Complex sentence: It was once thought to belong to the physical association of stars that comprise much of the constellation of Orion, but this is no longer believed to be the case, as Gamma Orionis is much closer to us than the rest of the group.
Split into simpler sentences: It was once thought to belong to the physical association of stars that comprise much of the constellation of Orion. But this is no longer believed to be the case, as Gamma Orionis is much closer to us than the rest of the group.