In 2007, Dutch schoolteacher Hanny van Arkel was volunteering for Galaxy Zoo, an online project that invited ordinary people to help astronomers classify thousands of galaxies photographed by telescopes.

While examining one of the images, van Arkel noticed something that looked completely different from the galaxies around it: a bright, mysterious green cloud floating near a distant galaxy.

She posted about the strange object online, and professional astronomers soon began investigating. The object was eventually named Hanny’s Voorwerp, with voorwerp meaning “object” in Dutch.

What made the discovery particularly strange was that the cloud appeared to be glowing even though there was no obvious powerful light source nearby.

Further observations, including images from the Hubble Space Telescope, helped scientists develop an extraordinary explanation. The cloud is believed to be made largely of ionized gas that was illuminated by an extremely powerful quasar in the neighboring galaxy.

The quasar appears to have become dramatically fainter long ago, but its light continued traveling outward and interacting with the surrounding gas. In effect, astronomers were seeing an echo of an ancient cosmic event—almost like a delayed message from a quasar that had already changed.

Van Arkel’s discovery became a landmark example of citizen science. She wasn’t using a professional observatory or working as a trained astronomer. She simply noticed something unusual that thousands of other people had overlooked.