A protostar becomes a main sequence star when it ________.
A. starts fusing helium
B. starts glowing with infrared light
C. starts fusing hydrogen to helium
D. starts fusing heavier elements
Answer: C
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Evidence for dark matter in our Galaxy comes from
A. distribution of supernovae. B. orbital periods of stars in the Galaxy. C. locations of black holes. D. unexplained energetic events. E. distribution of nebulae.
An inventor suggests that a house might be heated by using a refrigerator to draw energy as heat from the ground and reject energy as heat into the house. He claims that the energy supplied to the house as heat can exceed the work required to run the refrigerator. This:
A. is impossible by first law B. is impossible by second law C. would only work if the ground and the house were at the same temperature D. is impossible since heat energy flows from the (hot) house to the (cold) ground E. is possible
Assuming a linear velocity distribution and a linear temperature distribution in the boundary layer over a flat plate, derive a relation between the thermal and hydrodynamic boundary-layer thicknesses and the Prandtl number.
GIVEN
Boundary layer over a flat plate
FIND
A relation between the thermal and hydrodynamic boundary-layer thicknesses and the Prandtl number
ASSUMPTIONS
Linear velocity and temperature distributions in the boundary layers
SKETCH
Based on the number of galaxies visible in the Hubble eXtreme Deep Field (Figure 20.1 in your text), the estimated number of galaxies in our observable universe is at least
A) 100 billion. B) 100 million. C) 50,000. D) infinity.