X-ray sources in the sky radiate "bar codes" revealing their properties. These bar codes consist of narrow peaks and troughs within the source's spectrum. The RGS instrument on the satellite XMM-Newton was built to find these bars—or spectral lines. Astronomer Junjie Mao and his colleagues at SRON and ESA have now created a catalogue filled with 12,000 X-ray lines, which astronomers can use to conduct large-scale surveys among the stellar population. They have reported its development in Astronomy & Astrophysics.