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Superstar race car driver Dale Earnhardt Jr. has abruptly announced today that he will retire at the end of the 2017 NASCAR racing season, capping off a career that has spanned 18 seasons and more than 600 races.

Although the announcement comes as a shock to fans, it's a decision that has been brewing since at least March 29, when Earnhardt Jr. first broached the subject with his team owner Rick Hendrick, according to a statement on the Hendrick Motorsports team's site.

The 42-year-old has been plagued by concussions over the last few years, the latest of which resulted in his having to sit out half of last year's racing season while recovering from a concussion believed to have been sustained during a crash in June 2016.