When light from a star like our sun is split into the spectrum of colours which make it up, dark lines are seen over the top of the rainbow of colours. These dark lines are due to absorption by different chemical elements in the star. An analogy can be made with barcodes which tell you the price and other details of a product in a shop. The stellar barcode tells you how hot the star is and how massive it is, as well as what stage in its lifecycle it is at. Astronomers use these stellar barcodes to work out the distance of stars and it is one of the key ways we know how large our galaxy actually is.

Dr David Jenkins
Department of Physics
The University of York