25.3.0 The Mass of the Galaxy

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18. Which of the following statements about dark matter in the Galaxy is FALSE? a. Most of it cannot at present be observed with our telescopes (we only know its existence from the gravity it exerts) b. While the dark matter cannot be observed with our present-day instruments, we still have a pretty good idea what it consist of c. Our measurements suggest that there may be as much as ten or twenty times more dark matter in the Galaxy than the matter we have been able to observe d. Dark matter appears to be distributed in a giant halo (outer layer) around the Galaxy e. We can pretty much rule out the idea that the dark matter is made mainly of vast clouds or regions of hydrogen gas

b. While the dark matter cannot be observed with our present-day instruments, we still have a pretty good idea what it consist of

16. How do astronomers measure the mass that the Galaxy contains inside the orbit of the Sun? a. they count the number of stars one by one and multiply by the average mass of a star b. they add up all the observations at 21-cm wavelengths (because the mass of hydrogen gas is far greater than the mass in all the stars) c. they measure the distance to the center of the Galaxy and the period of the Sun's orbit and then use Kepler's Third Law d. they measure the masses of many other spiral galaxies nearby and then take an average e. you can't fool me; there is no way to get the mass of something as big as our Galaxy

c. they measure the distance to the center of the Galaxy and the period of the Sun's orbit and then use Kepler's Third Law

15. Objects orbiting around the center of the Milky Way obey Kepler's 3rd Law. This means that: a. the pull of gravity gets stronger and stronger as you get further away from the center b. larger clusters of stars will orbit the center more quickly than smaller ones c. the closer a star is to the center, the longer it will take to go around d. a cloud of gas or star that is further from the center will generally take more time to orbit e. the orbits of all objects around the Galaxy are in the shape of a perfect circle

d. a cloud of gas or star that is further from the center will generally take more time to orbit

14. A "galactic year" as defined by astronomers is: a. the time it takes the Earth to go around the Sun (starting with when it's closest in its orbit to the center of the Galaxy) b. the time it takes the outermost part of the solar system (the Oort Cloud) to rotate once relative to the Galaxy) c. the time it took the Milky Way Galaxy to develop spiral arms d. the time it takes the Sun to revolve once around the center of the Milky Way Galaxy e. the time it will take the Sun to become a red giant and expand into the Galaxy

d. the time it takes the Sun to revolve once around the center of the Milky Way Galaxy

17. Recently, astronomers have observed stars and other objects that orbit the center of the Milky Way Galaxy farther out than our Sun, but move around faster than we do. How do astronomers think such an observation can be explained? a. all these faster-moving objects must be escaping from the gravity of the Milky Way and will soon be lost to our Galaxy b. each of the faster-moving outer objects must be the result of a supernova explosion (giving them extra speed) c. it is the Sun that is moving too slowly because of a collision billions of years ago; the outer objects are really moving at the appropriate speed for their distance from the center d. there must be a great deal of invisible dark matter outside the orbit of the Sun whose gravitational pull explains the faster motions we see out there e. no one can come up with any explanation for this puzzling observation

d. there must be a great deal of invisible dark matter outside the orbit of the Sun whose gravitational pull explains the faster motions we see out there

19. Astronomers making observations in our Galaxy have been able to rule out a number of suggestions for what the dark matter in the Galaxy might be. Which of the following have we NOT been able to rule out (which suggestion is still "in the running")? a. black holes b. hydrogen gas that is at low temperatures compared to stars c. Jupiter-mass planets or brown dwarfs d. large quantities of dust (like we find in nebulae) e. a new kind of subatomic particle

e. a new kind of subatomic particle


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