Astronomy 101 Thought Questions - Fall 2007

Thought Questions for Astronomy 101 (Fall 2007)
Instructor: Eric Sandquist

Animations about Blackbody Radiation (very useful)

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

Thought Questions for Final Exam:

Question Topics:

Light (from Midterm 2)
Atoms (from Midterm 2)
Parallax
Black Holes
Galaxies
The Universe
 Blackbody Radiation (from Midterm 2)
Star Lifetimes
Star Luminosity
The Milky Way
 Types of Spectrum (from Midterm 2)
Star Temperature
Star Size
Doppler Shift

Star Lifetimes

Which of these tanks will empty first?

  1. Tank A
  2. Tank B
  3. Tank C
  4. More than one tank will empty at the same time.
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.

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?

  1. 2 hours
  2. 2.5 hours
  3. 20 hours
  4. 25 hours
  5. 50 hours
  6. 125 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).

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?

  1. The small car.
  2. The large car.
  3. They will both run out of gas at the same time.
Answer: 3. Hint: The large car is using gas faster, but it has more gas to begin with.

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?

  1. The small car.
  2. The large car.
  3. They will both run out of gas at the same time.
Answer: 2. Hint: The large car has more gas, but in this case it uses fuel much faster than the small car.

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?

  1. Star A
  2. Star B
  3. Star C
  4. Stars A, B, and C all have approximately the same mass.
Answer: 3. Hint: More massive stars use their fuel much more quickly than lower mass stars.

The bright star Vega has about 3 times the Sun's mass, and 60 times the Sun's luminosity. Vega will live

  1. about 3 times longer than the Sun will.
  2. about 20 times longer than the Sun will.
  3. about 60 times longer than the Sun will.
  4. about 180 times longer than the Sun will.
  5. about 1/3rd as long as the Sun will.
  6. about 1/20th as long as the Sun will.
  7. about 1/60th as long as the Sun will.
  8. about 1/180th as long as the Sun will.
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.

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?

  1. Large number of logs, large logs, bright campfire
  2. Large number of logs, small logs, bright campfire
  3. Small number of logs, large logs, bright campfire
  4. Large number of logs, large logs, faint campfire
  5. Small number of logs, small logs, bright campfire
  6. Small number of logs, large logs, faint campfire
  7. Large number of logs, small logs, faint campfire
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?

  1. The cooler star.
  2. The hotter star.
  3. Neither - they will be the same brightness.
  4. You didn't give me enough information to decide.
Answer: 2. Hint: A hotter star emits more light at all wavelengths than a cooler star of the same size. Remember blackbody radiation.

If you look at both stars through a red filter, which star is going to appear the brightest?

  1. The cooler star.
  2. The hotter star.
  3. Neither - they will be the same brightness.
  4. You didn't give me enough information to decide.
Answer: 2. Hint: A hotter star emits more light at all wavelengths than a cooler star of the same size. Remember blackbody radiation.

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?

  1. You see more light through the blue filter than you see through the red filter.
  2. You see more light through the red filter than you see through the blue filter.
  3. You see the same amount of light coming through both filters.
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.

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?

  1. You see more light through the blue filter than you see through the red filter.
  2. You see more light through the red filter than you see through the blue filter.
  3. You see the same amount of light coming through both filters.
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?

  1. Your thumb seems to move less when you switch between eyes.
  2. Your thumb seems to move the same amount when you switch.
  3. Your thumb seems to move more when you switch between eyes.
  4. Your thumb doesn't seem to move when you switch.
Answer: 3. Hint: Do the experiment! This is one way your brain determines the distance to something you are looking at.

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?

  1. The star nearest the upper left corner.
  2. The 4th star from the right (in the top picture).
  3. Both stars are at the same distance.
  4. It isn't possible to tell.
Answer: 2. Hint: Larger shifts mean smaller distance from us (just like looking at your thumb --- see above).

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?

  1. Star 1.
  2. Star 2.
  3. Both stars are at the same distance from us.
  4. This cannot tell us which of the stars is closer.
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?

  1. One bulb.
  2. Two bulbs.
  3. Three bulbs.
  4. Four bulbs.
  5. More than four bulbs.
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.

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?

  1. 1/4th as bright.
  2. 1/2 as bright.
  3. The same brightness as before.
  4. 2 times as bright.
  5. 4 times as bright.
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.

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?

  1. Your front.
  2. Your back.
  3. You will get a nice even tan.
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.

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?

  1. Antares
  2. Spica
  3. They will be the same size.
  4. You didn't give me enough information to decide.
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.

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?

  1. Spica
  2. Sirius B
  3. They will be the same size.
  4. You didn't give me enough information to decide.
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.

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?

  1. A main sequence star more massive than the Sun.
  2. A main sequence star less massive than the Sun.
  3. A giant.
  4. A white dwarf.
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?

  1. The escape velocity gets larger - it is harder to escape.
  2. The escape velocity gets smaller - it is easier to escape.
  3. The escape velocity will remain the same.
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.

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?

  1. The Earth would travel in a straight line in the direction it was heading when the transformation occurred.
  2. The Earth would continue to follow the same orbit it was on before the transformation occurred.
  3. The Earth would immediately start to move toward the black hole more and more quickly until it was consumed.
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.

If you decided to dive into a black hole head first, what would the gravitational forces be like?

  1. The gravitational forces on all parts of your body would be equal.
  2. The gravitational force on your head would be greater than the gravitational force on your feet.
  3. The gravitational force on your feet would be greater than the gravitational force on your head.
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?

  1. A
  2. B
  3. C
  4. D
  5. E
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!

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?

  1. A
  2. B
  3. C
  4. D
  5. E
Answer: 1. Hint: This is still a small distance compared to the size of the Milky Way!

Roughly how big is the red dot in the picture at right?

  1. 1,000 light-years
  2. 10,000 light-years
  3. 25,000 light-years
  4. 100,000 light-years
  5. 500,000 light-years
Answer: 1. Hint: This is still a small distance compared to the size of the Milky Way!

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?

  1. The first (faster) car.
  2. The second (slower) car.
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.

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?

  1. There is a stronger gravitational force pulling on the slower star.
  2. There is a stronger gravitational force pulling on the faster star.
  3. The gravitational forces must be the same.
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?

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.

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?

  1. Main sequence stars with low mass, and giants.
  2. Main sequence stars with high mass, and giants.
  3. Main sequence stars with high mass, and white dwarfs.
  4. Main sequence stars with low mass, and white dwarfs.
Answer: 2. Hint: Both of giant and high-mass main sequence stars can be hundreds of times brighter than the Sun.

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?

  1. Main sequence stars with high mass.
  2. Main sequence stars with low mass.
  3. Giant stars.
  4. White dwarfs.
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?

  1. ``Galaxy B is not moving.''
  2. ``Galaxy B is moving toward me (in galaxy C).''
  3. ``Galaxy B is moving away from me (in galaxy C).''
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.

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?

  1. ``Galaxies A and B are moving away from me with equal speeds.''
  2. ``Galaxy A is moving away from me faster than galaxy B.''
  3. ``Galaxy B is moving away from me faster than galaxy A.''
Answer: 2. Hint: In an expanding universe, the space between each pair of galaxies will get larger, and more distant galaxies expand apart faster.

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?

  1. Both people are the same age but are at different distances from you.
  2. The people are actually different ages, but are at the same distance from you.
  3. The person that is closer to you is actually the older of the two people.
  4. The person that is farther from you is actually the older of the two peopple.
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.

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?

  1. 15 years ago
  2. 5.5 billion years ago
  3. 8 billion years ago
  4. 13.5 billion years ago
Answer: 3. Hint: If the galaxy is 8 billion light-years away, light will take 8 billion years to cover the distance.

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?

  1. 10 million years old.
  2. 40 million years old.
  3. 50 million years old.
  4. 100 million years old.
Answer: 1. Hint: We are forced to judge the star based on the light that we are receiving now.

How long will it be before we receive light from the supernova at this star's death?

  1. 10 million years from now.
  2. 40 million years from now.
  3. 50 million years from now.
  4. 100 million years from 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).

When will or when did the supernova occur?

  1. 40 million years ago.
  2. 60 million years ago.
  3. 100 million years ago.
  4. 40 million years from now.
  5. 140 million years from now.
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.

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?

  1. The temperature is hotter than the center of the Sun.
  2. The temperature is hotter than the surface of the Sun, but cooler than the center of the Sun.
  3. The temperature is hotter than body temperature, but cooler than the surface of the Sun.
  4. The temperature is cooler than body temperature.
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.

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?

  1. The temperature is going to appear to be lower than it actually was.
  2. The temperature is going to appear to be higher than it actually was.
  3. The temperature is going to appear to be exactly what it was originally.
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.

Imagine that the universe has always existed (and that there was no Big Bang). What would we observe today?

  1. The cosmic background radiation would be just like we see it today.
  2. We would see cosmic background radiation having larger wavelengths than we see today.
  3. We would see cosmic background radiation having shorter wavelengths than we see today.
  4. We would not see cosmic background radiation today.
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.)

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?

  1. The cosmic background radiation would be just like we see it today.
  2. We would see cosmic background radiation having larger wavelengths than we see today.
  3. We would see cosmic background radiation having shorter wavelengths than we see today.
  4. We would not see cosmic background radiation today.
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!

What was the order the universe was put together (from start to finish)?

  1. atoms, helium nuclei, carbon nuclei, stars
  2. helium nuclei, atoms, stars, carbon nuclei
  3. helium nuclei, carbon nuclei, atoms, stars
  4. atoms, stars, helium nuclei, carbon nuclei
  5. stars, atoms, helium nuclei, carbon nuclei
Answer: 2. Hint: Helium is mostly made in the Big Bang, but carbon is produced in stars.

Which of the universes graphed below was expanding ("stretching") fastest in the past?

  1. Universe A.
  2. Universe B.
  3. Universe C.
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.

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?

  1. The car would have been doing the speed limit.
  2. The car would have been going faster than the speed limit.
  3. The car would have been going slower than the speed limit.
Answer: 2. Hint: If the car is decelerating, it is slowing down, and it was going faster in the past.

For an accelerating universe, which of the following is true?

  1. The universe was stretching more quickly in the past.
  2. The universe was stretching more slowly in the past.
  3. The universe was stretching at the same rate as it is now.
Answer: 2. Hint: If the universe is accelerating, the stretching is speeding up, and it was going stretching slower in the past.

For an accelerating universe, which of the following is true?

  1. More distant galaxies have larger redshifts than closer ones.
  2. More distant galaxies have smaller redshifts than closer ones.
  3. More distant galaxies can have the same redshifts as closer ones.
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:

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).

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Last update: December 4, 2007