UNIX. When I started my PhD, we used VMS on VAXes, but these were superseded by these beauties -
SunOS, my first experience of UNIX. Soon after, we had the birth of penguin, and we could have real computing in the home with Linux.
People realised that that you can string computers together to build cheap supercomputers and computational astrophysics forged ahead. Many of the spectacular cosmological simulations were run on such supercomputers.
But if that was not enough, Ritchie give us one more thing, the C programming language. I'm going to moan and gripe like an old man that students today don't know how to code, and perhaps they can get away with Python, but I'm a fan of basic languages, C and fortran, for coding in supercomputer environments (and I'm not going to get all religious on programming language).
However, I started to learn C from Ritchie's own book