The rules for the Nobel Prize in Physics require that the significance of achievements being recognized has been "tested by time". In practice, that means that the lag between the discovery and the award is typically on the order of 20 years and can be much longer. For example, half of the 1983 Nobel Prize in Physics was awarded to Subrahmanyan Chandrasekhar for his work on stellar structure and evolution that was done during the 1930s. As a downside of this tested-by-time rule, not all scientists live long enough for their work to be recognized. Some important scientific discoveries are never considered for a prize, as the discoverers die by the time the impact of their work is appreciated.