Like fireflies “dancing” on a warm summer night, 10 distinct star clusters appear in observations from NASA’s James Webb Space Telescope. They are held in a cocoon of diffuse light emitted by other stars scattered around them. This galaxy, known as Firefly Sparkle, was taking shape around 600 million years after the big bang.

Researchers using Webb’s images and data concluded that the Firefly Sparkle had the same mass as our Milky Way galaxy would have if we could “turn back time” to weigh it as it was assembling.

How is this analysis possible? A natural effect known as gravitational lensing, which allows researchers to “zoom in” on extremely distant objects that perfectly align with Webb’s view. When combined with the telescope’s near-infrared images and data, astronomers can study distant objects in astonishing detail.

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