Chris Pratt and Katherine Schwarzenegger made their relationship official on Thursday.

The Guardians of the Galaxy actor posted a collage of photos of himself and his 29-year-old girlfriend for her birthday. In his caption he gushed about the birthday girl, writing that she "lights up the room."

"Happy Birthday Chief! Your smile lights up the room. I've cherished our time together," Pratt wrote. "Thrilled God put you in my life. Thankful for the laughs, kisses, talks, hikes, love and care."

Pratt's collage included photos of himself kissing Schwarzenegger, a picture of her giving him a pedicure and a snapshot of the couple sporting animal sheet masks. It also included three candid photos of Schwarzenegger smiling.

Their relationship, which dates back to around June, is heating up, a source recently told ET Online. They're said to be in a "serious  relationship and everything is going well." Don't expect to see these two planning a wedding anytime soon, however.

"Chris just finalized his divorce from Anna [Faris] a couple months ago, and he's happy with just dating Katherine for now. The couple has a lot of fun together and is fully involved in one another's life," the source said, adding that they're "in a committed relationship. There just isn't a rush to take it to the next step right now. Chris is enjoying the dating process."