The Big Bang Theory is undoubtedly one of the most iconic and beloved shows ever gracing our screens. The series, which aired from 2007 to 2019, followed a group of scientists and their socially awkward friend as they navigated the ups and downs of life, all while trying to understand the complexities of the universe.

While the show was a hit thanks to its cast of lovable characters, it also featured many celebrity guest stars, making these scientists' worlds a little more colorful. Here are some of the top celebrity guest stars on The Big Bang Theory that you may have forgotten about.

Justin Tallis/ AFP | AFP | Getty Images | Stephen Hawking worked on the physics of black holes.

Bob Newhart as Professor Proton

Fans of The Big Bang Theory were delighted to see comedy legend Bob Newhart guest star as Professor Proton in seasons 6, 7, and 9. Newhart won an Emmy award for his role and captured audiences' hearts as Sheldon's science hero and mentor. Professor Proton's character was so popular that the show even created an entire episode around his character called "The Proton Transmogrification."

Stephen Hawking

Theoretical physicist Stephen Hawking is one of the few celebrities whose guest appearances on The Big Bang Theory didn't involve a fictionalized version of himself. Instead, he portrayed himself as a robotic voice simulation of himself. Hawking appeared in seven episodes of the show throughout its run and served as a mentor and inspiration to its main characters.

DISTRACTIFY/ GETTY IMAGES | Nye now works as a visiting professor at the Planetary Society's C.E.O. and teaches at Cornell University.

Bill Nye as Himself

Bill Nye, sometimes called "The Science Guy," briefly appeared on The Big Bang Theory in season 7. Although short, his appearance solidified his status as a beloved science and pop culture icon. He played himself, appearing as a guest presenter at a science convention.

Carrie Fisher as Leia Organa

Carrie Fisher's guest appearance on The Big Bang Theory as Princess Leia Organa was one of the most memorable cameos of the entire series. Fisher appeared in the season 7 finale, where she played a fantasy version of Sheldon's subconscious. Her guest appearance cemented her status as a beloved pop culture icon and gave a nod to the Star Wars franchise that many Big Bang Theory fans love.

Neil deGrasse Tyson as Himself

Neil deGrasse Tyson's guest appearance on The Big Bang Theory was a true treat for science and pop culture fans. Tyson, widely known for his work as an astrophysicist, appeared as himself on the show's 12th and final season. His appearance helped solidify The Big Bang Theory's legacy as a show celebrating science and math, all while appealing to mainstream audiences.

DISTRACTIFY/ GETTY IMAGES | Neil Degrasse Tyson's estimated net worth is around $7 million

Elon Musk

Famous for his innovations and world-changing ideas, Elon Musk appeared in the show's tenth season. He played himself and was acknowledged as an inspiration by the cast and crew. His cameo was a brief glimpse into his personality, with some humorous references to his plans to colonize Mars.

Stan Lee

The Marvel Comics legend made several appearances on the show, and his cameo in the tenth season's "The Excelsior Acquisition" episode was especially memorable. In that episode, he was seen sitting beside Sheldon Cooper on a train while playing one of his iconic characters. His passing soon after his appearance was even more poignant for show fans.