ConceptCheck 13-1: Looking at other galaxies, astronomers have observed that globular clusters tend to cluster around the center of galaxies. If globular clusters appeared in every direction equally, then astronomers would assume that we were in the center of the globular clusters and, subsequently, the center of our Galaxy.
ConceptCheck 13-2: While far-infrared wavelengths best reveal the presence of warmed dust grains, near-infrared wavelengths, which can pass through dust, are more predominantly emitted by cool stars, making near-infrared a better choice.
ConceptCheck 13-3: As the view between the galactic center and the Sun is obscured by intervening dust, the better vantage point would be from outside the disk in the halo, where the path of light is largely unobstructed by dust.
ConceptCheck 13-4: There is virtually no star formation occurring in the surrounding halo, so the youngest stars would be found actively forming in the rich dusty regions along the Galaxy’s disk.
ConceptCheck 13-5: Because human eyes are most sensitive in the visible range, we would observe virtually no difference in the appearance of the Milky Way.
ConceptCheck 13-6: Neutral hydrogen gas emitting 21-cm photons will appear to be Doppler shifted to longer wavelengths.
ConceptCheck 13-7: Starting at the outside and moving inward, the sequence of galactic arms near the Sun are Perseus Arm, Orion Arm (where the Sun resides), and then the Sagittarius Arm.
ConceptCheck 13-8: The most important factor for how fast a star moves around the galaxy is how much mass is closer to the galactic center than the star.
ConceptCheck 13-9: In the absence of dark matter, the stars most distant from the galactic center should be moving much slower than those stars orbiting closer to the center.
ConceptCheck 13-10: Dark matter is the best explanation, to date, of what sort of unseen mass causes the outermost stars of galaxies to orbit around the galactic center as fast as they do.
ConceptCheck 13-11: Without density waves causing areas of the galaxy to bunch up here and there, the galaxy would have no discernible arms.
ConceptCheck 13-12: Because the arms of flocculent spiral galaxies are likely the result of star formation, this suggests that stars came before spiral arms in the early universe.
ConceptCheck 13-13: The gravitational effects of a supermassive black hole cause stars to move at very high rates of speed; if there was no supermassive black hole, then stars would be moving much, much slower.
ConceptCheck 13-14: The gases in the nebula would need to be energized by an external energy source, and it is proposed that a sudden intense release of X-rays from heated material moving into a supermassive black hole could have externally energized the nebula.
CalculationCheck 13-1: No matter how many hydrogen atoms are undergoing spin-flip transitions, only 21-cm photons are released. Fortunately, the more hydrogen atoms present results in a stronger signal received at radio telescopes.
CalculationCheck 13-2: Remembering that it takes 2.2 × 108 years for the Sun to make one orbit around the Galaxy, 4.5 × 109 years × (1 trip ÷ 2.2 × 108 years) = 20 orbits.
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