Symbolism, Imagery, Allegory
Of course it's a symbol—it appears in the stinkin' title. But the narrator is careful to tell us not to get excited: don't "expect stars in what follows, he says, "for nothing will scintillate" (1.20). But as it turns out, we do actually get stars in the text, and more than one of them.
Star of Stage and Screen
The first important "star" comes toward the beginning of the novella as the narrator explains how Macabéa knows what she needs to know, and she knows these things instinctively more than through intelligence. Experience teaches her how to know: "lots of things just as a dog knows how to wag its tail or a beggar how to feel hungry: things happen and you suddenly know" (3.63).
And the same thing is going to happen when she dies, "as if she had already learned by heart how to play the starring role. For at the hour of death you become a celebrated film star …" (3.63). And, guess what? Macabéa actually does have a dream of being a film star, just like Marilyn Monroe.
Star of Her Own Life
But at the end, Macabéa's death doesn't seem very celestial. The narrator even tries to deny it by turning Macabéa into the film star she secretly dreams of being: "To my great joy, I find that the hour has not come for the film-star Macabéa to die" (5.438). But alas, even against the narrator's wishes, Macabéa does eventually die.
And check out what happens: right before she dies, Macabéa feels like vomiting something luminous, "a star with a thousand pointed rays" (5.444). It turns out that the star is actually already inside her, and she doesn't need to become it. She carries around with her an eternal spiritual abundance, represented by this regurgitation of inner light.
So how do we make sense of these two contradictory meanings of "star"? One the one hand, Macabéa wants to be a film star, someone who glows up on screen. On the other hand, she already is a star: a star of light, hope, grace, and of, course, the star of this novel. Maybe being a film star is the only way Macabéa has of understanding the concept of the celestial.
Or, maybe—considering this text's root in the individualism of Existential philosophy—the star represents the way we're all the unique, shining centers of our own lives, no matter how pitiable.