A bright lantern in thick fog seems dim, while a weak flame in clear air can be seen a kilometer away. A similar oddity baffled astrophysicists almost half a century ago. In 1977, astronomer Jack Baldwin, while studying the spectrum (light spread out by color) of a quasar, noticed something: the more powerful the quasar — a supermassive black hole at a galaxy's center — the weaker the magnesium emission lines that appear against it. These lines arise in the hot gas surrounding the hole. One would think a powerful source should shine brighter at all wavelengths, but instead it specifically dimmed those lines.

The answer came when astronomers analyzed data from 41 159 quasars — collected by the Sloan Digital Sky Survey and the LOFAR radio telescope. It turned out to be all about gas density. This gas resides in what's called the broad-line region — a dense cloud swirling around the supermassive black hole. That's where the magnesium lines are born. Dense gas acts like fog: the thicker it is, the more the radiation scatters, and the weaker the lines appear against the bright continuum.

The key mechanism is the Eddington ratio. This number indicates how actively a black hole is accreting matter. The faster it grows, the more powerfully its radiation drives the gas outward. The gas becomes denser — and the magnesium lines blur, like details in a heavy fog. This is how the Baldwin effect weakens.

The discovery doesn't just solve an old mystery. Now, a quasar's spectrum is not just light from a distant beacon — it also tells a story about the fog it pierces through. By looking at the brightness of magnesium lines, astronomers can gauge how dense the gas near the black hole is, and thus how fast it is growing and how it affects the life of the entire galaxy. The lantern and the fog are now inseparable.