After careful consideration, we have divined four main reasons pop stars show up on television shows with such great frequency: 1) It’s a desperate ratings ploy by the network to give advertising execs and marketers something to hype. 2) It’s a desperate ploy by a music label to get a musician some additional exposure. 3) Singers often want to break into the acting biz after making a video or two. 4) The singer loves the show, the show loves the singer -- hey, we should get together and do something sometime!

There is an endless supply of pop stars making television cameos -- No Doubt (pictured) will perform on "Gossip Girl" and The Veronicas will rock the "90210" prom -- but only a few of these have been truly memorable, momentous or successful. Here are a few of the very best.

Compiled by Deborah Netburn, Patrick Day, Todd Martens, Stephanie Lysaght and Kate Stanhope, Los Angeles Times staff writers