Glow-in-the-dark stars feel like a staple of childhood. You may have stuck them to your bedroom ceiling in fifth grade and then discovered them, on a visit home decades later, still lighting up faithfully every evening. There are now glow-in-the-dark sheets, glow-in-the-dark pajamas, glow-in-the-dark paint, and so much more.

But what exactly is glowing inside of these objects? And is the eerie glow that makes them so alluring really safe?

Where the glow comes from

A number of minerals phosphoresce naturally; put them in the dark after they have been illuminated for a while, and they will glow. Zinc sulphide treated with copper is one substance commonly used in glow-in-the-dark toys, says Dean Campbell, a professor of chemistry at Bradley University in Peoria, Ill. Light striking it caus

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