This number is a prime.
The 509th Composite Group (an air combat unit of the U.S. Air Force) fulfilled its mission when the Enola Gay piloted by Colonel Tibbets dropped the first atomic bomb on Hiroshima. [McCranie]
509 was the number of the room in which the character Eve (played by Anne Baxter) in the movie "All About Eve" stayed in a hotel in New Haven. [Litman]
The sum of three consecutive squares (122 + 132 + 142). [Schlesinger]
In poker, there is one chance in 509 of receiving a flush (a mere flush, not a straight flush or a royal flush) when dealt 5 random cards from a deck of 52 playing cards. [Green]
The smallest multidigit octanacci prime number. (The sequence of octanacci numbers starts with eight predetermined terms, the first seven are zeros and the 8th is 1, each term afterwards being the sum of the preceding eight terms). [Loungrides]
The sum (p) of three consecutive squares up to and including the (5+0+9)th is 509. [Worrom]