Free Astronomy Magazine July-August 2020

46 JULY-AUGUST 2020 ASTROBIOLOGY which galaxies un- dergo profound transformations. Observations show that today’s large and quiescent ellip- tical galaxies may have been shocked billions of years ago by the quasar phenomenon pro- duced by super- massive black holes (SMBH) nestled at their centers. In those times, the el- liptical galaxies were, on average, more compact. As a result, the activity of the nucleus most likely led to the in- hibition of star for- mation and the sterilization of a large number of planets. Consequently, if the large elliptical galaxies relatively close to us may seem safer right now for possible forms of advanced life compared to conditions in spiral galaxies (in which the supernovae rate is decidedly higher), the fact re- mains that ellipticals were by no means safe when life would have had to conquer the mainland of any habitable planets. It is true that nearly all larger galaxies, in- cluding spirals, have gone through active phases during which their SMBHs have pro- duced a quasar. That said, the effects have been much more intense in elliptical galax- ies due to the proportionately greater mass of the black holes. In a paper published in Nature in 2017, Amedeo Balbi and Francesco Tombesi (Uni- versity of Rome “Tor Vergata”) investigated the habitability of the Milky Way during the active phase of our SMBH, Sgr A*, to under- stand up to which distances and to what ex- tent UV and X-ray radiation may have affected the loss of mass of planetary at- mospheres and the development of life. The two researchers found that only within a few kiloparsecs (1 kpc = 3,262 light-years) of Sgr A* would the radiation have been lethal for any life forms hosted on the surface of Earth-like planets. Beyond about 7 kpc, all A majestic group of galaxies domi- nated by the spi- ral NGC 7331 in the constellation Pegasus. [Cima Rest Astronomical Observatory, Ma- gasa, Valvestino, Italy]