• 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?

    Asked by Ella.G to Anna on 21 Nov 2014.
    • Photo: Anna Scaife

      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 🙂