The Mary Sue has a great post on optics and the reason why we draw stars with spikes.
Draw a star. It probably looks something like the ones in the picture above. Stars have points when we draw them, but why? Real stars, the ones in space, are round. So how did we get to the pointed star shape from balls of fire incandescent plasma?
One explanation might be diffraction spikes, optical artifacts created when light passes through a reflecting telescope. It results in what looks like rays shooting out from the center. The spikes are due to light bending around the support rod structure that holds the reflecting element of a telescope in place. Therefor, the pattern of the diffraction spikes mimics that of the support rods in a telescope.
Here’s a photo of the support structure of a reflecting telescope:
And this is an example of diffraction spikes in a photograph of stars:
Makes sense, right? Except, humans have drawn stars with points well before the invention of the reflecting telescope in the 17th century. The paint L’Adoration des Mages was done in the 15th century by Jean Fouquet. Check out those sweet diffraction spikes!
Since Jean Fouquet clearly didn’t have access to a reflecting telescope, what with dying more than 100 years before its invention and all, what’s happening here? Simpler telescopes that relied on lenses only predate the reflecting telescope. They began appearing in 1608, but they don’t predate this painting.
Any time light passes through a lens, it becomes somewhat distorted. If you wear glasses or have ever looked through the windshield while driving at night, you know this already. Glasses date back to the late 13th century, so maybe Fouquet used them? This certainly isn’t the only example of a spiky star in art, and some date back even further.