Nebulae or galaxies? The history of a change in astronomical terminology

Abstract

While today the term ‘galaxy’ is a household word primarily associated with the huge congregations of stars far outside the Milky Way, until about 1950 these structures were generally known as ‘nebulae’. Although the singular form ‘galaxy’ was commonly used, it was typically considered as just another word for the Milky Way. Edwin Hubble’s celebrated discovery in the 1920s of the extragalactic nature of the spirals was for a long time referred to as the discovery of ‘extragalactic nebulae’, whereas the plural term ‘galaxies’ was only employed by a minority of astronomers. In fact, it took until about 1960 before our current usage became commonly accepted and the spiral nebulae became spiral galaxies. The paper focuses on the words of and related to extragalactic astronomy as they were used historically in the period from about 1920 to 1960.