A New Catalog of Stellar Rotation Periods for over a Million Stars

- **Kepler Space Telescope** – a NASA observatory (2009–2018) that continuously monitored the brightness of ~200,000 stars in a single patch of sky to find exoplanets via the transit method. - **Gaia** – a European Space Agency space observatory launched in 2013 that measures the precise positions, motions, distances, and properties of nearly two billion stars in the Milky Way. - The paper combines Kepler's ultra-precise brightness measurements with Gaia's stellar properties (temperature, size, age) to determine rotation periods for over a million stars — a dramatic increase from previous samples of a few thousand. - **Why it matters**: A star's rotation slows predictably as it ages (like a spinning top) — the older a star, the longer its rotation period. This new catalog turns rotation into a powerful "clock" for figuring out the ages of millions of stars, which in turn helps us understand how planetary systems evolve and how the Milky Way formed.