(written from a Production point of view)
Arcturus is a real star described as the densely populated homeworld of the Arcturians. (The Making of Star Trek: The Motion Picture, pp. 131; Star Trek: The Magazine Volume 2, Issue 9, "Who is that Alien?", January 2002)
In an early story outline for TOS: "The Cage" (29 June 1964), when the lead was still Robert April, the captive creatures kept by the Talosians in their zoo-menagerie included something a race that was reminiscent of a "civilization April himself has seen –the intelligent Lemur-life of a Class M planet in the Arcturus system." (The Making of Star Trek, p. 59)
According to Star Trek: Star Charts (United Federation of Planets I), Arcturus, also known as Alpha Bootis, was the name of a star in the Alpha Quadrant. The primary was a class K star. The magnitude of this star was -3, which was 1,000 times brighter than Sol. Arcturus was also the name of a planet in the system and had, by 2378, become a member of the Federation.