What happens when a star dies in battle with the black thing

When it touched the darkness, the light disappeared, and the Medium assures them that “it can be overcome.” When Charles Wallace asks what is happening, Mrs. Whatsit tells them that it was a star doing battle with the Dark Thing. The star won, but lost its life in the process. Mrs. Whatsit then informs them that she herself had once been a star and had done battle with the Dark Thing, which is why she has fallen into her current form.