What is the most common thing you can see that emits optical light and what is the least common thing you can see that

Question: What is the most common thing you can see that emits optical light and what is the least common thing you can see that emits radio light and why?
- Keywords:
  light,
  optical,
  radio
Asked by Ella.G to Anna on 21 Nov 2014.
- Anna Scaife answered on 21 Nov 2014:
  
  Hi Ella
  
  Stars are probably the most common source of optical light (they’re like our sun but further away). The least common thing that emits radio emission… hmmm…. probably a “cosmic string”. Cosmic strings are defects in the fabric of the Universe that produce radio emission at mm wavelength – we haven’t found one yet, but we’re pretty certain they exist, although even then they’re probably the rarest thing in the universe 🙂

Question: What is the most common thing you can see that emits optical light and what is the least common thing you can see that emits radio light and why?