Ensign Carstairs was a male Starfleet officer who lived during the mid-23rd century. He served in the sciences division aboard the USS Enterprise under Captain James T. Kirk, during a historic five-year mission that starship undertook. In 2268, he served his first tour of duty as one of the ship's geologists.
That year, he was selected to be in the landing party to Alpha Carinae II by the M-5 multitronic unit but not by Captain Kirk. This was because M-5 knew that Carstairs had once visited the planet while being with the Merchant Marines. (TOS: "The Ultimate Computer")