One of the sky’s brightest lights is losing its shine.

Since the start of December, the star Betelgeuse — the gleaming right shoulder of the constellation Orion — has been rapidly growing dim. Just 650 light-years from Earth, it’s usually the ninth most luminous star in the sky. Right now, it wouldn’t even break the top 20.

Betelgeuse is a “variable” star, known for wild fluctuations in its brightness, but scientists have never recorded it changing quite so fast. Such strange behavior from a beloved star has them wondering: Is this a sign that Betelgeuse is about to explode?

Astronomers know that day is bound to come. Betelgeuse is at least 15 times more massive than the sun and wide enough that, if we moved it to our solar system, it would extend past the orbit of Jupiter. “Supergiants” like this tend to live fast and die young, and Betelgeuse’s red color indicates that it has already moved to one of the last stages of a star’s life: fusing helium atoms into ever-heavier elements, which it occasionally spews into space.