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Physics A stellar black hole may form when a massive star dies. The mass of the star collapses down to a single point. Imagine an astronaut orbiting a black hole having four times the mass of the Sun. Assume the orbit is circular. (a) Find the speed of the astronaut if his orbital radius is r = 1 AU. m/'s (b) Find his speed if his orbital radius is / = 5.9 km. m/'s (c) CHECK and THINK: Compare your answers to the speed of light in a vacuum. What would the astronaut's orbital speed be if his orbital radius were smaller than 5.9 km