"POP STAR" was written and composed by Hirai, and the lyrics have the protagonist wanting to become the titular pop star not particular for fame or money but just so that he can continue to have the undying love of his admirer (awww...). According to the J-Wiki article, the singer-songwriter really wanted to have that sense of brightness and optimism imbued into the song so when he went to consult with arranger Seiji Kameda(亀田誠治)about how it was to be brought together, Hirai had him listen to this particular aidoru classic.
And yet initially, he hadn't intended for "POP STAR" to be placed into any particular album but due to overwhelming popular demand, it finally got into his BEST compilation, "Ken Hirai 10th Anniversary Complete Single Collection '95-'05 Uta Baka" (Ken Hirai 10th Anniversary Complete Single Collection '95-'05 歌バカ...Singing Fool) which was released about a month after the single. As it was, though, "POP STAR" did perfectly fine on its own, going Platinum as it hit No. 1 on the Oricon weeklies and quickly becoming the 45th-ranked song of the year. Strangely enough, even the video apparently got a number of accolades, inside and outside of Japan, as Hirai took on 7 different characters including a raccoon. It's too bad that I couldn't find the music video online but it's nice to know that the man has a good sense of humour. As for that BEST album, it also hit the top of the charts and became the top-selling album for 2006. Perhaps I ought to check a bit more into Ken's discography after all.