Seven Stars, Seven Stones and One White Tree...

[Ardent]

Originally Posted by Gordis

"what hinders Aragorn's star from being ONE of the seven stars?"
We don't know of six others. Aragorn's star was unique, a symbol of Royalty, like a crown.

According to Bilbo's poem 'Earendil Was A Mariner', the star on Earendil's crown is the last of the three Silmarils [thanks for the edit Aganzir]. Aragorn instructed Bilbo to place a green gem on Earendil's breast, thus placing himself in the picture since this is what he wears.

It is unclear whether the actual jewel in his crown is the very star of Earendil, or just an emblem, but it is identified in the index of 'Lord of the Rings' (under star, as emblem) as a diamond shaped star on the banner of Gondor, along with the six other stars of Elendil and his captains.