In this study we compare the titles of scientific letters and those of research papers published in the field of astrophysics in order to identify the possible differences and/or similarities between both genres in terms of several linguistic and extra-linguistic variables (length, lexical density, number of prepositions, number of compound groups, number of authors and number of countries mentioned in the paper bylines). We also carry out a cross-genre and cross-journal analysis of the referred six variables. Our main findings may be summarized as follows: (1) When compared to research paper titles, scientific letter titles are usually shorter, they have a lower lexical density, they include a higher number of prepositions per number of words and a lower number of compound groups per number of words, although they have more up to 4-word compound groups, i.e. the simplest ones. As a consequence, scientific letter titles include less information, which is also less condensed, than research paper titles. (2) The predominance of compound adjectives over compound nouns in the titles of both genres highlights the scientificity of astrophysical discourse. (3) In general terms, our data show a positive correlation between title length and the number of countries mentioned in the bylines for both genres. The positive correlation between title length and number of authors is only met in the case of research papers. In light of these findings, it may be concluded that scientific letters are a clear example of a timeliness and more “immediate” science, whereas research papers are connected to a more timeless and “elaborate” science. It may also be concluded that two different collaboration scenarios are intertwining on the basis of three separate geographic and linguistic publication contexts (Mainland Europe, The United Kingdom and The United States of North America).