In Sumerian mythology, as well as in Babylonian and in Assyrian mythology, An or Anu is the supreme deity. (In Babylonian mythology, his grandson Marduk became the supreme deity later on). He was the great father and king of the Gods. His realm was the expanse of the Heavens, and his symbol was the star. He lived in his heavenly abode, but on occasions came over to Earth for a visit. One of his spouses was his half-sister, Antu. He was the father of Enki (oldest son but conceived with a different mother) and Enlil (son with Antu).
According to Sitchin, this Heavenly abode is on the "12th Planet," Nibiru.

Personally, I believe Anu in these stories is not just one person. I think 'he' represents the Lyran race, and its influence upon Earth.
(Lyssa Royal and Keith Priest however point out that there may be a connection with the Egyptian God Anu-bis, which is the dog headed god, suggesting a link with Sirius, the Dog-star. Where I can see a connection between Anubis and Sirius, I do not think this is the case with Anu. Still, Anu's symbol is the Star, and Sirius is the brightest star to be seen... The reason why I do not think there is a direct connection between Anu and Sirius, is because Enki has got that direct connection. See Enki for more details).