Free Astronomy Magazine July-August 2020

44 JULY-AUGUST 2020 ASTROBIOLOGY us), ventured into calculating the distance from the Earth to the star Sirius by assum- ing that the Sun was a typical star and rep- resentative of all stars. From the differences between the apparent brightness of the two stars and the assumption that both were similar, they obtained a distance of about half a light-year – a value proven to be underestimated (by just over eight light years) only after the subsequent awareness of the mul- tiplicity of the stellar classes. That said, the calculated distance of about half a light-year provided a realistic idea of the depth of the cosmos at the time. Just as the astronomers of the sev- enteenth century once did, even today’s astronomers are in the po- sition of having to apply the prin- ciple of mediocrity to a restricted class of an appropriately chosen reference – that of the technolog- ical species. The only representa- tive that we know to fall into this class is the human species, and it goes without saying that any extrapolation to hypothetical real- M 81, in Ursa Major, is a spiral galaxy slightly smaller than ours, hous- ing an SMBH al- most 20 times more massive than Sgr A* − still much lighter than those typical of large elliptical galaxies. Below, the famous Whirlpool Galaxy, M51, in Canes Ve- natici, the Hunt- ing Dogs constel- lation, a spiral galaxy par excel- lence. One of its arms interacts gravitationally with its compan- ion NGC 5195 − a clash between galaxies. [Cima Rest Astronomi- cal Observatory, Magasa, Valvesti- no, Italy]