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She may be one of the biggest names on the planet, but Katy Perry still gets starstruck.

The pop superstar straight up fangirled on July 19 as she headed to Las Vegas to see Mariah Carey perform at her residency show.

Katy's fandom was on full display on her Instagram as she put up not one, not two, but three posts from her evening.

"Lambily 4lyfe," she captioned a photo of her in the shadow at an oversized poster of Mimi at Caesars Palace (Lambily is the name of Mariah's fan community.) In another video of the two divas posing for pictures backstage together, Katy simply captioned the post, "moments."

A fan actually saw Katy inside the 90-minute show shooting a video of the "Heartbreaker" singer, as well.

Mariah, too, seemed all to ecstatic that Katy attended to her "#1 To Infinity" show.

"Thank you to @katyperry and her friends for coming to the show," Mimi captioned the same image that Katy posted. "You looked beautiful!"

By the way, Mariah is 45-years-old; Katy is 30. We're gonna go out on a limb here and say that by meeting Mariah, Katy just lived out a "teenage dream." But, did she see Mariah's closet?