Astronomy 101 Thought Questions - Spring 2015

## Thought Questions for Astronomy 101 (Spring 2015) 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

## 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.

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.)

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.

The spectrum produced by a planetary nebula must be

1. an emission line spectrum because ultraviolet light from the white dwarf causes material in the nebula to glow.
2. an absorption line spectrum because the white dwarf produces a continuous spectrum and atoms from the nebula absorb light of particular wavelengths.
3. a continuous spectrum because the nebula is hot, has high density, and is opaque.

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.

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

In nuclear fission, a large nucleus (like uranium) breaks up into smaller nuclei and releases energy. If you compared the mass of the large nucleus to the total mass of the smaller nuclei, what would you find?

1. The large nucleus has the same mass as the total of the smaller nuclei.
2. The large nucleus has more mass as the total of the smaller nuclei.
3. The large nucleus has less mass as the total of the smaller nuclei.

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.

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

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.

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.

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.

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.

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

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.

The HR Diagram below shows the results of surveying 100 nearby stars. What are most nearby stars like (compared to the Sun - the red X)?

1. hotter, less luminous
2. cooler, less luminous
3. hotter, more luminous
4. cooler, more luminous

The HR Diagram below shows the 100 brightest stars. What are most bright stars like (compared to the Sun - the red X)?

1. hotter, less luminous
2. cooler, less luminous
3. hotter, more luminous
4. cooler, more luminous

## 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.

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?

Which of the stars in the picture 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?

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.

## Star Luminosity

You are studying at your 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.

You are studying at your 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.

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.

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.

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.

## 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.

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.

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.

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.

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.

## 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.

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?

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.

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.

## 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

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

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

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.

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.

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.

## Doppler Shift

Imagine you listen to a car horn 1) from inside a moving car, and 2) from the side of the road as a the car passes by. What should you hear?
1. Constant pitch for both.
2. Constant pitch for #1, but pitch that changes from low to high for #2.
3. Constant pitch for #1, but pitch that changes from high to low for #2.
4. Constant pitch for #2, but pitch that changes from low to high for #1.
5. Constant pitch for #2, but pitch that changes from high to low for #1.

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?

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?

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?

## 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.

Imagine you are studying stars orbiting in different galaxies. The white circles below show their orbits, and the white arrows represent their speeds. Rank the galaxies from least massive to most massive.

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.

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

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.

The disks of spiral galaxies are flat and round. What are the stars in these places doing in order to create these shapes?

1. The stars stay in their places, spread around the center of the galaxy.
2. The stars orbit the galaxy in an orderly way, all moving in the same direction (clockwise or counterclockwise).
3. The stars orbit around the center of the galaxy, but their orbits take the stars in all different directions.

Which of the following galaxies is most likely to have stars that orbit around the center of the galaxy in the same direction in an orderly fashion?

1. An elliptical galaxy.
2. A spiral galaxy.
3. An irregular galaxy.

Which of the following mergers of spiral and elliptical galaxies is most unlikely?

## The Universe

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

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).''

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.''

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."

Which of the 4 universes below is homogeneous?

Which of the 4 universes below is isotropic according to the stick figure?

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?

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.

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

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.

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.

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.

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.

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.

Based on what happens to the universe and the light in it, what will happen to the density and temperature of the universe as time goes on?

1. The universe will become hotter and denser.
2. The universe will become hotter and less dense.
3. The universe will become cooler and less dense.
4. The universe will become cooler and denser.

Which of these abundance patterns is an unrealistic chemical composition for a star?

1. 70% H, 28% He, 2% other
2. 95% H, 5% He, less than 0.02% other
3. 75% H, 25% He, less than 0.02% other
4. 72% H, 27% He, 1% other

What was the order that most of the following kinds of things were put together (from first to last)?

1. atoms
2. helium nuclei
3. carbon nuclei
4. 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 45
1. Faster than 80 km/h
5. Slower than 40 km/h
6. It should come to a stop when it reaches you.

The diagrams below represent versions of Hubble's Law that we could have measured in the universe today. Which diagram represents the youngest universe?

The Sun holds the planets in their orbits. How do the orbit speeds of the different planets compare?

1. All of the planets orbit at the same speed.
2. The farther a planet is from the Sun, the larger its orbit speed.
3. The farther a planet is from the Sun, the lower its orbit speed.

Today we observe two supernova explosions that have the same luminosity, but one of the supernovas appears brighter to us than the other does. Which of the following is correct?

1. The fainter supernova blew up later in history.
2. The supernovas happened at the same time because their luminosities are the same.
3. The fainter supernova blew up earlier in history.

The three graphs below represent cars that are now at the same distance fromt he start and moving at the same speed. Which of the graphs represents the motion of a decelerating car?

## 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.

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

2. A soccer ball
3. A softball
4. A baseball
5. A ping pong ball
6. A marble
7. A BB
8. A grain of sand

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)

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).

K. Lee. ClassAction, astro.unl.edu/classaction, (University of Nebraska, Lincoln).

Last update: April 30, 2015