What does 'but not for twenty years' mean?

[joham]

Michelson was awarded a Nobel Prize in physics for the work--the first American so honored--but not for twenty years.

I'm not clear about what the phrase 'but not for twenty years' mean. Is it saying that Michelson was awarded a Nobel Prize in physics for the results of the work but not for his twenty years of work?

Thank you in advance.

 

[Barb_D]

It was 20 years later that he received the award. He did not receive the award until 20 years had passed.