Arnold Palmer, Jack Nicklaus

Arnold Palmer was golf's first celebrity star when the sport became mainstream television in the late 1950s and early 1960s. The second superstar was not far behind. Jack Nicklaus, ten years Palmer's junior, became a main rival to Palmer in the 1960s.

Although their competitiveness produced some of golf's greatest finishes, Palmer and Nicklaus separated their rivalry on the course from their relationship off the course. "The King" and "The Golden Bear" became close friends during and after their careers. Palmer, who passed away Sunday at age 87, served as a lifelong ambassador to the sport, while Nicklaus, 76, continues to promote the game of golf.

On Sunday evening, a somber, but thoughtful Nicklaus posted a tribute to Palmer on Facebook:

The highlights:

-- Nicklaus advises others not to get caught up in the cause of death. "At this point I don't know what happened, and I suppose it is not important what happened," Nicklaus says. "What is important is that we just lost one of the incredible people in the game of golf and in all of sports."

-- Nicklaus last spoke to Palmer on Sept. 10, Palmer's 87th birthday.

-- This is a great line: "Arnold transcended the game of golf. He was more than a golfer or even great golfer. He was an icon. He was a legend."

-- Nicklaus explains their balance of competitiveness and friendship: "We were great competitors, who loved competing against each other, but we were always great friends along the way. Arnold always had my back, and I had his. We were always there for each other. That never changed."

-- Nicklaus, whose 18 major championships are the most all time, more than double Palmer's seven major titles, respects Palmer's nickname prowess: "He was the king of our sport and always will be."

Tiger Woods, less than half Palmer's age, also posted about The King's sudden passing:

-- Follow Jeffrey Eisenband on Twitter @JeffEisenband.