509

Number
$509$ (five hundred and nine) is:


 * The $97$th prime number


 * The $26$th Sophie Germain prime after $2$, $3$, $5$, $11$, $23$, $29$, $41$, $\ldots$, $233$, $239$, $251$, $281$, $293$, $359$, $419$, $431$, $443$, $491$:
 * $2 \times 509 + 1 = 1019$, which is prime.