Chapter 16

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Basic Skills/ Definitions A: Which of the following best summarizes what we mean by dark matter? Matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light. B: Which of the following best summarizes what we mean by dark energy? It is a name given to whatever might be responsible for causing the expansion of the universe to accelerate with time. C: The text states that luminous matter in the Milky Way seems to be much like the tip of an iceberg. This refers to the idea that: The unseen dark matter represents much more mass and extends much further from the galactic center than the luminous matter in the Milky Way. ** Dark matter might make up 90% of the galaxy’s mass** D: What is a rotation curve? A graph showing how orbital velocity depends on distance from the center for a spiral galaxy E: What do we mean when we say that the rotation curve for a spiral galaxy is “flat”? Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward. F: What is the main way that we determine the mass distribution of a spiral galaxy? Constructing its rotation curve, which we make by looking for Doppler shifts in the 21 cm emission from atomic hydrogen in its disk. G: Although we know less about dark matter in elliptical galaxies than we do in spiral galaxies, what does current evidence suggest? Elliptical galaxies probably contain a similar proportion of their mass in the form of dark matter as do spiral galaxies. H: Which of the following is NOT one of the three main strategies used to measure the masses of galaxy clusters? Measuring the speeds of galaxies orbiting the cluster’s center Measuring the temperatures of stars in the halos of the galaxies ** Stellar temperatures do not indicate the mass of a galaxy cluster. Observing how the clusters bend light as gravitational lenses Studying the X-ray emission from hot gas inside the clusters I: When we say that a cluster a galaxies is acting as a gravitational lens, what do we mean? It bends or distorts the light coming from galaxies located behind it.
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