apparent optical magnitude of 21. How much
fainter is it than the faintest star you can detect
with your naked eye (magnitude = 6) ?
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100 x
1000 x
1,000,000 x
1,000,000,000 x
You detect a narrow width Lyman α absorption line in the
spectrum of a quasar which is at a significantly shorter
wavelength than the quasar’s observed Lyman α
emission line. You conclude that the absorbing cloud is
• Warm and located close to the quasar
• Cold and located close to the quasar
• Cold and located close to the Milky Way
• Warm and located close to the Milky Way
At high redshift, a larger fraction of galaxies are
“active” than at low redshift. From this, we can
safely conclude that
• All galaxies may become active more than once in their lifetime
• All galaxies go through an active phase, and fewer galaxies in the
past were active than now
• Some galaxies go through an active phase, and more galaxies in
the past were active than now
• Most galaxies never become active in their lifetime
Which cluster is the most likely to be relaxed?
Cluster A
Cluster B
Gravitational lenses are often detected at optical
wavelengths because
• Dark matter absorbs light at all other wavelengths
• Gravity bends spacetime only for optical wavelengths
• There is typically less obscuration in the optical
• Distant background galaxies emit most of their light in
the optical
Which of the following measurements can NOT be used
to measure the amount of dark matter in a galaxy
cluster?
• The deflection of light rays from a background object passing by
the outskirts of the cluster
• The average speed of galaxies orbiting the cluster center
• The dispersion in the speeds of the galaxies orbiting the cluster
center
• The properties of X-rays emitted by gas that has been heated by
falling into the cluster
Olber’s Paradox asks why the night sky is dark, when
every line of sight must eventually fall on a star. It is
dark primarily because the Universe is
• Expanding, so that distant stars are redshifted
• Infinite and mostly empty
• Young, so there are only stars out to a finite distance
• Clumpy, so that not every sightline hits a star
The near-perfect blackbody spectrum of the
cosmic microwave background implies that at
one time,
• the Universe was very much like a giant black hole
• the Universe was optically thick
• the mean free path of photons in the Universe was very
long
• the Universe was much colder than it is now
The cosmic background radiation is visible in
every direction since
• We are looking back to when the Universe was cold
• We are at the center of the Universe
• We are looking back to when the Universe was young in
every direction
• It has scattered in every direction over the age of the
Universe
Dark matter has so far not been detected in the lab
most likely because
• Radiation pressure from our Sun pushes it outward
• Dark matter is only found in distant galaxies
• It rarely interacts with normal matter
• It has a very short decay half life
The observed redshifts of galaxies mean that
• Photons lose energy as they travel through space
• We are at the exact center of an expanding Universe
• Dark matter is causing the Universe to expand faster than
it normally would
• Galaxies were moving much faster with respect to each
other when the Universe was very young
The consensus model of cosmology indicates
• The expansion of the Universe has stopped
• The expansion of the Universe is slowing
• The expansion of the Universe is accelerating
• The Universe is currently contracting
The lookback time to an object is the # of years
between when the object emitted its light and when
we see it. What piece of information surely does NOT
affect the lookback time for a distant object?
• The rate of expansion of the Universe
• Its distance
• The speed of light
• The wavelength of light being observed
As the Universe evolves over time
• The energy density of dark energy remains constant
• The energy density of dark matter increases
• The energy density of dark energy increases
• The energy density of dark matter remains constant
Since the time when the cosmic microwave background
was created, the Universe has expanded by
approximately a factor of
• 10
• 1000
• 100000
• 10 million
By observing the cosmic microwave background, the
Planck mission confirmed that on the whole, spacetime
in the Universe is
• Positively curved, like a sphere
• Negatively curved, like a saddle
• Flat
The luminosity distance is defined as the distance over
which
• The universe has expanded in the time the light from the
object takes to reach us
• The small angle formula applies
• The comoving coordinates stay constant
• The inverse square law of light applies
Which product name isn’t associated with an
astronomical phenomenon?
Starburst
Pulsar
Quark
Quark
Nova