Animations about Black Holes (more fun than informative)
Animations about the Expansion of the Universe (pretty good explanation)
Frequently Asked Questions about the Universe (a nice website explaining a number of tough questions, but some of the answers are mathematical)
Thought Questions from first Midterm
Thought Questions from second Midterm
You are going to start a barbecue marathon, but you have just one tank
of propane for your gas grill. The tank contains 50 pounds of propane.
To keep the grill at optimum cooking temperature, you have to burn 2.5
pounds of propane per hour. How long can your grilling marathon go on?
A small car has a 10 gallon gas tank, and it uses 1 gallon of gas
every hour of driving. A large car has a 20 gallon tank and uses
2 gallons of gas every hour of driving. Which will run out of gas first?
A small car has a 10 gallon gas tank, and it uses 1 gallon of gas
every hour of driving. A large car has a 20 gallon tank and uses
4 gallons of gas every hour of driving. Which will run out of gas first?
Consider the information given below about the lifetime of three main
sequence stars A, B, and C.
Star A will be a main sequence star for 45,000 million years.
Star B will be a main sequence star for 70 million years.
Star C will be a main sequence star for 2 million years.
Which star has the greatest mass?
The bright star Vega has about 3 times the Sun's mass, and
60 times the Sun's luminosity. Vega will live
Imagine you are camping, and are very afraid of the animal noises you
hear coming from near camp. You want to build a campfire that will
last all night and scare the animals off. In which of the following
cases would your campfire last the shortest time?
If you look at both stars through a red filter, which star is going
to appear the brightest?
Now imagine measuring the brightness of a star (that is much hotter
than the Sun) when you look at it separately through a red filter and
through a blue filter. What will you see?
Now imagine measuring the brightness of a star (that is much cooler
than the Sun) when you look at it separately through a red filter and
through a blue filter. What will you see?
Imagine that you are looking at the stars from Earth in January. The
picture in the frame shows a view of the distant stars as seen from
Earth. Which number would star A appear above as seen by someone on
Earth in January?
Answer: 3. Hint: Draw a line between Earth and star A and
continue it until it reaches the "Distant Stars". That shows where
star A will appear to be. (Anything on that line will appear to be
either directly in front of or directly behind star A.)
Which of the stars in the pictures below shows a measurable parallax?
Answer: The star nearest the upper left corner and the 4th star
from the right in the top picture. Hint: Look for the shift
between pictures. All of the other stars remain in the same
pattern.
Which of the stars in the pictures above is closest to us?
Two stars are photographed at the same time (the ones labeled "A")
and then six months later (the ones labeled "B"). Between those two
times the positions of the two stars appear to change. Which of the
two stars is probably closer to us?
You are studying at you desk under an unshaded lamp with a 100 watt
light bulb. Your roommate moves the lamp so that it is twice as far
away from you, but replaces it with a 200 watt bulb. How bright is your desk compared to earlier?
You are on a planet with two stars, lying on your side to try and tan
both sides at once. The stars have the same temperature as our Sun and
are the same height in the sky. The star illuminating your front is
one-third as luminous as the star illuminating your back, but the star
illuminating your back is also three times farther from the planet. and
the other is illuminating your back. Which side is going to tan
first?
For each pair of hot plates shown, which one that will boil
water more quickly? If there is no way to tell, state that.
Answers: a) Left. b) Right. c) Left. d) No way to tell. Hint:
The amount of heat the hot plate puts out depends on its temperature
and its size. This is very similar to the case for stars. For
d), one hot plate is hot but small, and one is cooler but larger, so
it is not possible to tell which will boil water faster without more
information.
The stars Antares and Spica each have the same luminosity. Antares is
cooler than Spica. Which star is larger?
The star Spica is much more luminous than Sirius B. Spica and Sirius B
have the same temperature. Which star has the greater surface area?
A star is about 100 times brighter than the Sun, and about three times
hotter at its surface than the Sun is. Which of the following types is
this star likely to be?
Imagine that a black hole was passing by Earth. As the black hole
passes by, it almost gets between you and your favorite star (as shown
in the diagrams). Which of the following pictures shows where your
favorite star would appear to be when the black hole is in the
position shown?
Answer: 3. Hint: The star will appear to be in the direction
that the light is coming from when it reaches your eyes. If the path
of the light was bent along the way, the star will not actually be in
that direction. (Think of a mirror: you can appear to be on the
opposite side of the mirror, but that is just because light is coming
to your eye from that direction after it bounces off of the mirror.)
If the Sun was instantaneously compressed and turned into a black hole
of the same mass, what would happen to Earth's orbit?
If you decided to dive into a black hole head first, what would the
gravitational forces be like?
In the same picture above, Betelgeuse (a star that will probably go
supernova in the future) is about 430 light-years away. Which dot best
represents where it is?
Roughly how big is the red dot in the picture at right?
Imagine you are watching two identical cars drive into a sharp
turn. The first car is driving faster than the second car. Which of
the cars is more likely to skid out during the turn?
Imagine you are studying two stars orbiting at the same
distance from the centers of two different galaxies. One star
is orbiting faster than the other. What does this tell you?
Answer: 2. Hint: "Moving away" produces a redshift, so
only objects 1 and 2 are possible answers (the line patterns have
become redder). Faster motion makes larger color shifts.
A policeman's radar gun is not always able to measure your speed
accurately. In which of the situations below would the cop be able to
measure your speed using only the radar gun?
Answer: 4. Hint: A radar gun uses the Doppler shift to
measure speed, but it can only motions that make distance get
larger or smaller! So you have to be moving almost directly towards or
away from the cop for him to measure your speed accurately. In #2, the
cop is moving in the same direction, so that the distance between you
is not changing, and again there would be no Doppler shift!
Imagine you are trying to accurately measure the speed of a star
moving around our galaxy using ONLY the Doppler shift of the lines in
its spectrum. For which of the stars in the diagram below would you
be able to do this?
Answer: 4. Hint: Star 4 is the only one that is moving directly
toward or away from the Sun, but the Sun itself is not moving toward
or away from the star.
The light from a galaxy you are studying has a strong reddish tint to
it. Which of the following types of stars is probably releasing most
of the light that we see?
Imagine that you are located in galaxy A and observe that galaxies B
and C are both moving away from you (with galaxy C moving faster). If
you asked an alien in galaxy C to describe what it sees, how would it
answer?
Imagine you simultaneously receive transmissions from two people that
live on planets orbiting two different stars. The two pictures show
the people at their 21st birthday parties. Which of the following is
most likely?
Fifteen years ago, a galaxy was discovered that was found to be 8
billion light years away. If our universe is approximately 13.5
billion years old, when did the galaxy emit the light that we observe?
Imagine you were observing a distant star located in a galaxy 100
million light-years away. By analyzing the starlight, you are able to
tell that the star appears to be 10 million years old. You are able to
predict that the star will have a lifetime of 50 million years.
How old does the star appear to be to us on Earth?
How long will it be before we receive light from the supernova at this
star's death?
When will or when did the supernova occur?
The cosmic microwave background is composed of photons with
wavelengths longer than visible light and longer than infrared light.
If this background is blackbody radiation, what does this imply about
the temperature?
If the expansion of the Universe makes the wavelengths of all light
stretch the longer they travel, what is that going to do to the
apparent temperature of the light reaching us from early in the
Universe's history?
Imagine that the universe has always existed (and that there was no
Big Bang). What would we observe today?
Imagine that the ``Big Bang'' occurred right here where the Milky Way
is (and nowhere else), and that the light and matter created by the
Big Bang moved away from here after that. What would we observe today?
What was the order the universe was put together (from start to finish)?
Which of the universes graphed below was expanding ("stretching")
fastest in the past?
Imagine you are a policeman using a radar gun in the proper way. You
measure the speed of a car that is near you, and find that it is going
the speed limit. If the car was decelerating, what would you
have found if you had measured its speed a few seconds earlier?
For an accelerating universe, which of the following is
true?
For an accelerating universe, which of the following is
true?
J. P. Adams, D. J. Loranz, E. E. Prather, and
T. F. Slater. Lecture Tutorials for Introductory Astronomy --
Instructor's Guide, 2002 (Prentice Hall). E. Mazur. Peer
Instruction: A User's Manual, 1997 (Prentice Hall).
Number of accesses:
Last update: December 4, 2007
Answer: 2. Hint: Even though Tank B has the smallest amount of fuel, the fuel is being used up very slowly - it will take 20 hours.
Answer: 3. Hint: The "lifetime" of the tank will be the amount of fuel
divided by the rate of fuel use (pounds per hour).
Answer: 3. Hint: The large car is using gas faster, but it has
more gas to begin with.
Answer: 2. Hint: The large car has more gas, but in this case
it uses fuel much faster than the small car.
Answer: 3. Hint: More massive stars use their fuel much more
quickly than lower mass stars.
Answer: 6. Hint: 3 times more fuel (the star's mass) would mean
3 times as long a life, but 60 times more luminosity (which tells us
about the rate at which the star uses fuel) shortens the lifetime to
1/60th what it would be. Both factors affect the lifetime.
Answer: 5. Hint: Less fuel (small logs and small number of
logs) and larger luminosity (bright campfire) contribute to using up
the star's supply of fuel quickly.
Star Temperature
Two stars have identical sizes and are the same distance away from
you, but their surface temperatures are different. If you look at both
stars through a blue filter (which only allows blue light through),
which star is going to appear the brightest?
Answer: 2. Hint: A hotter star emits more light at all
wavelengths than a cooler star of the same size. Remember blackbody
radiation.
Answer: 2. Hint: A hotter star emits more light at all
wavelengths than a cooler star of the same size. Remember blackbody
radiation.
Answer: 1. Hint: The hotter a star gets, a larger fraction of
its light has short wavelengths (bluer). This is responsible for the
change in the color of a blackbody from red to yellow to white to
blue-white as it gets hotter.
Answer: 2. Hint: The cooler a star gets, a larger fraction of
its light has long wavelengths (redder). See the previous question.
Parallax
If you hold your thumb at arm's length and look at it with your left
eye and your right eye separately, your thumb seems to move compared
to the background. What happens if your thumb is closer to your head?
Answer: 3. Hint: Do the experiment! This is one way your brain determines the distance to something you are looking at.
Answer: 2. Hint: Larger shifts mean smaller distance from us
(just like looking at your thumb --- see above).
Answer: 2. Hint: You can do this experiment with your thumb.
Hold it close to your face, and switch back and forth between eyes. Do
the same thing with your thumb farther from your face.
Star Luminosity
You are studying at you desk under an unshaded lamp with a 100 watt
light bulb. Your roommate moves the lamp so that it is twice as far
away from you. How many 100-watt light bulbs would have to be used in
the lamp to light your desk as bright as it was before?
Answer: 4. Hint: When one light bulb is moved twice as far
away, it will appear 1/4 as bright to you. So you would need 4 light
bulbs to illuminate you the same amount as the one closer light
bulb.
Answer: 2. Hint: When one light bulb is moved twice as far
away, it will appear 1/4 as bright to you. But the bulb's luminosity
is twice what it was before (it is putting out twice as much light as
the first bulb), and this partly makes up for the more distant bulb.
Answer: 2. Hint: Even though the star lighting your back is
less luminous (emits one-third as much total light), it is closer
(making its apparent brightness 3 times larger than the other
star).
Star Size
You are comparing the abilities of electric hot plates of different
sizes and temperatures to bring identical pots of water to a boil. The
pots are all as large as the largest hot plate. When a hot plate is at
one of the temperature settings (low, med, high), the hot plate is
depicted as a shade of gray. The lighter the shade of gray, the higher
the temperature.
Answer: 1. Hint: For two stars of the same size, the cooler one
will be less luminous. So unless a cool star is much larger (and has
much more surface area), it can't be as luminous as a hotter star.
Answer: 1. Hint: Both temperature and size affect how much
light a star will release (its luminosity). If they both have
the same temperature, then the size is the only thing that can be
responsible for the difference in luminosity.
Answer: 1. Hint: If a star is bright and hot, it
doesn't have to be much different in size than the Sun. It is
probably a main sequence star.
Black Holes
You are on the launch pad in your rocket ship, getting ready to escape
from the surface of a dying planet. Before you can take off, the
planet begins to shrink in size (although the mass does not change). What
happens to the escape velocity as a result?
Answer: 1. Hint: The force of gravity gets stronger the closer
two masses are together (like you and the center of the planet). As
the planet collapses, the force of gravity on you will get larger.
Answer: 2. Hint: As long as the mass of the Sun is not changed
in the process, the gravitational force we feel at Earth will be
unchanged, and our orbit will continue as before. The gravitational
forces near the black hole will be very strong because it is possible
to get very close to all of that mass. We are fairly far away.
Answer: 2. Hint: The gravitational force depends on the
distance between the two masses (your head or feet, and the black
hole).
The Milky Way
Imagine the Sun is located at the red dot in the picture below. If the
bright star Sirius is 9 light-years away, which dot best represents
where it is?
Answer: 1. Hint: For scale, the distance between the center of
the Milky Way and the Sun is about 28,000 light-years, and the
diameter of the Milky Way is about 100,000 light years!
Answer: 1. Hint: This is still a small distance compared to the
size of the Milky Way!
Answer: 1. Hint: This is still a small distance compared to the
size of the Milky Way!
Answer: 1. Hint: This should be common sense - but why? More
friction force between the tires and road is needed to keep the faster
car in the curve than is needed for the slower car. If there is not enough
friction, the car will skid.
Answer: 2. Hint: The faster the star is moving, the more
gravitational force is needed to hold it in its orbit. This is just
like the previous question with the cars.
Doppler Shift
The laboratory spectrum below shows emission lines from hydrogen. If
the spectrums of the other objects also show hydrogen, which is moving
away from us fastest?
Galaxies
Most of the light released by a galaxy comes from the most luminous stars.
Which of the following kinds of stars have the largest luminosities?
Answer: 2. Hint: Both of giant and high-mass main sequence stars
can be hundreds of times brighter than the Sun.
Answer: 3. Hint: The stars that are releasing most of the light
for this galaxy need to be bright and cool.
The Universe
Imagine that you are located in galaxy A and observe that galaxies B
and C are both moving away from you (with galaxy C moving faster). If
you asked an alien in galaxy C to describe what it sees, what would it
say about galaxy B?
Answer: 3. Hint: In an expanding universe, the space between
each pair of galaxies will get larger, so that these galaxies will
get farther apart from each other.
Answer: 2. Hint: In an expanding universe, the space between
each pair of galaxies will get larger, and more distant galaxies
expand apart faster.
Answer: 4. Hint: It has taken longer for light to travel here
from the more distant person, so what we see know actually happened
farther in the past.
Answer: 3. Hint: If the galaxy is 8 billion light-years away,
light will take 8 billion years to cover the distance.
Answer: 1. Hint: We are forced to judge the star based on the
light that we are receiving now.
Answer: 2. Hint: If the star appears to us to be 10 million
years old now, it will appear to die in another 40 million years (at
an age of 50 million years).
Answer: 2. Hint: Because the star was 100 million light years
away, and the star will appear to die in 40 million years, the light
must already be on its way. The light emitted at the death of the star
is only 40 million light-years away now, so it must have already been
travelling for 60 million years.
Answer: 4. Hint: Hot objects emit lots of short wavelength
(bluer) light. (Remember blackbody radiation.) The surface of the Sun
is hot enough to emit visible light. Human bodies are much cooler, and
they emit infrared light. So, the universe must be cooler than humans.
Answer: 1. Hint: Hot objects emit lots of short wavelength
(bluer) light. (Remember blackbody radiation.) If the wavelength of the
light is stretched, it will appear to be cooler.
Answer: 4. Hint: Cosmic background radiation is created during the
Big Bang. If the Big Bang didn't happen, there would be no radiation from it.
(If it happened infinitely long ago, it would have cooled down so much that
it would be undetectable.)
Answer: 4. Hint: If the Big Bang happened here, the radiation
created during the Big Bang would immediately start to travel away
from us. If there is nothing to reflect it back to us, we would not
see it afterwards!
Answer: 2. Hint: Helium is mostly made in the Big Bang, but
carbon is produced in stars.
Answer: 3. Hint: The galaxies in universe C have separated by
the same amount as in the other two universes, but took less time to
do it.
Answer: 2. Hint: If the car is decelerating, it is slowing
down, and it was going faster in the past.
Answer: 2. Hint: If the universe is accelerating, the stretching
is speeding up, and it was going stretching slower in the past.
Answer: 1. Hint: The redshift of a distant galaxy tells us about
the total amount of stretching the universe has done during the
time the light has been traveling. Even though the stretching was slower
in the past, it was still stretching...
Some of the questions above are taken from the following
sources: