When two neutron stars collide, a kilonova is born — a cosmic explosion that shines not only from heat but also from the decay of freshly created heavy elements. Among them is neodymium, a rare-earth metal that hides in minerals deep within the Earth, but in space, for a brief time, becomes the main conductor of light.

Astrophysicists simulated such a flash on a computer using three different sets of data on how neodymium atoms absorb and emit light. The result was surprising: the brightness of the explosion in the infrared range changed by almost 1.5 times depending on the chosen tables. The spectrum, the rainbow stripe by which scientists determine composition, also became noticeably distorted, especially in the near and mid-infrared light.

The cause turned out to be in the description of neutral neodymium (Nd I)—an atom with all its electrons in place. Even small inaccuracies in calculating its energy levels lead to large changes in the spectrum. The scientists also tried calibrating the data against the NIST reference database, and again saw strong differences in the infrared part.

This means that for accurate modeling of kilonovae, including the famous AT2017gfo, special attention must be paid to the atomic physics of each rare element. Otherwise, we risk misinterpreting how much gold, platinum, or uranium was born in such an explosion.