A light-year is:

If you heat a gas so that collisions are continually bumping electrons to higher energy levels, when the electrons fall back to lower energy levels the gas produces 

What quantities does angular momentum depend upon? 

Why do some of the bright stars (such as the one indicated by the arrow) in this photo have cross-shaped spikes over them?

According to the conservation of angular momentum, what would happen if a star orbiting in a direction opposite the neutron’s star rotation fell onto a neutron star?

A radio telescope and an optical telescope of the same size have the same angular resolution. 

Which of the following correctly states the luminosity-distance formula?

If your mass is 60 kg on Earth, what would your mass be on the Moon? 

All the stars in this photo are at about the same distance from Earth (some 25,000 light-years away). Which stars in this picture are the largest in size (radius)?

This photo shows an object located in the halo of our Milky Way galaxy. What kind of object is it?