Free Astronomy Magazine July-August 2020

31 JULY-AUGUST 2020 GALAXIES have been born as a result of that close passage of the dwarf galaxy. The Sun is located in the thin disk and could therefore fall within that 24%. The subse- quent passages of Sagittarius pro- duced less relevant effects in the thin disk, and almost imperceptible ef- fects in the thick disk, but they nonetheless con- tributed to increas- ing the stellar mass of our galaxy. The opposite hap- pened instead to Sagittarius. In fact, an analysis of its stellar content shows that, between five and seven billion years ago, the formation of new stars and chemical enrichment were those typical of dwarf galaxies. After moving to the peri- center 5.7 billion years ago, the star forma- tion rate collapsed, a sign of a remarkable loss of gas that was likely acquired by the Milky Way. At this point, can we say that the spark that gave rise to the Sun, the Earth and ourselves arose from a close passage of Sagittarius? Not with certainty, due to the difficulties re- searchers encounter in representing the or- bital dynamics of the dwarf galaxy. However, it is undeniable that the current shape of the Milky Way and its stellar mass are at least, in part, the result of the action of a crowd of satellite galaxies (about sixty are known), among which Sagittarius has certainly played an important role − more than that of the Magellanic Clouds, which are only on their first approach. Sagittarius could also have indirectly started the for- mation of the solar system by contributing to the birth of that massive star that would later produce the supernova many as- tronomers considered the source of our ex- istence. All hypotheses remain open. S tar formation history in the two kpc bubble around the Sun, with distinction between the thin and thick galactic disks. The star for- mation rate is expressed in arbi- trary units for both disk sub- structures. The inset shows the spatial distribu- tion of the stars belonging to each substructure in the x-z plane. Some regions close to the galac- tic plane were not included in the analysis due to the strong extinc- tion of the signal in those direc- tions. The stars were divided into thin disk or thick disk stars based on their tangential velocity. The four star formation peaks of the last six billion years are evident, in- cluding the cur- rent one, still not well-defined. [T. Ruiz-Lara et al.] ! If the results of the Ruiz-Lara team are cor- rect, we can deduce that Sagittarius played a non-marginal role in the building up of the stellar mass of the Milky Way’s disk, and that probably the Sun and its planetary system were formed as a consequence of a pericen- tric passage of the dwarf galaxy and not be- cause of the explosion of a supernova. The passage of about 5.7 billion years ago could be the one that created the conditions for the subsequent birth of the solar system. It is true that the Sun was formed about a bil- lion years after the apex of that event, but several factors may have intervened to delay its formation. For example, not a short time must have elapsed before gravitational per- turbations generated a sufficient degree of inhomogeneity in the distribution of gas and dust to trigger the collapse of clouds of mat- ter and the consequent birth of new stars. Although it seems impossible to accurately reconstruct the sequence of events, the star formation burst caused by Sagittarius be- fore, during and after the pericenter passing of 5.7 billion years ago is very evident both in the thick disk and in the thin disk. In these two kinematic components of our galaxy, the Ruiz-Lara team found that, respectively, 16% and 24% of the component stars could