How do astronomers know that the luminous regions of active nuclei of active galaxies are very small?
a. Their nuclei are much brighter than expected.
b. The orbits of stars near the center of the galactic nuclei are faster than expected.
c. The luminosity of the nuclei varies over short times.
d. The color of the nuclei is different than expected.
c
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On a Hertzsprung-Russell diagram, where would we find white dwarfs?
A) upper right B) lower right C) upper left D) lower left
Work-Energy Theorem: The kinetic friction force that a horizontal surface exerts on a 60.0-kg object is 50.0 N. If the initial speed of the object is 25.0 m/s, what distance will it slide before coming to a stop?
A. 15.0 m B. 30.0 m C. 375 m D. 750 m
Motional Emf: A long vertical wire carries a steady 70 A current. As shown in the figure, a pair of horizontal rails are 0.20 m apart. A 20-? resistor connects points a and b, at the end of the rails. A bar is in contact with the rails, and is moved by an external force with a constant horizontal velocity of 0.90 m/s to the right, as shown. The bar and the rails have negligible resistance. At the instant that the bar is 0.20 m from the wire, what are the induced current in the resistor and its direction through the resistor? (?0 = 4? × 10-7 T ? m/A)
A. 0.63 ?A, from a to b B. 0.63 ?A, from b to a C. 0.32 ?A, from a to b D. 0.32 ?A, from b to a E. 1.9 ?A, from b to a
When we see x-rays from an accretion disk in a binary system, we can't immediately tell whether the accretion disk surrounds a neutron star or a black hole. Suppose we then observe each of the following phenomena in this system. Which one would rule out the possibility of a black hole?
A) intense x-ray bursts B) spectral lines from the companion star that alternately shift to shorter and longer wavelengths C) visible and ultraviolet light from the companion star D) bright x-ray emission that varies on a time scale of a few hours