Many, many centuries before the RigVeda and before Zoroaster, a name that sounds like "Asura" was the proper name of a very mighty god. Some scholars say his name was "Ansu" but this is not sure. So that is why they write the name as "*Ansu". That '*' means it is not sure. *Ansu was the god of life from which the name 'asura' and 'ahura' comes. We know that this god existed because far, far away in Northern Europe there was also a god named 'Aesir', who was also a giver of life. It sometimes can happen that a similar name appears in two or three places. But it is very rare that the same name with the same function appears. So, that is why 'Asura' and 'Ahura' and 'Aesir' must be all come from '*Ansu'. And we know that such a god must have been very powerful because he was very popular for so very long.