There are several artist named RockStar or Rockstar:
1 Rockstar is a cover band featuring Tim Ripper Owens (Judas Priest, Iced Earth, ...), Keri Kelli (Alice Cooper, Slash's Snakepit), James Kottak (Scorpions), Rudy Sarzo (Whitesnake, Ozzy Osbourne, Dio, ...) and Teddy "Zig Zag" Andreadis (Guns N' Roses).
2. A Hard Rock/Sleaze band from Italy.
3. A group from the Philippines.
4. Jony Rockstar
5. An alias for Martin Sponnich.
6. An alias for Michael Dowell.
7. Bollywood 2011 Movie Rockstar
It may also be in incorrect artist tag for video game soundtracks like the Grand Theft Auto titles. If so, do yourself and Last.fm a favour and fix your tags.