I think this is "The Quatermass Xperiment", which is actually British but had an American lead actor playing the main character of professor Bernard Quatermass. From the wiki summary, two astronauts on a rocket expedition were killed, their bodies completely destroyed, but one survived:
The rocket's hatch is finally opened, and the space-suited Carroon stumbles out. There is no sign of the other two crew.
Carroon is slowly mutating over the course of the movie, eventually becoming a tentacled blob (which might be remembered as starfish-like) killed by electricity:
On a police tip from a vagrant, Lomax and his men track the Carroon mutation to Westminster Abbey, where it has crawled high up on a metalwork scaffolding. It is now a gigantic shapeless mass of combined animal and plant tissue with eyes, distended nodules, and tentacle-like fronds filled with spores. Quatermass arrives and orders London's electrical power centres be combined and the generated power quickly diverted to the Abbey. Heavy duty electrical cable is run and attached to the bottom of the metal scaffolding. The alien creature is cremated by electrocution before it can release its spores.
From this blog post, an image of the final form of the alien: