Light from a distant quasar, the bright core of a galaxy, passes by a massive galaxy and bends, like a flashlight beam through uneven glass. If the lens and source align almost perfectly, we see not one quasar but several copies. Astronomers have found the smallest such system: a radius of just 0.2 arcseconds, like spotting a coin from 200 kilometers away.

The lensing galaxy sits at redshift 1.055, meaning we see it as it was 5.5 billion years after the Big Bang. Its mass is only about 20 billion solar masses, very light for a lensing galaxy. This very lightness and compactness allowed scientists to reconstruct with unprecedented precision what stars populate the galaxy's center.

It turns out the stellar mass distribution in this galaxy's center resembles what we see in the Milky Way: many low-mass stars, few high-mass ones. This contradicts the classic picture where galactic cores form quickly and remain unchanged, accumulating many massive stars. So either the core grew slowly, adding stars gradually, or early collisions and mergers 'reshuffled' its composition, ejecting massive stars or preventing their birth.

That tiny lens, which might seem like a curiosity, became a window into the early history of galaxies. Light that bent around invisible mass told us how stellar cores, the building blocks that grew into modern giants, were assembled.