What substance is causing the Sun to dim in Project Hail Mary?

Astrophage is the substance causing the Sun to dim in Project Hail Mary.

In the story, astrophage is a fictional single-celled organism that feeds on electromagnetic radiation. It reproduces near stars and absorbs their energy, causing the affected stars to grow progressively dimmer. The Sun’s declining output threatens Earth with catastrophic cooling, creating the crisis behind Project Hail Mary.

The name combines the Greek-rooted ideas of “star” and “eating,” essentially meaning “star-eater.” Ryland Grace studies the organism and helps establish how it behaves, including its connection to the infrared phenomenon called the Petrova Line.

The distractors belong to different parts of the story or other science-fiction traditions. Taumoeba is a separate organism that preys on astrophage, while xenonite is a fictional material used in spacecraft systems. “Red matter” is associated with the Star Trek universe, not Project Hail Mary. The astrophage concept is central to both Andy Weir’s 2021 novel and the 2026 film.