From Duke to Stardust, the many faces of David Bowie birthed movements in musical culture – each one different from the last. As he disappeared into the night this week, he did so far from quietly, leaving behind his final opus. Backstar is, in many ways, the mark of a new Bowie, the latest incarnation of our favorite eccentric. But at the center of the final chapter, Bowie placed a feature which many of us have come to remember him by: his eyes.

Like most everything about the man who fell to Earth, Bowie’s eyes were at odds. The right, a clear blue. The left, a moody green – as if they too, hailed from different planets. Most people assume Bowie to be among the few with complete heterochromia, a rare condition by which a person (or, more commonly, a non-human animal) has different-colored irises. Chances are, you’ve seen this before in a husky or house cat, but if you look closely, you’ll notice that Bowie’s ocular anomaly was something else entirely. His eyes weren’t different colors at all.

The left eye, rather, had a permanently dilated pupil, the mark of a condition known as anisocoria.

Your irises do more than lend color to your eyes. The structures consist mostly of smooth muscle, which can contract and relax to control how much light enters the pupil, like a camera’s aperture. If you shine a light into your eyes, you’ll see this happen – the iris contracts, the pupil closes, and as a result, you aren’t blinded by the change in brightness.

Bowie’s left eye, however, was unable to react in this way. As story has it, the damage was caused by friend George Underwood, who socked Bowie during an adolescent show of bravado back in 1962. Just how much of this is true, we’ll never know, but trauma like this can certainly cause anisocoria if the muscles that contract the iris are damaged. In fact, the same thing happened to my uncle, as seen in the photo below.

Because the pupil is fixed in the open position, the injured eye as a whole appears much darker than the unaffected eye. This tricks your brain into perceiving the iris as being darker as well.

Underwood went on to perform countless collaborations with Bowie, who reportedly thanked his friend for the injury. Those otherworldly eyes became, like so many things along the way, a part of who David Bowie was.

“I always had a repulsive need to be something more than human,” he said. “I felt very puny as a human. I thought, ‘F**k that. I want to be a superhuman.’” And he was.

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IMAGES: David Bowie/YouTube