There is a specific symbol that appears on many pages, but it's not a star. It's a circle with a line through it, resembling a crossed circle. This symbol is called a "Restricted Section" symbol and is used throughout the books to indicate pages that are part of the Restricted Section of the Hogwarts library.
These pages contain information that is deemed too dangerous or forbidden for most students to access, making the symbol a recognizable marker of secrecy and danger within the Harry Potter universe.