Usually when astronomers analyze the spectra of stars, the chemical fingerprint of iron can be spotted. The more iron in the star, the younger it is. Each subsequent generation of star fuses more and more iron in their cores. As each generation of star reaches the end of its life and explodes as a supernova, the iron (and other heavy elements) from its interior is blasted throughout space. This iron intermingles with other interstellar gases that clump together, collapse and ignite to create the next generation of stars.
The iron fingerprint can therefore be used to "age" any stellar object, much like the rings in a log can be used to age a tree.
"We can use the iron abundance of a star as a qualitative ‘clock' telling us when the star was formed," said Keller.
SMSS J031300.36-670839.3, however, has no detectable sign of iron. Even within the margins for error, and astronomers assume an upper limit on the quantity of iron it contains, the star is still dated 13.6 billion years old. Previous "oldest star" record breakers have been dated to 13.2 billion years old.