Answers

ConceptChecks

ConceptCheck 22-1: Looking at other galaxies, astronomers have observed that globular clusters tend to form a spherical distribution centered on the center of galaxies. If our own globular clusters appeared in every direction equally, then astronomers would assume that we were near the center of the globular clusters and, subsequently, near the center of our Galaxy.

ConceptCheck 22-2: Far-infrared wavelengths best reveal the presence of warmed dust grains, and weak emission from stars is not easily detected with dust around. Near-infrared wavelengths, which can pass through dust, are emitted more strongly from cool stars than far-infrared, making near-infrared a better choice.

ConceptCheck 22-3: 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 22-4: 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 22-5: Because human eyes are only sensitive in the visible range, we would observe virtually no difference in the appearance of the Milky Way.

ConceptCheck 22-6: Neutral hydrogen gas emits 21-cm photons, and this is the wavelength observed when there is no relative motion between the gas and the observer. However, if the gas is moving away from the observer, its emitted light will be Doppler shifted and detected at longer wavelengths.

ConceptCheck 22-7: The Perseus arm (see Figure 22-16).

ConceptCheck 22-8: 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 22-9: 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 22-10: Astronomers detected only a small amount of matter in the form of MACHOs, yet they would have detected more if it were there. Therefore, the source of dark matter still remains to be discovered. Until WIMPs are detected, or ruled out, they remain a good candidate.

ConceptCheck 22-11: Without density waves causing areas of the galaxy to bunch up here and there, the galaxy would have no discernible arms.

ConceptCheck 22-12: 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 22-13: The gravitational effects of a supermassive black hole cause surrounding stars to move at very high speeds; if there were no supermassive black hole, then nearby stars would be moving much more slowly.

ConceptCheck 22-14: 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.

CalculationChecks

CalculationCheck 22-1: No matter how many hydrogen atoms are undergoing spin-flip transitions, only 21-cm photons are released from each atom. Fortunately, greater numbers of hydrogen atoms results in more photons and a stronger signal received at radio telescopes.

CalculationCheck 22-2: Remembering that it takes 2.2 × 108 years for the Sun to make one orbit around the Galaxy, 4.5 × 109 years × (1 orbit/2.2 × 108 years) = 20 orbits.

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