Astronomy 101 Thought Questions - Spring 2013

Thought Questions for Astronomy 101 (Spring 2013)
Instructor: Eric Sandquist


Animations about Thermal (Blackbody) Radiation (very useful)

Animations about Black Holes (more fun than it is 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
Thermal Radiation (from Midterm 2)
Star Lifetimes
Star Luminosity
The Milky Way
The Universe
Types of Spectrum (from Midterm 2)
Star Size
Doppler Shift
Scale Models

Star Insides

Three people stand on each other to form a stack. In order to prevent the stack from collapsing, the person on the bottom has to be able to support...

  1. no weight.
  2. the weight of the person on the top.
  3. the weight of the person in the middle.
  4. the weight of both people above him.
Answer: 4. Hint: To prevent the stack from collapsing, each person has to support the weight of everything above him.

Which of the cut-away views below best represents what is happening in the core of a star at the end of its main sequence phase? ("→" means nuclear reactions are happening, H = hydrogen, "He" = helium, "C" = carbon.)

Answer: D. Hint: During the main sequence, H is being reacted to make He. When the H runs out at the center of the star, that is the end of the main sequence phase.

What happens when a star like the Sun becomes a red giant?

  1. The surface temperature and luminosity increase.
  2. The surface temperature and luminosity decrease.
  3. The luminosity decreases and the surface temperature increases.
  4. The luminosity increases and the surface temperature decreases.
  5. Only the luminosity increases.
  6. Only the luminosity decreases.
Answer: 4. Hint: If the star becomes a "red" giant, the temperature must be decreasing. Giant stars are much bigger than main sequence stars, and if a star is big (even if it is cool), it wil generally have large luminosity. The star is radiating light from a bigger surface.

How high will the smallest superball bounce when I drop the stack of four superballs from a foot above the ground?

  1. It won't bounce - it will come to a dead stop.
  2. It will bounce back to its starting point (1 foot).
  3. It will bounce about 4 times as high (about 4 feet).
  4. It will bounce about 8 times as high (about 8 feet).
  5. It will bounce about 16 times as high (about 16 feet).
  6. It will hit the ceiling.
Answer: 5. Hint: All of the superballs are compressed when they hit the ground, and when they return to their original shapes, they all work together to push the smallest superball upwards. This kind of thing happens in a massive star at the beginning of a supernova explosion.

Gas from a supernova explosion is expanding 0.1 light-years every decade, and the edges of the gas cloud are about 10 light-years from the center. How long has it been since the cloud started expanding?

  1. 0.1 year
  2. 1 year
  3. 10 years
  4. 100 years
  5. 1000 years
  6. 10,000 years
Answer: 5. Hint: If it maintains its speed, it will expand 1 light year every 10 decades (one century).


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: 3. 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.

If the Sun doesn't have a source of heat inside, what does this say about the past history of the solar system?

  1. The Sun must have been much hotter and brighter in the past, and the planets must have been much warmer.
  2. The Sun and the solar system must have been about the same as they are today.
  3. The Sun must have been much cooler and fainter in the past, and the planets must have been much cooler.
Answer: 1. Hint: Without a source of heat, the Sun must be cooling down, so it was hotter in the past.

The HR Diagram below shows the 30 nearest stars. What are most nearby stars like (compared to the Sun - the yellow dot)?

  1. hotter, less luminous
  2. cooler, less luminous
  3. hotter, more luminous
  4. cooler, more luminous
Answer: 2. Hint: Though there are some stars with other characteristics, BY FAR faint red stars outnumber stars like the Sun!


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 picture is closest to us?

Answer: F. Hint: Only two stars move during the year (A and F). Compare the pictures in the upper left and lower right to see the biggest shifts.

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.

I am sunbathing on a planet with two stars that have the same temperature. I am lying on my side and trying to tan both sides at once. Star A lights my back. Star B lights my front, and it is 3 times as luminous as star A but it is 3 times farther from the planet. What will happen?

  1. My front tans more.
  2. My back tans more.
  3. I will get a nice even tan.
Answer: 2. Hint: Even though the star A is less luminous (emits one-third as much total light), it is closer (making its apparent brightness 3 times larger than the other star).

An Earth-sized planet orbits 2 AU from another star. The star has the same temperature as the Sun, but is 3 times more luminous than the Sun. What is the temperature on the planet probably like?

  1. It is probably warmer than Earth.
  2. It is probably cooler than Earth.
  3. It is probably about the same temperature as Earth.
Answer: 2. Hint: Even though the Sun is less luminous (emits one-third as much total light every second), it is closer to Earth than the star is to the other planet. As seen from the planet, the star will appear to be about 1/3rd as bright as the Sun appears from Earth.

If you know two stars have the same luminosity, but star A appears 100 times fainter than star B, how do their distances compare?

  1. Star A is 10,000 times farther away than star B.
  2. Star A is 100 times farther away than star B.
  3. Star A is 10 times farther away than star B.
  4. Star B is 10 times farther away than star A.
  5. Star B is 100 times farther away than star A.
  6. Star B is 10,000 times farther away than star A.
Answer: 3. Hint: This is again related to the inverse square law for light. The apparent brightness of the star decreases as the distance (squared) increases.


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.

The star Spica is both hotter and more luminous than the Sun. Which star is larger?

  1. Spica
  2. The Sun
  3. They will be the same size.
  4. You didn't give me enough information to decide.
Answer: 4. Hint: Either temperature or size could be responsible for the larger luminosity of Spica. Without more information, it isn't possible to tell which is more important.

If you took a star that had the same mass as the Sun and made its diameter 10 times smaller, its density would

  1. remain the same.
  2. become 10x smaller.
  3. become 10x larger.
  4. become 100x smaller.
  5. become 100x larger.
  6. become 1000x smaller.
  7. become 1000x larger.

Answer: 7. Hint: If the diameter of a sphere is made 10 times smaller, the volume of the sphere is 1000 times (10x10x10) smaller.


Black Holes

Which of the following planets would be hardest to escape from (if you start from the planet's surface)?

  1. Earth.
  2. A planet with Earth's mass but half the size.
  3. A planet with Earth's mass but twice the size.
  4. Mars (1/10th Earth's mass, and half Earth's size).
  5. A planet with Mars' mass but twice the size.
Answer: 2. Hint: The force of gravity gets stronger the closer two masses are together (like you and the center of the planet). A large planet mass and a small size make for the largest escape velocity.

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 but remained the same mass, what would happen to Earth's orbit?

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

The picture of the Milky Way is about 1 meter across (when shown on the screen in class). The Andromeda Galaxy is about the same size as the Milky Way and 2,500,000 light years away. In a scale model, where would we have to place Andromeda?

  1. Next to the screen.
  2. Near the door to the classroom.
  3. At the library.
  4. On the opposite side of campus.
  5. In downtown San Diego.
Answer: 2. Hint: The Milky Way is about 100,000 light years across, so Andromeda should be about 25 Milky Ways away.

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

Imagine you are studying two stars orbiting in two different galaxies at the same distance from the centers. One star is orbiting faster than the other. What does this tell you?

  1. There is a stronger gravitational force on the slower star.
  2. There is a stronger gravitational force on the faster star.
  3. The gravitational forces are the same.
Answer: 2. Hint: To hold the faster star in its orbit, its galaxy has to be exerting a larger gravitational force on it --- otherwise the star would be able to get farther from the center of the galaxy.

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 blue tint. Which of the following types of stars is probably releasing most of the light you see?

  1. low-mass main sequence stars
  2. high-mass main sequence stars
  3. giant stars
  4. supergiant stars
  5. white dwarfs
Answer: 2. Hint: The stars that are releasing most of the light for this galaxy need to be both luminous and hot.

Which of the following types of stars are you likely to find in a galaxy ONLY if there are large amounts of gas and dust that can be used to form stars?

  1. Main sequence stars with high mass.
  2. Main sequence stars with low mass.
  3. Giant stars.
  4. White dwarfs.
Answer: 1. Hint: Gas and dust are the raw materials for stars. Main sequence stars with high mass live for such short times that you will not find them unless stars are being born in the galaxy today.


The Universe

Which of the graphs below best represents Hubble's Law?

Answer: C. Hint: Hubble's law is the equation of a line, where expansion velocity increases with distance from us. At zero distance, the expansion velocity is zero, and only answer C matches that.

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.

The drawing below represents the same group of galaxies at two different points in time during the history of the universe.

How would an alien in galaxy A describe what it sees?

  1. "Galaxies B and E are moving away from me with equal speeds."
  2. "Galaxy B is moving away from me faster than galaxy E."
  3. "Galaxy E is moving away from me faster than galaxy B."
Answer: 3. Hint: If you measure the changes in distances between the galaxies, the biggest change occurs for the most distant galaxies.

The ovals below show imaginary maps of the entire sky (similar to an Earth map, with the poles at the bottom and top. If we could see the glow of heat released during the Big Bang, which of the maps would represent the glow best?

Answer: A. Hint: The Big Bang was something that happened everywhere at once. So no matter the direction you look in, you would be looking back at the glow of the Big Bang.

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 long does it take light to travel to us from the star?

  1. 10 million years.
  2. 50 million years.
  3. 100 million years.
Answer: 3. Hint: When a distance is given in light-years ("100 million light-years"), that tells us how long it takes light to travel that space.

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 picture below might help visualize this:

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.

When were the FIRST of each of these kinds of things put together in the history of the universe? (Put the answers from earliest to latest.)

  1. atoms
  2. helium nuclei
  3. carbon nuclei
  4. stars
Answer: 2143. Hint: helium nuclei are in the first 3 minutes of history, atoms form after about 380,000 years after the universe cools down, stars after about 100 million years once gas clouds have a chance to collapse to make them, and carbon nuclei can only be made inside stars. Imagine you are using a radar gun to measure a car's speed and distance as it moves straight toward you. The table shows your measurements. How fast will the car probably be moving when it reaches you?

distance (m) speed (km/h)
400 80
300 60
200 50
100 80
  1. Faster than 80 km/h
  2. About 80 km/h
  3. About 60 km/h
  4. About 40 km/h
  5. Slower than 40 km/h
  6. It should come to a stop when it reaches you.
Answer: 5. Hint: As each 100 m passes, the car's speed decreases, but it decreases by smaller and smaller amounts. It is approaching a value near 40 km/h.


Scale Models

If Earth is roughly 1/10th the width of Jupiter, and the Sun is about 10 times the width of Jupiter, roughly how does the Sun compare to Earth in size?

  1. The Sun is roughly the same width as Earth.
  2. The Sun is roughly 10 times wider than Earth.
  3. The Sun is roughly 100 times wider than Earth.
  4. The Sun is roughly 1000 times wider than Earth.
  5. The Sun is roughly 10,000 times wider than Earth.
Answer: 3. Hint: 10 Earths fit across Jupiter and 10 Jupiters fit across the Sun.

If the Sun is 100 times larger than Earth, roughly how big should a shrunken Earth be compared to the basketball-Sun?

  1. A basketball
  2. A soccer ball
  3. A softball
  4. A baseball
  5. A ping pong ball
  6. A pea
  7. A grain of sand
Answer: 6. Hint: You could fit many more than 100 grains of sand across a basketball, and the other objects are too large.

If the Sun is the size of a basketball, Proxima Centauri is the size of a large superball, but where should the star be (if the scale is the same)?

  1. downtown San Diego (5 miles)
  2. Los Angeles (110 miles)
  3. St. Louis (1500 miles)
  4. London (5500 miles)
  5. the Moon (240,000 miles)
Answer: 4. Hint: The star would be slightly closer than this, but London would be closest.


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).


Last update: May 6, 2013