(covers information from several alternate timelines)
Alpha Carinae was a star.
In 2293, Alpha Carinae's location in the Milky Way Galaxy was labeled in the star chart "The Explored Galaxy", which was on display in Captain James T. Kirk's quarters aboard the USS Enterprise-A. (Star Trek VI: The Undiscovered Country okudagram)
In 2366, in an alternate timeline, during the Federation-Klingon War, the location of Alpha Carinae was labeled on a tactical situation monitor in the ready room aboard the USS Enterprise-D. (TNG: "Yesterday's Enterprise" okudagram)
In the canonical Star Trek universe, Canopus and Alpha Carinae were depicted as two distinct stars, with their own solar systems. This was contrary to the Star Trek Encyclopedia (4th ed., vol. 1, p. 115) and the real word universe, in which Canopus and Alpha Carinae referred to the same star.
According to the Star Trek: Star Charts (p. 65) and the Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), Canopus (Alpha Carinae) was a star in the Beta Quadrant. The primary was a F-class star.
For Star Trek: First Contact, Alex Jaeger created a ship size comparison chart for Industrial Light & Magic. On this chart, one of the names considered for the Norway-class starship was the USS Canopus.