Hawking's cause of death was not immediately released.

He was diagnosed with ALS in 1963 when he was 21 years old. The progression of the illness led to his eventual paralysis. The only way Hawking was able to communicate in his later years was through the help of a device attached to a cheek muscle that used a text-to-speech program that provided a voice for him.

Hawking was best known for his work as a theoretical physicist, cosmologist, author and even sometime actor. He made multiple appearances on "The Big Bang Theory," The Simpsons, Futurama and his life story was portrayed by Eddie Redmayne in the 2014 "The Theory of Everything."

He also held the position as Cambridge University's Lucasian Professorship of Mathematics, a position that has been held by only 17 people in history - including the father of physics, Issac Newton.

His scientific work on gravitational theories and black holes led to a revolution in the field of cosmology. His book "A Brief History of Time" appeared on the best-seller list for a record-breaking 237 weeks.

People were quick to express the loss they were feeling about Hawking's passing online: