What Is A Star Note?

By GovMint :

A Star Note is a piece of paper currency produced by the U.S. Bureau of Engraving and Printing as replacements for banknotes that were found to have errors during the printing process. Instead of discarding the flawed notes and their corresponding serial numbers, the Bureau issues Star Notes as replacements, which are identical in every way except for a tiny but significant detail—a star symbol at the end of the serial number. This star differentiates them from standard notes and adds an intriguing layer to their story.

Sources Used:

https://money.howstuffworks.com/why-do-some-us-bills-have-a-star-instead-of-a-letter-at-the-end-of-the-serial-number.htm