The stars is in the building,
Jay and Ye have arrived. Coincidentally, Kanye has a sad history of considering “stars” to be a singular noun
Note that the Watch the Throne album release party was held at a planetarium in New York’s Natural History Museum
This line may also be playing on the phrase “Elvis has left the building,” which is often said when one means to bring events to an effective end. Elvis Presley was one of the earliest popular music icons and was notable for causing hysteria among his fans. Kanye is equating his and Jay-Z’s popularity and the effect they have upon their fans with that of Elvis. Moreover, by turning the phrase on its head (“left the building” vs “is in the building”), Kanye is suggesting that the show is about to begin (figuratively speaking) and people should get excited.
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