In the Lighthouse model:
A) pulsars are observable only if they lie in the galactic plane.
B) pulsars are navigational devices created by interstellar navigators as discovered by Jocelyn Bell in 1967.
C) all pulsars have their poles pointed directly at us or they would be not observable.
D) if the beams sweeps across us, we can observe the pulse.
E) the period of pulsation must speed up as the neutron star continues contracting.
D
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A sphere is made up of two layers: The first is lead (density 11340
and is of radius R. The second is tin, (7310
and is concentric with the first, from radius R to radius 2R. The sphere is placed in a pool of mercury (density 13600 
How much of the volume of the sphere is below the surface?
A. 68.5% B. 57.5% C. 57.0% D. 64.2%
If the composition of the upper atmosphere were altered to permit a greater amount of terrestrial radiation to escape, Earth would be
A) cooler. B) warmer. C) quite unaffected. D) none of the above
Standing waves of frequency 57 Hz are produced on a string that has mass per unit length 0.0160 kg/m. With what tension must the string be stretched between two supports if adjacent nodes in the standing wave are to be 0.71 meters apart?
What will be an ideal response?
An astronaut on a spaceship moving at 0.927c says that the trip between two stationary stars took 7.49 y. How long does this journey take as measured by someone at rest relative to the two stars?
A) 20.0 y B) 2.81 y C) 4.05 y D) 22.1 y