What are Supermassive Black Holes?
Supermassive black holes are gravitational features located at the center of many galaxies, including the Wilky Way. Supermassive black holes can be thought of the galactic sun: in the same way that planets in the solar system orbit the Sun, the 200-400 billion solar systems in the Milky Way orbit the central black hole. Supermassive black holes contain between 105 and 1010 (hundreds of thousands and tens of billions) solar masses. Our galaxy’s supermassive black hole is suspected to be a within compact object called Sagittarius A*, which contains 3.7 million solar masses within a volume with radius no larger than 6.25 light-hours (45 AU) or about 4.2 billion miles. In comparison, Pluto orbits the Sun at 5.51 light-hours or 3.7 billion miles. Theoretically, it is possible that the Sagittarius A* region contains more than one supermassive black hole. If so, they will combine to form one object in less than 100 years. Sometime around 2015-2025, the technology of astronomical interferomet
Definition: gravitationally collapsed objects that preclude escape from their surface Mass: ~ 100 million solar masses Size: less than the solar system Location: the nuclei of many galaxies, both active and dormant Identification: via the rapid motion of radiating matter tightly orbiting around the black hole as revealed by optical, radio and X-ray spectral line features, especially definitive with upcoming X-ray missions
Ordinary black holes are formed when massive stars die, and they typically have the mass of about 5–10 Suns. A supermassive black hole, however, has a mass millions of times greater, and is formed when huge gas clouds collapse. Supermassive black holes seem to be the power source of high-energy active galaxies, such as quasars. Astronomers believe that a supermassive black hole lurks at the center of our own galaxy.
What is the main observational evidence for the existence of supermassive black holes? What other observational evidence supports their existence? See Cosmic Perspective p.672. • Historical development of understanding of the Milky Way. Give an account of the history of the historical development of our understanding of the Milky Way. How did Galileo, Herschel, Parsons, Leavitt, Slipher, Shapley, Hubble, Oort and Lindblad contribute? How did the development of ever-better telescopes contribute? What observation finally settled the issue of the nature of the spiral nebulae? What observations finally settled the question of where was the center of the Milky Way? • Milky Way. Sketch a diagram of the Milky Way, indicating the bulge, the disk, and the halo. Name some objects observed in each component. See Cosmic Perspective p.597. • Globular clusters. What are globular clusters? About how many are there in the Milky Way? Whereabouts in the Milky Way are they? What is it about their age and
