Caught in the act: Astronomers spot star swallowing a planet for first time

Roughly 5 billion years from now, our Sun will end, not with a bang but with a whimper. That’s when it finally burns through all the fuel in its core and puffs outward into a red giant, swallowing all the inner planets of our Solar System in the process, including Earth. But no star has ever been caught in the act of gulping down a planet this way—until now. Astronomers have spotted a white-hot flash from a distant star in our Milky Way galaxy and concluded that it came from the final stage of this process, according to a new paper published in the journal Nature. Yes, it’s a literal “Death Star,” announced on the eve of Star Wars Day (May 4).

Like all stars, our Sun is a giant nuclear reactor, made up of hydrogen and helium gas. To produce light, it converts hydrogen into helium through fusion reactions, and that energy pushes back against the crushing weight of its outer layers. When all the hydrogen is turned into helium, the Sun will no longer generate sufficient energy to counteract the pull of gravity. The core will contract inward, heating up the interior to sufficient temperatures to fuse helium into carbon. Hydrogen fusion will migrate to the outer layers. The Sun will puff outward and turn into a red giant, expanding those outer layers as the core collapses. The outer layers will engulf the nearest planets, including Earth, which will plunge into the core and be vaporized, triggering an outburst of energy and matter. The Sun will continue to burn for a few more billion years before it ultimately cools into a black dwarf star.

This process only occurs a few times a year in the Milky Way. Astronomers have observed the early stages of the process (planets so close to their host stars that they will inevitably be engulfed when those stars expand) along with the aftermath of this stellar evolution (when the stars have puffed up and seem to have peculiar properties, such as their rotational speed or chemical composition). But scientists have never witnessed the actual devouring. That’s what makes this discovery so exciting, according to co-author Kishalay De, an MIT postdoc: This is the first direct evidence of a crucial stage of stellar evolution.