(covers information from several alternate timelines)
Antares was a star.
In 2366, in an alternate timeline, during the Federation-Klingon War, the location of Antares was labeled on a tactical situation monitor in the ready room aboard the USS Enterprise-D. (TNG: "Yesterday's Enterprise", okudagram)
It was probable that this star was in the Antares sector.
According to Star Trek: Star Charts ("United Federation of Planets II"), the Antares (Alpha Scorpii) star system was located in the Beta Quadrant. This was a binary system with a M-class star and a B-class star.