Out of order

摘要

Galaxies are like people: the better you get to know them, the more peculiar they often seem. Classifying and explaining the appearance of objects that occur in such an array of shapes, masses and sizes is a major challenge. As astronomer Allan Sandage wrote recently: "The danger in devising a classification from scratch is that, if the classifier has preconceived notions of what those explanations might be, then the classification becomes circular if it is later used to provide the explanation." To complicate matters, the Hubble Space Telescope (HST) has shown that the appearance of galaxies, like that of people, changes systematically with age.