Intro to astrophysics:
(a) somewhere in our galaxy resides a cloud of neutral hydrogen gas with a radius r. the gas density is n. (a) show that the number of 21-cm photons the cloud emits every second is given by:
n_{ph}=\frac{\pi r^{3}n}{t}
(b) if the cloud is a distance d from the sun, show that the energy flux of this radiation at the sun is given by:
f=\frac{hc r^{3}n}{4d^{2}\lambda t}

This is important 1. what happens to the speed of molecules in water vapor when it condenses to form a liquid? (2 points) the molecules slow down. the molecules speed up. the molecules slow down and then speed up. the molecules speed up and then slow down. 2. how might human activities change to compensate for a decrease in water supplies? (2 points) humans might build a factory. humans might build drainage systems. humans might extract more water from the ground. human behavior does not change because of the water cycle. i'll be grateful if anyone can answer these 2 questions !