The Star of David (a star with six points, not five) is a pretty in-your-face symbol. Star of David = Judaism. Voilà.
Since we're talking about Number the Stars here, it's important to remember that the Star of David is often associated with the Holocaust and World War II. Here's why: in many occupied countries, the Nazis singled out Jewish people by making them wear yellow Stars of David pinned on their clothes. To be seen wearing the star put you in danger. But to be caught taking one off would make things even worse.
Taking off a Star of David… hmmm… sounds familiar. Oh yeah! That's exactly what Ellen does when the Nazi soldiers come to the Johansens' house. She has to remove the necklace in order to hide her identity from the Nazis.
But Ellen is still Ellen—and she's definitely still Jewish—after she takes off the necklace. That serves as a helpful reminder for us readers: a symbol is just a symbol. And it's a symbol that Annemarie can adopt, too. By wearing the necklace in Ellen's place, Annemarie is making a statement: it's okay for her friend to stop hiding and honor those parts of her identity.
Ellen's Star of David necklace reminds her of her Jewish faith and her relationship with her father who gave her the necklace. And by focusing on the Star of David as a representation of Ellen's identity, Lois Lowry protects this important symbol from the Nazis' cruelties. In using a corrupted version of the symbol, the Nazis hoped to repress the Jewish faith. But Lowry turns that very idea on its head, reminding us that no matter what, the Star of David is a source of pride, not a tool of oppression.
(We've got a lot more for you to chew on, don't worry. For more on Ellen's necklace itself and what it means to Ellen, check out "Tools of Characterization." For more on why stars in general are so important in the book, check out "What's Up With the Title?" But don't forget to come back to Symbols—this book is loaded with 'em.)