 Theoretical physicist Brian Greene appeared in the fourth season, as well as astrophysicist, science popularizer, and physics outreach specialist Neil deGrasse Tyson, who also appeared in the twelfth season.Cosmologist Stephen Hawking made a short guest appearance in a fifth-season episode; in the eighth season, Hawking video conferences with Sheldon and Leonard, and makes another appearance in the 200th episode.In the fifth and sixth seasons, NASA astronaut Michael J. Massimino played himself multiple times in the role of Howard's fellow astronaut.