Meet G117-B15A: the most stable optical clock in the Universe

Not too far away in galactic terms — 187 light years from us — is the remains of a dead star. It was once like the Sun, though perhaps six or so times more massive, and 400 million years ago it reached the end of its mainstream life. It swelled into a red giant, cast off its outer layers, and eventually lost so much material it exposed its hot, dense core to space.

This kind of object is known as a white dwarf, and is the end state for some 90% of all stars in the sky. But this particular one, ...