There's a star called J0815+4729. It's very old and contains almost no iron, an element that forms in stars. Astronomers use the carbon abundance in such stars to infer what the first stars in the Universe looked like.
Previously, carbon in these stars was measured using lines from the CH molecule in the spectrum, which are like fingerprints that reveal chemical composition. But J0815+4729 has so much carbon that it could also be measured two other ways: via atomic lines and via C2 molecules. It turned out that CH shows less carbon than the other two methods.
This means that all previous measurements of carbon in the oldest stars, based solely on CH, may have been too low. Now astronomers need to recheck these data and figure out why CH is wrong. There's no practical application outside astronomy yet, but it's very important for understanding the early Universe.
J0815+4729 is so carbon-rich that its spectrum shows not only molecular bands but also weak atomic lines of neutral carbon: a rare opportunity for a star with [Fe/H]=−5.49. Among the 16 known stars with [Fe/H]<−4.5, it has the highest carbon abundance.