Murphy died from pneumonia on Dec. 20, 2009.
The 24-year-old actress posted a shot of her and Murphy in their 2003 film Uptown Girls, which tells the story of a childish woman (Murphy) who starts nannying a mature 8-year-old girl (Fanning).
“Brittany Murphy was a ray of sunshine that made every day of filming magical for me,” Fanning wrote on the flashback post. She added the hashtag “#uptowngirls” with a sunshine emoji.
Murphy remains best known for Clueless (1995) and 8 Mile (2002) with Eminem, but also costarred in Girl, Interrupted (1999) with Angelina Jolie, and led the romantic comedies Little Black Book (2004) and Just Married (2003) — the latter in which she starred alongside her real-life boyfriend at the time, Ashton Kutcher.
The Atlanta-born star also made a splash on the music scene with DJ Paul Oakenfold via their single “Faster Kill Pussycat,” which became a club hit and reached No. 1 on Billboard’s Hot Dance Club Play chart in 2006.
On Dec. 20, 2009, Murphy collapsed in the bathroom of her Hollywood Hills home and died after battling flu-like symptoms for a few days. She was 32.
The L.A. County coroner concluded that Murphy died as a result of pneumonia combined with anemia and “multiple drug intoxication” from prescription and over-the-counter medication.
“She was really sick with pneumonia, very anemic, and she was taking medication,” Coroner Asst. Chief Ed Winter told PEOPLE at the time, “and all that combined killed her.”
Just five months after Murphy’s death, Murphy’s mom Sharon found Murphy’s husband Simon Monjack dead at 40 in the same bedroom where her daughter died. His cause of death read exactly the same as his Murphy’s — a case of pneumonia and anemia.
As news of her death broke, her fans, friends and costars began sharing tribute messages to the late star.
One of the most heartfelt came from Kutcher. “[Today] the world lost a little piece of sunshine. My deepest condolences go out [to] Brittany’s family, her husband, & her amazing mother Sharon. See you on the other side kid,” he wrote on Twitter.