The life cycles of stars are linked to their masses. Small stars, like red dwarfs, may shine for trillions of years, whereas the most massive stars explode just a few million years after their births. As stars like the sun start to die after billions of years, they transform into a class called red giants that expand hundreds of times in size, consuming anything within their advancing borders.

Signs of engulfment events are littered across the Milky Way. The light of some stars is polluted with the chemical signatures of planets, suggesting that whole worlds are being digested before our eyes. Scientists have also spotted hundreds of planets with small orbits that are doomed to fall within the radius of red giants in the future.

But while stars clearly consume the occasional planet, capturing this moment is challenging because the light sparked by these events is faint and ephemeral. In fact, Dr. De was using the Zwicky Transient Facility, a camera on a telescope at the Palomar Observatory in California, in May 2020 to look for something completely different — merging stars, called red novas. It was in those observations that he stumbled across a curious burst of visible light.

What unfolded was like a “detective story,” Dr. De said. To identify the burst, his team obtained visible-light observations of the source captured in November 2020 by the W.M. Keck Observatory in Hawaii. Those images revealed a star chilling at about 5,000 degrees Fahrenheit, about 10 times colder than the searing temperatures expected from red novas.

Puzzled, Dr. De and his colleagues observed the star again, this time in infrared light, using another camera at the Palomar Observatory and NASA’s NEOWISE space telescope. The system turned out to be brilliant in infrared, a band of the light spectrum that is ideal for spotting faint objects that don’t emit much energy. It dawned on the researchers that they were most likely watching a star gulping down a planet in real time.