Symbolism, Imagery, Allegory
You've already heard that The Hours is big on "time," and it stands to reason that a novel obsessed with the passage of time might have a thing or two to say about mortality and immortality, too.
It may surprise you to hear that celebrity, of all things, is its most potent symbol of immortality in The Hours. If that seems a little too shallow for your tastes, keep in mind that the symbol comes from the mind of Clarissa Vaughan—a woman who recognizes and who chooses to embrace her slightly shallow qualities.
Clarissa is fascinated by celebrity, and she seems to find it comforting to imagine that even after her own generation and others after it have faded into history, movie stars like Meryl Streep, Vanessa Redgrave, and Susan Sarandon will still be remembered and known (4.9). She finds it even more comforting to imagine that her dear friend Richard Brown might still be remembered long after his death, with his name and his poetry enshrined for keeps in the literary canon (4.112).
This kind of immortality is the only form of life after death that any of the novel's characters imagine, and so celebrity stands as the novel's best and brightest symbol of resistance to the passage of time.