Stars had some importance in Tolkien's mythology. Earendil (or rather his ship Vingilot) became a star, and his son Elros was the first King of Numenor.
Revelation 12:3-4: . . . an enormous red dragon . . . its tail swept a third of the stars out of the sky and flung them down to the earth.
Many people online have said that " the stars" in this chapter are referring to the angels and the "red dragon" is Satan. The latter part of this teaching is apparently talking obout how when Satan was banished from heaven he took a third of the angels with him down to earth. This may be a link to Numenor, as you said it is in the shape of a star and when the Numernoreans disobeyed Illuvatar the star shaped island was flung beneath the waves.
Or it could just be because Illuvatar was grateful for their services and thought a star shaped island emphasised that.
If there is much significance to the appearance of Númenor then it's probably closer to your second thought.