How long are astronomical papers remembered?

For the 165 papers published in the Astrophysical Journal and Supplements in 1954, we counted the citations during 1955-1994. They show an exponential decay with a half-life of 29 yr. Papers receiving more citations than others during the first five years do not have longer half-lives. This sample shows that observational papers have an average half-life (35 yr) that is somewhat longer than for theoretical papers (22 yr). Longer papers, such as Supplement ones, are cited more frequently on the average than short ones, such as Journal papers and, especially, Notes. Excluding Notes, 57% of the 1954 papers are still cited in 1990-1994. These durations are so long primarily because the field has been growing so rapidly-by a factor of 24 in 40 years. If normalized to a constant literature output, the average half-life would have been 6 yr. Another pertinent effect is caused by the changing fields of interest. Because studies of extragalactic objects, relative to other fields, are now seven times more frequent than in 1954, citations to them have remained nearly constant in 40 years while citations to papers in other fields have dropped off more quickly.