Homework 1 PDF

Homework 1 Thursday, October 3, 2024 10:26 PM   What do astronomers mean by the Big Bang? - the event that marked the beginning of the expansion of the universe - Based on careful study of the current expansion of the universe (and of the cosmic microwave background, which we\'ll discu...

Homework 1 Thursday, October 3, 2024 10:26 PM   What do astronomers mean by the Big Bang? - the event that marked the beginning of the expansion of the universe - Based on careful study of the current expansion of the universe (and of the cosmic microwave background, which we\'ll discuss later in the text), the Big Bang occurred about 14 billion years ago.   A typical galaxy is a \_\_\_\_\_\_\_\_\_\_. - collection of millions or billions of stars, bound together by gravity and orbiting a common center - Our own Milky Way Galaxy is relatively large, containing more than 100 billion stars.   These photos show two different astronomical objects. Which object is bigger, and by about how much?\\ - Object 2 is more than a trillion times as large as Object 1. - Object 2: milky way - Object 1 planet - Object 1 is the planet Saturn and Object 2 is an entire galaxy, and the diameter of a galaxy is indeed more than a trillion times the diameter of a planet like Saturn.   In any particular place on Earth, certain constellations are visible in the evening only at certain times of the year because \_\_\_\_\_\_\_\_\_. - our evening view of space depends on where Earth is located in its orbit around the Sun - As we orbit the Sun, the direction we are looking in space at any particular time of day is constantly changing.   Part complete Suppose that Galaxy 1 is twice as far away from us as Galaxy 2. Which of the following must be true? - We are seeing Galaxy 1 as it looked at an earlier time in the history of the universe than Galaxy 2. - Remember that the farther away we look in distance, the further back we look in time. Because Galaxy 1 is farther away, we are seeing it at a time further into the past, which means earlier in the history of the universe.   I live in the United States, and during my first trip to Argentina I saw many constellations that I\'d never seen before. - This statement makes sense, because the constellations visible in the sky depend on latitude. Since Argentina is in the Southern Hemisphere, the constellations visible there include many that are not visible from the United States.   The Andromeda Galaxy is faintly visible to the naked eye. When you look at the Andromeda Galaxy, the retina of your eye is absorbing light that has traveled through space for \_\_\_\_\_\_ to reach you. - about 2 ½ million years - The distance to the Andromeda galaxy is about 2 ½ million light-years, which means light takes about 2 ½ million years to travel from the Andromeda galaxy to us. Therefore, the light that you see from Andromeda travelled through space for 2 ½ million years before being absorbed by your retina.   Suppose that someone in the Andromeda galaxy had a super-telescope through which they were looking at Earth right now. They would see Earth \_\_\_\_\_\_. - as it was about 2 ½ million years ago - The approximately 2 ½-million-light-year distance between Earth and the Andromeda galaxy means that light takes about 2 ½ million years to travel this distance. Therefore, just as we see the Andromeda galaxy as it was about 2 ½ million years ago, someone looking right now from Andromeda would see us as we were about 2 ½ million years ago.   Choose the correct explanation why does the statement make sense (or is clearly true) or does not make sense (or is clearly false):   NASA will soon launch a spaceship that will photograph our Milky Way Galaxy from beyond its halo. - The statement doesn\'t make sense. Even if NASA could build a spaceship that traveled close to the speed of light, it would take tens of thousands of years to get into the halo and then the same time for image to get back to Earth.   Using the ideas discussed in your text, in what sense are we \"star stuff?\" - Nearly every atom from which we are made was once inside of a star. - According to present understanding, the early universe contained only the elements hydrogen and helium. All other elements, including carbon, oxygen, and other major ingredients of life, were produced by stars.   Which of the following statements about the celestial sphere is not true? - The celestial sphere is another name for our universe. - The celestial sphere is not another name for our universe. Rather, it is an imaginary device for locating objects by their direction in the sky as seen from Earth   Which of the following celestial objects do not rise in the east and set in the west? (Assume that all of these objects are visible from your location on Earth, and that none of them are circumpolar.) - All of these objects rise in the east and set in the west.   Which of the following best explains why a solar day is longer than a sidereal day? - Because Earth orbits the Sun at the same time it rotates, Earth must make slightly more than one full rotation between noon one day and noon the next.   Suppose it is January and the Sun remains above your horizon all day long. Where must you be located? - somewhere between the Antarctic Circle and the South Pole - January is summer for the Southern Hemisphere, so for the Sun to remain above your horizon all day long you must be within the Antarctic Circle.   The star Betelgeuse is about 600 light-years away. If it explodes tonight, - we won\'t know about it until about 600 years from now.   On this map of Earth, which numbered circle represents the place where the Sun appears directly overhead at noon on the December solstice? - Circle 3 is the Tropic of Capricorn, which has latitude 23.5ºS and hence is the latitude at which the Sun passes directly overhead at noon on the December solstice. - \   Which of the following is a general difference between a planet and a star? - Stars shine with energy released by nuclear fusion while planets shine primarily with light reflected from stars. - Nearly all the visible light that we see from planets is light that they are reflecting from their stars. Note, however, that planets generally emit their own infrared light.   This diagram represents a simplified model of the celestial sphere. The unlabeled circle that is highlighted in purple represents: - the celestial equator - And the blue circle is the ecliptic, which is tilted by 23.5 degrees to the celestial equator because that is the tilt of Earth\'s axis.   What is the basic reason that we have seasons on Earth? - Earth's axis is tilted relative to the ecliptic plane. - As shown in the video, the tilt of Earth's axis means that sunlight hits the two hemispheres differently as we orbit the Sun, leading to opposite seasons in the Northern and Southern Hemispheres. What happens to Earth's axis as we orbit around the Sun over the course of each year? - It remains pointed in the same direction at all times. - As shown in the video, the axis remains pointed in the direction of Polaris at all times. That is why the orientation relative to the Sun changes over the course of the year. Which of the following statements are true about summer compared to winter? - You have more daylight and less darkness in summer. - Temperatures tend to be warmer in summer. - The Sun follows a longer and higher path through your sky in summer. - The key features of summer are a longer and higher path for the Sun through the sky, which means more concentrated sunlight --- generally leading to higher temperatures --- and shorter shadows. What is special about the June solstice? - It's the day on which the Northern Hemisphere receives its most direct sunlight. - The June solstice is when the Northern Hemisphere is most directly oriented toward the Sun, which is why this hemisphere gets its most direct sunlight on this day.   Suppose you live at a mid-latitude in the Southern Hemisphere (such as in Australia, New Zealand, Chile, or South Africa). When will you have 12 hours of daylight and 12 hours of darkness? - on both the March and September equinoxes - Recall that equinox means "equal night," so the two equinoxes are the dates on which all locations (except the poles) have equal amounts of daylight and darkness. Note: More technically, these are the dates on which the Sun is above the horizon for 12 hours and below it for 12 hours; of course, it does not instantly become dark when the Sun is below the horizon, so there is still some "daylight" in the evening and predawn hours.   Why is the writing on maps and globes usually oriented so that the Northern Hemisphere is at the top, even though there is no up or down in space? How does this relate to the fact that the June solstice has traditionally been called the summer solstice? - North is at the top on the most maps traditionally, maps are mostly made by people in the Northern hemisphere. The June solstice is traditionally called the summer solstice for the same reason.   Why do scientists suspect that most of our galaxy\'s mass consists of dark matter? - Dark matter is invisible to our telescope, because it Does not emit light But we can detect it because of its gravity that affects observable objects. Stars orbit the galactic center significantly faster than they should if we could see most of our galaxy\'s mass. Large amount of unseen mass is present in our galaxy, mostly residing in the halo of the galaxy. What makes dark matter and dark energy so mysterious and so important? - We don\'t exactly know what they are, and they make up the majority of the total energy content of the universe.   What do astronomers mean by a constellation? - A constellation is a region in the sky as seen from Earth. - Every location in the sky belongs to some constellation; sky maps or models of the celestial sphere often show constellation borders. Relative to the age of the universe, how old is our solar system? - It is about one third the age of the universe. - The universe is about 14 billion years old and the solar system about 4 1/2 billion years ago, and 4 1/2 billion years is about one-third of 14 billion years.   The south celestial pole is 35 ∘ above your southern horizon. What does it tell you? - You are at latitude 35 ∘S   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? - Several thousand years - There are at least 100 billion stars in the Milky Way Galaxy, so it would take at least 100 billion seconds to count them, and 100 billion seconds is more than 3,000 years.   Which of the following lies in the ecliptic plane? - Earth's orbital path around the Sun - The ecliptic plane is defined by Earth's orbital path around the Sun.   In January, Earth's rotation axis points in the direction of the star Polaris. Where does it point in July? - Toward the star Polaris - Earth's axis does not noticeably change its orientation in space over the course of each year, so it remains pointed toward Polaris at all times. Note: The axis is not quite perfectly pointed at Polaris, but points within 1° of it. Moreover, as you'll learn in Chapter 2, the orientation of Earth's axis in space gradually changes due to a motion known as precession. However, the rate of precession is so slow that it would take several centuries before the change was noticeable to the naked eye.   Rank the following objects from largest to smallest. - Largest: galaxy, solar system, sun, earth Smallest. - Galaxies are gigantic collections of millions, billions, or even trillions of stars, and stars are far larger than the planets that orbit them. It\'s important to understand the relative sizes of common astronomical objects so you\'ll have the proper context for everything we\'ll learn from now on.   Regarding the history of the universe, which of the following is true? - Key elements of which Earth and life are made, including carbon, oxygen, and iron, did not exist when the universe was born and were created later in stars. - Evidence indicates that the Big Bang produced essentially only the elements hydrogen and helium, and all other elements were made subsequently by stars.   Which of the following are real motions of our \"spaceship Earth\"? - Earth and our solar system moves with the Milky Way galaxy relative to other galaxies in our Local Group. - Earth moves with the Sun on an orbit around the center of the Milky Way galaxy. - Earth orbits the Sun. - It may feel like the surface of the Earth is stationary, but we are all moving relative to other objects in the universe in numerous ways on our \"spaceship Earth.\"   Nearly all of the objects that you can see in this photograph are: - galaxies - This photo is a portion of an image from the James Webb Space Telescope, which shows galaxies in one tiny region of the sky. Almost all the object in this photo are galaxies (with the exception of the objects with spikes, which are stars). - \   Suppose Earth\'s axis tilt was significantly greater than its current 23.5 degrees, but Earth\'s rotation period and orbital period were unchanged. Which statement below would not be true? - The length of each season (for example, the number of days from the summer solstice to the fall equinox) would be significantly longer than it is now. - The length of the seasons would be unaffected, because this is determined by Earth\'s orbital period.   Use a scale on which 1 centimeter represents 180,000 light-years. What would be separation of galaxies on this scale? - 15 cm     How does the separation between galaxies compare to the separation between stars? - Relative separation between stars is much greater than relative separations between galaxies.   What is the likelihood of galactic collisions in comparison to the likelihood of stellar collisions? - Galaxies can collide relatively often, and it is almost impossible for two stars to collide.   The point directly over your head is called \_\_\_\_\_\_\_\_\_\_. - the zenith - Note that the zenith is therefore defined only for the local sky.   An astronomical unit (AU) is \_\_\_\_\_\_\_\_\_\_. - the average distance between Earth and the Sun - This distance is about 150 million kilometers. (More precisely, it is 149.6 million kilometers.)   What do we mean when we say that the universe is expanding? - Average distances between galaxies are increasing with time. - Note that while distances between galaxies are increasing (on average), galaxies themselves are not expanding because they are held together by gravity. (Many clusters of galaxies are also held together by gravity and hence not expanding, which is why we say that average distances between galaxies are increasing with time.)   In the demonstration, 360 ∘ of rotation (one full rotation) represents a sidereal day. You can actually measure the length of the sidereal day by measuring the time from when \_\_\_\_\_\_\_\_\_\_ crosses your meridian on one day (or night) until it crosses the meridian on the next day (or night). - the star Sirius - the star Vega - In fact, any star can be used to measure the length of the sidereal day, because the daily motion of stars across the sky is due only to Earth\'s daily rotation. As shown in the figure, a solar day requires about an extra 1° of rotation, or a total of about 361° of rotation for Earth. Therefore, a solar day is longer than a sidereal day by about \_\_\_\_\_\_\_\_\_\_. - 1/360 of 24 hours - This works out to be about 4 minutes, so the sidereal day is about 23 hours, 56 minutes.   Which of the following changes would cause the solar day to be shorter (rather than longer) than the sidereal day? - Reversing the direction of Earth\'s rotation - In our solar system, Venus is the only planet that rotates \"backward\" relative to its orbit, so it is the only planet with a solar day that is shorter than its sidereal day.     Suppose that Earth orbited the Sun 10 times as fast as it actually does but kept the same rotation period it has now. Which of the following would be true? - The sidereal day would still be 23 hours, 56 minutes, but the solar day would be a little over 24 1/2 hours.   The Moon takes about 1 month to orbit Earth (more precisely, about 27 1/3 days), and it travels in the same direction that Earth orbits the Sun. Therefore, during the time it takes the Moon to complete one orbit around Earth, Earth moves \_\_\_\_\_\_\_\_\_\_ around its orbit of the Sun. - about 1/12 of the way - Earth takes a year to orbit the Sun, and there are about 12 months in a year, so during the 1 month that it takes the Moon to orbit Earth, Earth must move about 1/12 of the way around its orbit of the Sun.   Based on your answer to Part E, the time from one new moon to the next must be \_\_\_\_\_\_\_\_\_. - longer than the Moon\'s actual orbital period by about 1/12 of the 27-day orbital period - Just as the solar day is longer than the sidereal day because Earth rotates and orbits in the same direction, the fact that the Moon orbits in the same direction as Earth means that its synodic period (from new moon to new moon) must be longer than its actual orbital period. The difference is 1/12 of the 27-day orbital period, which is just over 2 days. That is why the Moon\'s cycle of phases lasts about 29 1/2 days on average, or about 2 days longer than the Moon\'s orbital period of 27 1/3 days.     Why do the patterns of the stars in our sky look the same from year to year? - Because the stars in the constellations are so far away. - Although nearby stars move relative to us at typical speeds of tens to hundreds of thousands of kilometers per hour, they are so far away that it would take thousands of years for their motion to be noticeable to the eye.   Which statement about motion in the universe is not true? - The mysterious dark matter is the fastest-moving material in the universe. - This is the correct answer because the statement is not true. In fact, dark matter is thought to reside galaxies and clusters of galaxies, and hence moves with these galaxies and clusters; it does not move faster than them.   A television advertisement claiming that a product is light-years ahead of its time does not make sense because \_\_\_\_\_\_\_\_\_\_. - it uses \"light-years\" to talk about time, but a light-year is a unit of distance - You can see how the advertisement fails by remembering that a light-year is about 10 trillion kilometers or 6 trillion miles. The ad therefore implies that the product is \"6 trillion miles ahead of its time,\" which clearly does not make sense.   What do we mean when we talk about the Milky Way in our sky? - The patchy band of light that outlines the plane of the Milky Way Galaxy as seen from Earth. - The Milky Way therefore goes all the way around the celestial sphere, although we can see only a portion of it at any given time from any given place on Earth.   Could we see a galaxy that is 20 billion light-years away? (Assume that we mean a \"lookback time\" of 20 billion years.) - No, because it would be beyond the bounds of our observable universe. - The universe is about 14 billion-years old, so we cannot observe objects more than about 14 billion light-years away.   Usually we assume that the number of stars in the Milky Way Galaxy is 100 billions. How your value from the previous part compares to that value? - We underestimated the number of stars in part A.   What is the most possible reason why estimation differs from the actual number? - Stars are distributed non-uniformly in the galaxy.   Listed below are motions of the Sun that you might observe in your local sky on particular dates. Place each in the correct bin corresponding to whether you would see this motion everywhere on Earth, nowhere on Earth, only in the Northern Hemisphere, or only in the Southern Hemisphere. Ignore the North and South poles. - \   The Sun reaches the zenith (directly overhead) at midday \_\_\_\_\_\_\_\_\_\_\_. - on the equator only on the March and September equinoxes - everywhere within the tropics (between latitudes 23.5°N and 23.5°S) on two dates each year - The Sun passes directly overhead only for latitudes within the tropics, and even then only on two dates each year. The dates vary with latitude: For the equator the two dates are the equinoxes. In the tropics north of the equator they are two dates between the March and September equinoxes, and for the tropic of Cancer (latitude 23.5°N) only the date of the June solstice. In the tropics south of the equator they are two dates between the September and March equinoxes, and for the tropic of Capricorn (latitude 23.5°S) only the date of the December solstice.   Where does the Sun remain above the horizon all day (never setting) on the December solstice? - Everywhere between the Antarctic Circle (latitude 66.5°S) and the South Pole - For the Antarctic Circle (latitude 66.5°S), the Sun remains above the horizon all day long only right around the time of the December solstice. (It would be only on that date, except bending of light by the atmosphere means it occurs for a longer period around that date.) As you go farther south, it remains above the horizon longer, and remains up for a full 6 months at the South Pole.   What is the ecliptic? - the path the Sun appears to trace around the celestial sphere each year - Note that this path is also the projection of the ecliptic plane (the plane of the Earth\'s orbit) onto the celestial sphere.   When traveling north from the United States into Canada, you\'ll see the North Star (Polaris) getting \_\_\_\_\_\_\_\_\_. - higher in the sky - Remember that the altitude of the north celestial pole in the local sky is equal to your latitude, so traveling north (to higher latitude) means the North Star will move higher in your sky.