What does a US 100 dollar star note look like?

A "star note" of any denomination looks just like an ordinary bill of the same series, except that the beginning or ending letter in the serial number is replaced by a star to indicate that the bill is a replacement for one that was damaged during printing.

Bills are printed in bulk so it's impractical for the Treasury to reprint replacement bills with the same serial numbers as those that were damaged. Instead, the damaged bills are destroyed and an equal number of replacements are printed with a new set of serial numbers. One letter is replaced with a star to indicate that the entire run consists of replacement notes.