Haining Li

A galaxy is like a cosmic highway where stars, like cars, race along circular orbits. Their speed is set by the invisible "cargo" of mass enclosed within the orbit: the more massive the central region, the faster the stars at the periphery.

15 175 107 stars. Each one got an age. But no number is free of uncertainty: a star's age isn't a nail hammered into the timeline: it's a probability cloud, a fog hiding the true value. For galactic evolution, such a timestamp is priceless: without it, the map of the Milky Way remains flat.

In the vast choir of galactic nuclei, some voices ring out louder than the rest. In quasars whose spectral nitrogen lines are anomalously strong, the chemical makeup of the gas in the broad-line region (the gas whipping around the black hole at enormous speeds) long remained a puzzle. Astronomers argued: is all the gas there simply rich in metals, or has nitrogen in particular been added beyond measure?