# ASTR 102 - TEST - MC01 & SA&TF

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### What is the meaning of the word cosmos? A) the origin of Earth and life upon it B) the light from a distant astronomical object C) the Milky Way D) the sum total of all matter and energy, that is, everything within and between all galaxies E) the dark sky

D) the sum total of all matter and energy, that is, everything within and between all galaxies

### Which of the following has your "address" in the correct order? A) you, Earth, solar system, Local Group, Local Supercluster, Milky Way B) you, Earth, solar system, Milky Way, Local Supercluster, Local Group C) you, Earth, solar system, Local Group, Milky Way, Local Supercluster D) you, Earth, Local Group, Local Supercluster, solar system, Milky Way E) you, Earth, solar system, Milky Way, Local Group, Local Supercluster

E) you, Earth, solar system, Milky Way, Local Group, Local Supercluster

### About where is our solar system located within the Milky Way Galaxy? A) at the center of the galaxy B) about 10 percent of the way from the center of the galaxy to the outskirts of the galactic disk C) about two-thirds of the way from the center of the galaxy to the outskirts of the galactic disk D) near the far outskirts of the galactic disk E) in the halo of the galaxy above the galactic disk

C) about two-thirds of the way from the center of the galaxy to the outskirts of the galactic disk

B) 100 billion

### Modern telescopes are capable of seeing bright galaxies up to about A) 1 million light-years away. B) 10 million light-years away. C) 1 billion light-years away. D) 10 billion light-years away. E) 1 trillion light-years away.

D) 10 billion light-years away.

### Suppose we imagine the Sun to be about the size of a grapefruit. How big an area would the orbits of the nine planets of the solar system cover? A) the size of a typical dorm room B) the size of a typical campus building C) the size of a typical campus D) the size of a small city E) the size of a western state (e.g., Colorado)

C) the size of a typical campus

### What do we mean when we say that the universe is expanding? A) Average distances are increasing between star systems within galaxies. B) Everything in the universe is gradually growing in size. C) Average distances are increasing between galaxies. D) The statement is not meant to be literal; rather, it means that our knowledge of the universe is growing. E) Individual galaxies are gradually growing in size.

C) Average distances are increasing between galaxies.

### The age of the universe is A) between 10 million and 16 million years. B) between 100 million and 160 million years. C) between 1 billion and 1.6 billion years. D) between 10 billion and 16 billion years. E) between 100 billion and 160 billion years.

D) between 10 billion and 16 billion years.

### How are galaxies important to our existence? A) Without galaxies, there could not have been a Big Bang. B) Without galaxies, the universe could not be expanding. C) Galaxies prevent planets from leaving their orbits around stars; e.g., our galaxy prevents Earth from leaving its orbit of the Sun. D) Galaxies recycle heavy elements produced in stars into future generations of stars. E) Galaxies provide the gravity that prevents us from falling off Earth.

D) Galaxies recycle heavy elements produced in stars into future generations of stars.

### Earth is made mostly of metals and rocks. Where did this material come from? A) It was produced in the Big Bang. B) It was created by chemical reactions in interstellar space. C) It was produced by nuclear fusion in stars. D) It was made by our Sun. E) It was made by nuclear fission of uranium and other radioactive materials.

C) It was produced by nuclear fusion in stars.

### What is nuclear fusion? A) an explosion caused by putting together two volatile chemicals B) the process of splitting nuclei to produce energy C) the process of turning matter into pure energy D) the process of combining lightweight nuclei to make heavier nuclei E) a process that only occurs in bombs

D) the process of combining lightweight nuclei to make heavier nuclei

### Why does Carl Sagan say that we are star stuff? A) The composition of most stars (mostly hydrogen and helium) is about the same as the composition of our bodies. B) Cosmic rays reaching Earth from distant astronomical sources may be one source of mutations that help evolution along. C) Nearly every atom from which we are made once (before the solar system formed) was inside of a star. D) Nearly every atom from which we are made was once inside our star, the Sun. E) Sagan thought that all of us have the potential to be movie (or TV) stars like he was.

C) Nearly every atom from which we are made once (before the solar system formed) was inside of a sta

### Which of the following statements does not use the term light-year in an appropriate way? A) It's about 4 light-years from here to Alpha Centauri. B) It will take me light-years to complete this homework assignment. C) A light-year is about 10 trillion kilometers. D) It will take the Voyager spacecraft about 20,000 years to travel just 1 light-year. E) The Milky Way Galaxy is about 100,000 light-years in diameter.

B) It will take me light-years to complete this homework assignment.

### One light-hour is the distance that light travels in an hour. How far is this, in kilometers? (Recall that the speed of light is 300,000 km/s.) A) 300,000 km B) 18 million km C) 100 million km D) 1.08 billion km E) 9.46 trillion km

D) 1.08 billion km

### 15) Suppose we look at a photograph of many galaxies. Assuming that all galaxies formed at about the same time, which galaxy in the picture is the youngest? A) the one that is farthest away B) the one that is reddest in color C) the one that is bluest in color D) the one that is closest to us E) the one that appears smallest in size

A) the one that is farthest away

### What do we mean by the observable universe? A) the part of the universe that we can see with the naked eye B) the part of the universe that we can see through telescopes C) the part of the universe that could be observed in principle, including things that may require future technologies D) the compendium of all objects that we have observed to date E) the entire universe, since it is inconceivable that there could be parts of the universe that we cannot observe

C) the part of the universe that could be observed in principle, including things that may require future technologies

### Suppose we imagine the Sun to be about the size of a grapefruit. Which of the following describes the size and distance of Earth on the same scale? A) Earth is the size of a point about 1 meter away from the Sun. B) Earth is the size of a golf ball about 1 meter away from the Sun. C) Earth is the size of a point about 15 meters away from the Sun. D) Earth is the size of a golf ball about 15 meters away from the Sun. E) Earth is the size of a marble about 25 miles away from the Sun.

C) Earth is the size of a point about 15 meters away from the Sun.

### What is the Sun mainly made of? A) hydrogen and oxygen B) hydrogen and helium C) carbon and nitrogen D) oxygen and carbon E) nearly equal portions of all the elements

B) hydrogen and helium

### Which of the following is smallest? A) size of a typical planet B) 1 light-second C) 1 AU D) size of a typical star

A) size of a typical planet

### Which of the following is largest? A) size of a typical galaxy B) size of Pluto's orbit C) distance to the nearest star (other than our Sun) D) 1 light-year

A) size of a typical galaxy

### On the 1-to-10-billion scale, about how far is it to the nearest stars besides the Sun? A) 4 kilometers B) 400 kilometers C) 1,000 kilometers D) 4,400 kilometers E) 10,000 kilometers

D) 4,400 kilometers

D) 2,500 miles

B) 100 meters

### Which of the following best describes the Milky Way Galaxy? A) a spiral galaxy with a disk about 100,000 light-years in diameter and containing between 100 billion and 1 trillion stars B) a spiral galaxy with a disk about 1 billion kilometers in diameter and containing between 100 million and 1 billion stars C) a spiral galaxy with a disk about 100,000 light-years in diameter and containing about 100,000 stars D) a spherically shaped collection of stars including our solar system and about a dozen other solar systems, stretching about 4 light-years in diameter E) a spherically shaped collection of about 1 million stars that is about 100 light-years in diameter

A) a spiral galaxy with a disk about 100,000 light-years in diameter and containing between 100 billion and 1 trillion stars

### How long would it take to count all the stars in the Milky Way Galaxy at a rate of one star per second? A) several days B) several weeks C) several years D) several thousand years E) hundreds of thousands of years

D) several thousand years

### How many galaxies are there in the observable universe? A) roughly (within a factor of 10) the same as the number of stars in our galaxy B) roughly a thousand times more than the number of stars in our galaxy C) about as many as the number of stars we see in the sky with our naked eyes D) about as many as the number of grains of sand on all the beaches on Earth E) an infinite number

A) roughly (within a factor of 10) the same as the number of stars in our galaxy

### If you represented each star by a grain of sand, how much sand would it take to represent all the stars in the universe? A) all the sand in a typical playground sandlot B) all the sand on Miami Beach C) all the sand on the beaches of California D) all the sand on the beaches in the United States E) more than all the sand on all the beaches on Earth

E) more than all the sand on all the beaches on Earth

D) a few seconds

### On a cosmic calendar, in which the history of the universe is compressed into 1 year, when did the dinosaurs become extinct? A) in late December B) in late November C) in late October D) in late September E) in late August

A) in late December

A) 1 second ago

B) 0.2 second

B) 1,300 km/hr

### What is an astronomical unit? A) the average speed of Earth around the Sun B) the length of time it takes Earth to revolve around the Sun C) the average distance from Earth to the Sun D) the diameter of Earth's orbit around the Sun E) any basic unit used in astronomy

C) the average distance from Earth to the Sun

### Which of the following statements about the ecliptic plane is not true? A) It is the plane of Earth's orbit around the Sun. B) It is the plane of the Moon's orbit around Earth. C) During a solar eclipse, the Moon lies in the ecliptic plane. D) During a lunar eclipse, the Moon lies in the ecliptic plane. E) The nodes of the Moon's orbit lie in the ecliptic plane.

B) It is the plane of the Moon's orbit around Earth.

### Patterns of stars in constellations hardly change in appearance over times of even a few thousand years. Why? A) Stars are fixed and never move. B) Stars move, but they move very slowly—only a few kilometers in a thousand years. C) Although most stars move through the sky, the brightest stars do not, and these are the ones that trace the patterns we see in the constellations. D) The stars in our sky actually move rapidly relative to us—thousands of kilometers per hour—but are so far away that it takes a long time for this motion to make a noticeable change in the patterns in the sky. E) Stars within a constellation move together as a group, which tends to hide their actual motion and prevent the pattern from changing.

D) The stars in our sky actually move rapidly relative to us—thousands of kilometers per hour—but are so far away that it takes a long time for this motion to make a noticeable change in the patterns in the sky.

### How long does it take our solar system to complete one orbit around the Milky Way Galaxy? A) 10 thousand years B) 230 thousand years C) 1 million years D) 100 million years E) 230 million years

E) 230 million years

### Which of the following statements about the Milky Way Galaxy is not true? A) It contains between 100 billion and 1 trillion stars. B) Our solar system is located very close to the center of the Milky Way Galaxy. C) Our view of distant objects is obscured by gas and dust when we look into the galactic plane. D) The galaxy is about 100,000 light-years in diameter. E) One rotation of the galaxy takes about 200 million years.

B) Our solar system is located very close to the center of the Milky Way Galaxy.

### Which of the following correctly lists speeds from slowest to fastest? A) Earth's speed of revolution about the Sun, typical speeds of stars in the local solar neighborhood relative to us, Earth's speed of rotation on its axis, the speed of our solar system orbiting the center of the Milky Way Galaxy, the speeds of very distant galaxies relative to us B) Earth's speed of rotation on its axis, Earth's speed of revolution about the Sun, typical speeds of stars in the local solar neighborhood relative to us, the speed of our solar system orbiting the center of the Milky Way Galaxy, the speeds of very distant galaxies relative to us C) the speeds of very distant galaxies relative to us, typical speeds of stars in the local solar neighborhood relative to us, Earth's speed of rotation on its axis, Earth's speed of revolution about the Sun, the speed of our solar system orbiting the center of the Milky Way Galaxy D) the speed of our solar system orbiting the center of the Milky Way Galaxy, Earth's speed of revolution about the Sun, Earth's speed of rotation on its axis, the speeds of very distant galaxies relative to us, typical speeds of stars in the local solar neighborhood relative to us E) Earth's speed of revolution about the Sun, Earth's speed of rotation on its axis, the speed of our solar system orbiting the center of the Milky Way Galaxy, typical speeds of stars in the local solar neighborhood relative to us, the speeds of very distant galaxies relative to us

B) Earth's speed of rotation on its axis, Earth's speed of revolution about the Sun, typical speeds of stars in the local solar neighborhood relative to us, the speed of our solar system orbiting the center of the Milky Way Galaxy, the speeds of very distant galaxies relative to us

### Most of the mass in the Milky Way Galaxy is located A) in the halo (above/below the disk). B) within the disk. C) in the stars in the spiral arms. D) in the gas and dust. E) in the central bulge of the galaxy.

A) in the halo (above/below the disk).

### The distribution of the mass of the Milky Way Galaxy is determined by A) counting the number of stars. B) determining the amount of gas and dust. C) studying how stars are distributed in the Milky Way. D) studying the rotation of the galaxy. E) weighing various parts of the Milky Way.

D) studying the rotation of the galaxy.

### From the fact that virtually every galaxy is moving away from us and more distant galaxies are moving away from us at a faster rate than closer ones, we conclude that A) the Milky Way Galaxy is expanding. B) we are located at the center of the universe. C) the farthest galaxies will eventually be moving faster than the speed of light. D) the universe is expanding. E) the universe is shrinking.

D) the universe is expanding.

### By studying distant galaxies in the 1920s, Hubble made the following important discovery that led us to conclude that the universe is expanding: A) all galaxies contain billions of stars, and all galaxies have spiral shapes. B) all galaxies were born at the same time, and all will die at the same time. C) all galaxies outside the Local Group are moving away from us, and the farther away they are, the faster they're going. D) all galaxies outside the Local Group are orbiting the Local Group. E) all galaxies outside the Local Group are moving away from us, and all are moving away at nearly the same speed.

C) all galaxies outside the Local Group are moving away from us, and the farther away they are, the faster they're going.

### Imagine that we put a raisin cake into the oven, with each raisin separated from the others by 1 cm. An hour later, we take it out and the distances between raisins are 3 cm. If you lived in one of the raisins and watched the other raisins as the cake expanded, which of the following would you conclude? A) All raisins would be moving away from you at the same speed. B) More distant raisins would be moving away from you faster. C) More distant raisins would be moving away from you more slowly. D) It depends: If you lived in a raisin near the edge of the cake, you'd see other raisins moving away from you, but they'd be coming toward you if you lived in a raisin near the center of the cake.

B) More distant raisins would be moving away from you faster.

### Which scientists played a major role in overturning the ancient idea of an Earth-centered universe, and about when? A) Copernicus, Kepler, and Galileo; about 400 years ago B) Aristotle and Copernicus; about 400 years ago C) Newton and Einstein; about 100 years ago D) Huygens and Newton; about 300 years ago E) Aristotle and Plato; about 2,000 years ago

A) Copernicus, Kepler, and Galileo; about 400 years ago

### Which of the following is not a general difference between a planet and a star? A) Planets are smaller than stars. B) Planets are dimmer than stars. C) All planets are made of rock and all stars are made of gas. D) Planets orbit stars, while stars orbit the center of the galaxy.

C) All planets are made of rock and all stars are made of gas.

### Our solar system consists of ________. A) the Sun and all the objects that orbit it. B) the Sun and the planets, and nothing else C) a few hundred billion stars, bound together by gravity D) the Sun and several nearby stars, as well as the planets and other objects that orbit these stars

A) the Sun and all the objects that orbit it.

### A typical galaxy is a ________. A) collection of a few hundred million to a trillion or more stars, bound together by gravity B) large, glowing ball of gas powered by nuclear energy C) nearby object orbiting a planet D) relatively small, icy object orbiting a star

A) collection of a few hundred million to a trillion or more stars, bound together by gravity

### Which of the following best describes what we mean by the universe? A) the sum total of all matter and energy B) a vast collection of stars that number as many as the grains of sand on all the beaches on Earth C) all the galaxies in all the superclusters D) The universe is another name for our Milky Way Galaxy.

A) the sum total of all matter and energy

### What do astronomers mean by the Big Bang? A) the event that marked the beginning of the expansion of the universe B) a gigantic explosion that blew all the galaxies in the universe to smithereens C) the explosion of a massive star at the end of its life D) the event that marked the birth of our solar system

A) the event that marked the beginning of the expansion of the universe

### What do we mean when we say that the universe is expanding? A) Everything in the universe is gradually growing in size. B) Within galaxies, average distances between star systems are increasing with time. C) The statement is not meant to be literal; rather, it means that our knowledge of the universe is growing. D) Average distances between galaxies are increasing with time.

D) Average distances between galaxies are increasing with time.

### Based on observations of the universal expansion, the age of the universe is about ________. A) 14,000 years B) 14 million years C) 14 billion years D) 14 trillion years

C) 14 billion years

### A television advertisement claiming that a product is light-years ahead of its time does not make sense because ________. A) it doesn't specify the number of light-years B) it uses "light-years" to talk about time, but a light-year is a unit of distance C) a light-year is an astronomically large unit, so a product could not possibly be so advanced D) light-years can only be used to talk about light

B) it uses "light-years" to talk about time, but a light-year is a unit of distance

### The term observable universe refers to ________. A) that portion of the universe that we have so far photographed through telescopes B) the portion of the universe that can be seen by the naked eye C) the portion of the universe that is not hidden from view by, for example, being below the horizon D) that portion of the universe that we can see in principle, given the current age of the universe

D) that portion of the universe that we can see in principle, given the current age of the universe

### On a scale in which the distance from Earth to the Sun is about 15 meters, the distance from Earth to the Moon is ________. A) small enough to fit within your hand B) about 1 meter C) about 5 meters D) about 30 meters

A) small enough to fit within your hand

### On a scale where the Sun is about the size of a grapefruit and the Earth is about 15 meters away, how far away are the nearest stars besides the Sun? A) 100 meters B) about the distance across 50 football fields C) about the distance across the state of Delaware D) about the distance across the United States

D) about the distance across the United States

### The number of stars in the Milky Way Galaxy is approximately ________. A) a few hundred B) a few hundred thousand C) a few hundred billion D) a few hundred million

C) a few hundred billion

### An astronomical unit (AU) is ________. A) any very large unit, such as a light-year B) the average distance between Earth and the Sun C) the current distance between Earth and the Sun D) the average distance between any planet and the Sun

B) the average distance between Earth and the Sun

### What is the ecliptic plane? A) the plane of Earth's orbit around the Sun B) the plane of Earth's equator C) the plane of the Sun's equator D) the plane of the Milky Way Galaxy

A) the plane of Earth's orbit around the Sun

A) one year

### 1) Which of the following has your "cosmic address" in the correct order? A) You, Earth, solar system, Local Group, Local Supercluster, Milky Way Galaxy, universe. B) You, Earth, solar system, Milky Way Galaxy, Local Group, Local Supercluster, universe. C) You, Earth, Local Group, Local Supercluster, solar system, Milky Way Galaxy, universe D) You, Earth, solar system, Local Group, Milky Way Galaxy, Local Supercluster, universe. E) You, Earth, Milky Way Galaxy, solar system, Local Group, Local Supercluster, universe

B) You, Earth, solar system, Milky Way Galaxy, Local Group, Local Supercluster, universe.

### Using the ideas discussed in your textbook, in what sense are we "star stuff"? A) The overall chemical composition of our bodies is about the same as that of stars. B) Movie stars and other people are all made of the same stuff, so we all have the potential to be famous. C) Nearly every atom from which we are made was once inside of a star. D) We could not survive without light from our star, the Sun.

C) Nearly every atom from which we are made was once inside of a star.

### How are galaxies important to our existence? A) Without galaxies, there could not have been a Big Bang. B) Without galaxies, the universe could not be expanding. C) Deep in their centers, galaxies created the elements from which we are made. D) Galaxies recycle material from one generation of stars to the next, and without this recycling we could not exist.

D) Galaxies recycle material from one generation of stars to the next, and without this recycling we could not exist.

### When we look at an object that is 1,000 light-years away we see it ________. A) as it was 1,000 years ago B) as it was 1,000 light-years ago C) as it is right now, but it appears 1,000 times dimmer D) looking just the same as our ancestors would have seen it 1,000 years ago

A) as it was 1,000 years ago

### uppose we look at two distant galaxies: Galaxy 1 is twice as far away as Galaxy 2. In that case, ________. A) Galaxy 1 must be twice as big as Galaxy 2 B) we are seeing Galaxy 1 as it looked at an earlier time in the history of the universe than Galaxy 2 C) we are seeing Galaxy 1 as it looked at a later time in the history of the universe than Galaxy 2 D) Galaxy 2 must be twice as old as Galaxy 1

B) we are seeing Galaxy 1 as it looked at an earlier time in the history of the universe than Galaxy 2

### Suppose we make a scale model of our solar system, with the Sun the size of a grapefruit. Which of the following best describes what the planets would look like? A) The planets are all much smaller than the Sun. Four planets are within about 20 meters of the Sun, while the rest planets are spread much farther apart. B) The planets are all much smaller than the Sun and are spread out evenly over a distance about the length of a large classroom. C) The planets are all much smaller than the Sun. Four planets are located within a few centimeters of the Sun, and four planets are located at distances ranging up to about a meter. D) The planets range in size from about the size of a marble to the size of a baseball. They are spread out over a region about the size of a football field.

A) The planets are all much smaller than the Sun. Four planets are within about 20 meters of the Sun, while the rest planets are spread much farther apart.

### If you could count stars at a rate of about one per second, how long would it take to count all the stars in the Milky Way Galaxy? A) several days B) several weeks C) several years D) several thousand years

D) several thousand years

### The total number of stars in the observable universe is about ________. A) 100 billion B) the same as the number of grains of sand in a school sandbox C) the same as the number of grains of sand on all the beaches on Earth D) the same as the number of atoms that make up the Earth

C) the same as the number of grains of sand on all the beaches on Earth

### Where is our solar system located within the Milky Way Galaxy? A) very near the center of the galaxy B) at the far edge of the galaxy's visible disk C) roughly halfway between the center and the edge of the visible disk of the galaxy D) in the halo of the galaxy

C) roughly halfway between the center and the edge of the visible disk of the galaxy

### If we imagine the history of the universe compressed into one year, dinosaurs became extinct ________. A) about 6 months ago B) about 3 weeks ago C) yesterday morning D) about an hour ago

C) yesterday morning

### Relative to the age of the universe, how old is our solar system? A) It is about 1% as old as the universe. B) It is between about 5% and 10% as old as the universe. C) It is about one-third the age of the universe. D) It is nearly the same age as the universe.

C) It is about one-third the age of the universe.

### How do the speeds at which we are moving with Earth's rotation and orbit compare to the speeds of more familiar objects? A) Earth's rotation carries most people around the axis faster than a commercial jet travels, and Earth's orbit carries us around the Sun faster than the Space Shuttle orbits Earth. B) Earth's rotation carries most people around the axis at about the speed of a commercial jet, and Earth's orbit carries us around the Sun at about the speed of a military jet. C) Earth's rotation carries most people around the axis at about the speed of a car on the freeway, and Earth's orbit carries us around the Sun at about the speed of a commercial jet. D) Earth's rotation carries most people around the axis at about the speed at which the Space Shuttle orbits Earth, and Earth's orbit carries us around the Sun at nearly the speed of light.

A) Earth's rotation carries most people around the axis faster than a commercial jet travels, and Earth's orbit carries us around the Sun faster than the Space Shuttle orbits Earth.

### Why do the patterns of the stars in our sky look the same from year to year? A) because the stars in the constellations are so far away B) because the stars in the constellations are not moving C) because the stars in the constellations all move at the same speeds and in the same directions, so they don't change their relative positions D) because the stars in the constellations move so slowlytypically about the speed of a snailthat their motions are not noticeable.

A) because the stars in the constellations are so far away

### Astronomers infer that the universe is expanding because distant galaxies all appear to ________. A) growing in size B) be moving away from us, with more distant ones moving faster C) be made mostly of dark matter D) rotate rapidly

B) be moving away from us, with more distant ones moving faster

### Which statement about motion in the universe is not true? A) The mysterious dark matter is the fastest-moving material in the universe. B) Some stars in the Milky Way Galaxy are moving toward us and others are moving away from us. C) Except for a few nearby galaxies, all other galaxies are moving away from us. D) Your speed of rotation around Earth's axis is faster if you live near the equator than if you live near the North Pole.

A) The mysterious dark matter is the fastest-moving material in the universe.

### When did humans learn that the Earth is not the center of the universe? A) within the past 500 years B) about 2,500 years ago C) about 1,000 years ago D) We haven't; there is still considerable scientific debate about whether Earth is the center of the universe.

A) within the past 500 years

False

False

False

True

True

False

True

True

False

False

### Briefly explain what we mean by the statement "The farther away we look in distance, the further back we look in time."

It means that when we look at a distant object, we see it as it was some time in the past, rather than as it is now. This is because the light we see has taken time to travel from the object to us.

### Starting from the Big Bang, briefly explain how our solar system came to contain the chemical elements necessary to make Earth and living organisms.

The Big Bang produced hydrogen and helium. Over time, stars have converted about 2 percent of this material into heavier elements, including all the elements of which we and Earth are made. Stars expel this material through winds and explosions, and the galaxy recycles it into new generations of stars. When a new star system forms, it therefore contains the ingredients needed to make planets and living organisms.

### Briefly explain why an expanding universe implies a beginning (called a Big Bang).

The fact that the universe is expanding means the average distance between galaxies is growing, which implies that this average distance was smaller in the past. Extrapolating back in time, there must have been a time when the distance between galaxies (or their precursors) was zero, which must be the beginning of the universe.

### Consider the following statement, and explain whether or not it is sensible: NASA hopes to build a new telescope that will allow us to see 100 million light-years into the past.

Not sensible: It uses light-years as a length of time.

### Consider the following statement, and explain whether or not it is sensible: NASA hopes to build a new telescope that will allow us to see some galaxies as they appeared 8 billion years ago.

Sensible: By looking to a distance of 8 billion light-years, we can see objects as they looked 8 billion years ago.

### Consider the following statement, and explain whether or not it is sensible: The universe is between 10 and 16 billion light-years old.

This statement does not make sense because it uses the term light-year as a length of time, rather than as a distance.

### Consider the following statement, and explain whether or not it is sensible: It will take me light-years to complete this homework assignment.

This statement does not make sense because it uses the term light-year as a length of time, rather than as a distance.

### Consider the following statement, and explain whether or not it is sensible: Someday we may build spaceships capable of traveling at a speed of 1 light-second per hour.

This statement is fine. A light-second is 300,000 kilometers, so it simply says that we'll someday build spaceships that can travel at a speed of 300,000 km/hr.

### Briefly explain how the Sun generates energy.

The Sun generates energy through nuclear fusion in its core, converting hydrogen into helium. This process releases energy because a small amount of the mass of the hydrogen is converted to energy.

### Explain why it is so difficult to see planets around other stars.

Planets are very faint compared to the stars they orbit. Moreover, they are very close to their parent star compared to the distance between stars. On the 1-to-10 billion scale, where the Sun is the size of a grapefruit and Earth is a pinhead about 15 meters way, the nearest star is several thousand kilometers away. Together, this makes it extremely difficult to distinguish the faint light of a planet from the star it orbits. (Nevertheless, massive Jupiter-like planets have been indirectly detected orbiting around nearby stars.)

### Based on the idea of "spaceship Earth," write one or two paragraphs explaining why it is not the case that we are "just sitting here."

Far from just sitting still, we on Earth are moving relative to the Sun, planets, stars, and even other galaxies. The rotation of Earth causes the most noticeable changes in the sky. This motion around Earth's axis causes the Sun and stars to appear to rise and set, producing what we call a "day." The revolution of Earth about the Sun produces the monthly changes of the constellations, the seasonal weather changes due to Earth's tilt, and the parallax of some stars. The precession of Earth's axis, a very slow movement that has a period of 26,000 years, causes the movement of the North Star, and the changing position of the equinoxes and solstices.

The motion of the Sun relative to the stars in the local solar neighborhood is at an extremely fast speed, although barely noticeable. Over time, this movement causes the patterns of the stars in the sky to change. The rotation of the galaxy means that the entire solar system is also orbiting the center of the Milky Way. This also produces motions of stars and clouds of gas. The expansion of the universe, the fact that the space between most galaxies is increasing with time, means that almost all galaxies outside the Local Group are moving away from us, with the more distant ones moving away faster. All of these motions, although not felt by us on Earth, are observed by watching the sky and prove that we are not "just sitting here."

### Consider the following statement, and explain whether or not it is sensible: Earth is always precisely 1 AU from the Sun.

Not sensible: One AU is the average distance between Earth and the Sun.

Example: