Why are there few quasars at low redshifts and at high redshifts but many at redshifts of about 2?
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Quasars are most common with redshifts over 2 and less common with redshifts above 3 . The largest quasar redshifts are over 6, but such high-redshift quasars are rare. Evidently astronomers see few quasars at high redshifts because they are looking back to an age when the Universe was so young it had not yet formed many galaxies. At redshifts between 2 and 3, galaxies were actively growing and colliding, and quasars were about 1000 times more common than they are now.
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When is a comet's orbital speed at its maximum?
A) when it is closest to the Sun B) when it is farthest from the Sun C) Its orbital speed does not change.
Why is the Moon's cycle of phases (the synodic month) longer than the Moon's orbital period (the sidereal month)?
A) The Moon completes the cycle of lunar phases before it completes a full orbit around Earth. B) The Moon has to complete more than one full orbit around Earth to complete the cycle of lunar phases. C) The Moon orbits Earth faster than Earth orbits the Sun. D) The Moon orbits Earth faster than Earth rotates. E) The lunar month is based on the Moon's orbit, while the sidereal month is based on Earth's orbit.
If, in a parallel universe, ? has the value 3.14149, express ? in that universe to four significant figures
A) 3.141 B) 3.142 C) 3.1415 D) 3.1414
A DC motor of internal resistance 6.0 ? is connected to a 24-V power supply. The operating current is 1.0 A
(a) What is the start-up current? (b) What is the back emf when the motor is running at full speed? (c) What is the back emf when the motor is running at half speed?