- Science Marches On: Several, given that the book is 60 years old now in 2017.
- The lightning-fast progress in astronomy linked to the radiotelescopic observation, computer-aided optics and space-based instruments means that we now know infinitely more about the space and how it works, and our observations have even reached the depths that simply couldn't be imagined 60 years ago. For starters, the Iron Star happens to sit in the "uncharted area" very close to the solar system, in the several lightyears range the area so thoroughly studied and mapped nowadays, that it is literally impossible to overlook anything that massive.
- The Iron Star itself. Back in Efremov's days such objects were believed to exist aplenty. Now it's commonly agreed that while they are theoretically possible, they have so VERY long formation times (way more then the whole current age of our Universenote ) that not even one had formed yet. The book was written when the Big Bang theory wasn't yet commonly accepted, and the commonly held theories of cosmic evolution were largely of the Steady State type, so it was believed that such stars would already exist. The formation of an iron star also presupposes the stability of proton, which so far hasn't been either proven or rejected.
Trivia / Andromeda Nebula