 Since real time conversations are impossible , the humans send sections of the expedition 's library . After reading an astronomy article , a cheela realizes that the supernova half a million human years ago created both their races . Many cheela generations later , but only a few hours for humans , <unk> develop gravity manipulation . A few generations later , a cheela spacecraft visits the human one . Although they still need extreme gravity fields to survive , the cheela can now control them precisely enough for both races to see each other face @-@ to @-@ face in safety . The cheela have decided that transferring their technologies , now far advanced of humans ' , would stunt humanity 's development . However the cheela leave clues in several challenging locations , before going their separate ways . 
