survey – Flashcard
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What is the diameter of the disk of the Milky Way?
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light years
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What is the thickness of the disk of the Milky Way?
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light years
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What kinds of objects lie in the halo of our galaxy?
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globular clusters
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What makes up the interstellar medium?
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gas and dust
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If you were to take a voyage through the entire disk of the Milky Way, what kind of material would you spend most of your time in?
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rarefied clouds of atomic hydrogen
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How does the interstellar medium obscure our view of most of the galaxy?
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The small mixture of dust grains in the interstellar medium absorbs visible light.
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How can we see through the interstellar medium?
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by observing at wavelengths (high-energy X-rays and long wavelength radio waves that are not absorbed by interstellar dust
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Harlow Shapley concluded that the Sun was not located at the center of the Milky Way Galaxy by
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mapping the distribution of globular clusters in the galaxy.
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Approximately how far is the Sun from the center of the galaxy?
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, light-years
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What elements do astronomers consider heavy elements?
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all elements besides hydrogen and helium
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Where are most heavy elements made?
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In stars and supernovae
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Why are we unlikely to find Earth-like planets around halo stars in the Galaxy?
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Halo stars formed from gas containing few heavy elements, the elements needed to create terrestrial planets like the Earth.
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How are interstellar bubbles made?
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by the winds of massive stars and supernovae explosions
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Sound waves in the interstellar medium
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can travel through the gas, but the very low density of the interstellar medium makes them inaudible.
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What is a shock wave?
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A pressure wave that moves faster than the speed of sound
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What is the most common form of gas in the interstellar medium?
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atomic hydrogen
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What produces the -cm radio line that we can be used to map the Milky Way Galaxy?
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atomic hydrogen
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Where do most dust grains form?
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in the winds of red giant stars
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Suppose you read somewhere that percent of the matter in the Milky Way is in the form of dust grains. Should you be surprised? If so, why?
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The fraction of percent cannot be correct. Dust grains are solid and only percent of the matter in the galaxy is made of anything besides hydrogen and helium.
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What is the most abundant molecule in interstellar clouds besides molecular hydrogen?
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carbon monoxide
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The image of our galaxy in radio emission from carbon monoxide maps the distribution of molecular clouds. Which of the following would give a similar picture of our galaxy?
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infrared emission from interstellar dust grains.
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Compared to our Sun, most stars in the halo are
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old, red, and dim and have fewer heavy elements.
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Compared with stars in the disk, orbits of stars in the halo
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are elliptical, with random orientations.
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Approximately how long does it take the Sun to orbit the center of the Milky Way Galaxy?
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million years
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Why do we believe that most of the mass of the Milky Way is in the form of dark matter?
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The orbital speeds of stars far from the galactic center are surprisingly high, suggesting that these stars are feeling gravitational effects from unseen matter in the halo.
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Over time, what is the net effect of the star-gas-star cycle in the Milky Way?
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The total mass in the galaxy's interstellar medium is gradually reduced, and the remaining gas is continually enriched in heavy elements.
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Where does most star formation occur in the Milky Way today?
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in the spiral arms
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The galactic center lies in the direction of which constellation?
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Sagittarius
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How do we learn about the conditions at the center of our own galaxy, the Milky Way?
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While we cannot see the galactic center with visible or ultraviolet light, radio and X-rays from the center can be detected and used to determine the conditions there.
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Which of the following does not accurately describe what we observe toward the Galactic center?
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at optical wavelengths, we see a cluster of old, red stars
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What evidence supports the existence of a black hole at the center of our galaxy?
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The motions of the gas and stars at the center indicate that it contains million solar masses within a region only light-years across.
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How does the diameter of the disk of Milky Way Galaxy compare to its thickness?
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The diameter is about times as great as the thickness.
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What do we call the bright, sphere-shaped region of stars that occupies the central few thousand light-years of the Milky Way Galaxy?
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the galaxy's bulge
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The Sun's location in the Milky Way Galaxy is ________.
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in the galactic disk, roughly halfway between the center and the outer edge of the disk
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What do we mean by the interstellar medium?
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the gas and dust that lies in between the stars in the Milky Way Galaxy
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What are the Magellanic Clouds?
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two small galaxies that probably orbit the Milky Way Galaxy
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How do disk stars orbit the center of the galaxy?
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They all orbit in roughly the same plane and in the same direction.
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How do we know the total mass of the Milky Way Galaxy that is contained within the Sun's orbital path?
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by applying Newton's version of Kepler's third law to the orbits of the Sun or other nearby stars around the center of the Galaxy
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What do we mean by the star-gas-star cycle?
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It is the continuous recycling of gas in the galactic disk between stars and the interstellar medium.
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What are cosmic rays?
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subatomic particles that travel close the speed of light
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The primary way that we observe the atomic hydrogen that makes up most of the interstellar gas in the Milky Way is with ________.
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radio telescopes observing at a wavelength of centimeters
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Which of the following molecules is the most abundant in molecular clouds?
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H
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Interstellar dust consists mostly of ________.
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microscopic particles of carbon and silicon
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Which of the following analogies best describes how the structure of the galaxy's spiral arms is maintained?
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Like cars slowing in traffic to look at an accident, stars slow as they pass through the spiral arms.
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What do we mean by a protogalactic cloud?
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a cloud of hydrogen and helium that contracts to become a galaxy
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Most stars in the Milky Way's halo are ________.
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very old
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What is an ionization nebula?
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a colorful cloud of gas that glows because it is heated by light from nearby hot stars
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What do halo stars do differently from disk stars?
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They orbit the galactic center with many different inclinations, while disk stars all orbit in nearly the same plane.
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Where does most star formation occur in the Milky Way Galaxy?
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in the spiral arms
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Based on observations, which of the following statements about stars in the Milky Way is generally true?
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The older the star, the lower its abundance of heavy elements.
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What kind of object do we think lies in the center of the Milky Way Galaxy?
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a to million solar mass black hole
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If we could see our own galaxy from million light-years away, it would appear ________.
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as a flattened disk with a central bulge and spiral arms
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How does the interstellar medium affect our view of most of the galaxy?
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It prevents us from seeing most of the galactic disk with visible and ultraviolet light.
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About what percentage of the mass of a molecular cloud is in the form of dust?
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%
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How would you expect a star that formed recently in the disk of the galaxy to differ from one that formed early in the history of the disk?
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It should have a higher fraction of elements heavier than hydrogen and helium.
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Suppose a scientist holds a press conference at which he claims that % of the matter in the Milky Way is in the form of dust grains. Does his claim seem reasonable? Why or why not?
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The % figure is too high because there are not enough heavy elements to make that much dust.
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The most common form of gas in the disk of the Milky Way Galaxy is ________.
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atomic hydrogen gas
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How should we expect the Milky Way's interstellar medium to be different in billion years than it is today?
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The total amount of gas will be much less than it is today.
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Over time, the star-gas-star cycle leads the gas in the Milky Way to ________.
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have a greater abundance of heavy elements
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Suppose you want to observe and study the radiation from gas inside an interstellar bubble created by a supernova. Which of the following observatories will be most useful?
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the Chandra X-Ray Observatory
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If you could watch a time-lapse movie of the interstellar medium over hundreds of millions of years, what would you see?
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Gas that is often moving at high speed, particularly after one or more supernovae, and constantly changing form between molecular clouds, atomic hydrogen, and hot, ionized bubbles and superbubbles.
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All the following types of objects are found almost exclusively in the disk (rather than the halo of the Milky Way except ________.
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globular clusters
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Red and orange stars are found evenly spread throughout the galactic disk, but blue stars are typically found ________.
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only in or near star-forming clouds
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Which of the following statements comparing halo stars to our Sun is not true?
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Most stars in the halo have either died or are in their final stages of life, while the Sun is only in about the middle of its lifetime.
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Most nearby stars move relative to the Sun at speeds below about km/s. Suppose you observe a nearby star that is moving much faster than this (say, km/s. Which of the following is a likely explanation for its high speed?
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It is probably a halo star that is currently passing through the disk.
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Why do we believe that most of the mass of the Milky Way is in the form of dark matter?
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The orbital speeds of stars far from the galactic center are surprisingly high.
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Spiral arms appear bright because ________.
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they contain more hot young stars than other parts of the disk
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How did star formation likely proceed in the protogalactic cloud that formed the Milky Way?
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The stars that formed first could orbit the center of the galaxy in any direction at any inclination.
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If we could watch spiral arms from a telescope situated above the Milky Way over million years, what would we see happen?
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Stars will move through the spiral arms, bunching up closer as they pass through. Young hot stars will form and die within the arms before having a chance to move out.
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What is the best evidence for an extremely massive black hole in the center of the Milky Way?
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The orbits of stars in the center of the galaxy indicate that the presence of to million solar mass object in a region no larger than our Solar System.
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Which of the following statements is not true of the object known as Sgr A* in the center of our Galaxy?
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It is by far the brightest source of visible light lying in the direction of the galactic center.
