ConceptChecks
ConceptCheck 24-1: Here are three differences: A quasar has emission lines, whereas a star has absorption lines. A quasar also emits more strongly than a star in the radio and X-ray wavelengths. The Doppler shift of spectral lines (whether emission or absorption) is much larger for quasars than for the stars in our Galaxy, indicating that quasars recede at much higher speeds than stars in our Galaxy.
ConceptCheck 24-2: The universe was around 2 billion years old when quasar activity peaked, and that time corresponds to a redshift z of around 3.
ConceptCheck 24-3: The farther away an object is observed, the lower its apparent brightness. The redshifts in Figure 24-5 all correspond to very distant objects that can only be observed because they were very luminous at the early times and remote distances when they shined bright.
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ConceptCheck 24-4: Radiation pressure would push outward on potentially accreting gas-fuel more than gravity could pull it in. This would slow the inward flow of gas (slowing accretion), and since accretion of gas fuels the quasar’s luminosity, the quasar would soon become less luminous. This reduction in luminosity would actually allow accretion to then rise to its original Eddington limit.
ConceptCheck 24-5: Orbits closer to the central mass have higher speeds so that an inner blob rubs against an outer blob in the disk, causing the inner blob to lose energy and spiral inward. Also, as the blobs rub past each other they produce heat. Thermal radiation from this heat produces much of a quasar’s visible light.
ConceptCheck 24-6: The blob travels 5 light-years (even though we see only the 3 light-years of its sideways, transverse motion). The blob takes 6 years to travel this distance (and it travels slower than the speed of light).
ConceptCheck 24-7: The answer is “no” to both questions. The object is a dim dusty torus that can block light, depending on the viewing angle. The intense visible light from a quasar comes from the much-smaller accretion disk, which appears pointlike in images.
ConceptCheck 24-8: Viewing angle #2. At these intermediate angles, visible light can be seen directly from the accretion disk without getting blocked by the torus. This line of sight also receives Doppler-shifted light from fast-moving clouds in a turbulent region near the black hole; these clouds produce the broad emission lines.
ConceptCheck 24-9: Yes, the supermassive black hole at our Galaxy’s center is probably a dead quasar. However, since a supermassive black hole is expected to capture a star and produce a dead quasar flare only about once every 10,000 years, we are unlikely to witness this event.