What three main types of celestial objects are sources of radio waves?

Question

What three main types of celestial objects are sources of radio waves?

Answer 1

Hydrogen 21-cm line is a radio wave. That is very important.



AGN (Active Galactic Nuclei) - like radi galaxies and quasars are major radio emitters.



The CMB (Cosmic Microwave Background) is considered a radio source by some.



Lesser than these would be pulsars if you don't consider CMB to be a radio source.