It's official: Zach Sobiech is a rock star.

The Stillwater High School senior — whose song "Clouds" went to the top of the iTunes chart just as he was about to be buried last week — now has the No. 1 single on Billboard's Rock Digital Songs chart as well as No. 26 on the big one, the Billboard Hot 100.

"Clouds" was written as a farewell to family members and friends after doctors determined that Sobiech had a terminal case of osteosarcoma, a rare form of cancer. The budding singer/songwriter died May 20, just 2½ weeks after his 18th birthday.

"The past few days have been very sad, exhausting and on the level of the fantastic all at once," said Scott Herold, founder of Rock the Cause, the St. Paul-based music nonprofit that is acting as Sobiech's record label. "I've been updating Zach's family, saying, 'I hate to talk business at a time like this.' But their response is, 'No, this brings us happiness. This is what Zach wanted.' "

Proceeds from sales of the single and a seven-song album he released in February are going to the Children's Cancer Research Fund in Sobiech's name. More than $100,000 had been raised by last week.

Clearly, that was just the start. Because of the project's nonprofit status, Sobiech's recordings earn a higher-than-usual royalty rate.

"Clouds" was rising in popularity when news broke last week of Sobiech's death.

Two weeks earlier, "The Office" star and former Guthrie Theater actor Rainn Wilson released a video he put together with other TV celebrities and musicians singing parts of "Clouds," including Jason Mraz, Jenna Fischer, Bryan Cranston, the Lumineers and Sarah Silverman.

Tribute on 'Katie' Thursday

Originally recorded on his cellphone with an acoustic guitar, "Clouds" became a viral hit on YouTube last summer. It was then formally recorded with help from Rock the Cause and released as an iTunes single in December.

That was followed in February by "Fix Me Up," an album released under the band name A Firm Handshake that includes "Clouds."

Sobiech's collaborator in the group, fellow teen musician Sammy Brown, is scheduled to appear Friday with some of his other friends on "The Katie Couric Show," which airs at 2 p.m. on KSTP, Channel 5.

Thanks to media reports and widespread social media attention — though little radio play, which is a factor in Billboard charting — "Clouds" amassed 156,000 downloads in the week ended Sunday, according to Billboard, which ranked it at No. 7 overall among digital songs. "Fix Me Up" also sold 19,000 to rank at No. 20 on Billboard's album chart and No. 1 on the folk albums chart, where it ranks above such hot acts as Mumford and Sons and the Lumineers.

New York music executive Jeff Stempeck, whose international distribution company the Orchard tallied the sales of Sobiech's recordings, called his success an "incredible and unique" feat.

"Generally we see an artist's sales sprint out of the gates in the first or second week of release, but 'Clouds' grew and surged as Zach's story did," Stempeck said. "The song didn't need millions of dollars in marketing or radio support to resonate. It just needed people to hear Zach's story, and it clicked."

Sobiech's uncle Luke Friedrich, who is also a musician, said the family is still mourning, "and hearing that song on the radio can be pretty difficult."

However, he said they are thrilled with the numbers "because it offers physical, tangible evidence of how many people were affected by Zach's music, and how much good he is going to do to help end [osteosarcoma] or come up with a treatment for it."

Chris Riemenschneider • 612-673-4658