Free Astronomy Magazine July-August 2020

30 JULY-AUGUST 2020 GALAXIES Direct observations of very peripheral stars of our galaxy and computer simulations lead us to believe that part of the mass lost by Sagittarius is scattered in large arcs along the orbits so far described. It was reasonable to expect that mass transfer and gravita- tional perturbations would have produced star formation events in the Milky Way, but convincing evidence was needed to associ- ate their timing with that of the transits of the pericenter by Sagittarius. A decisive step in this direction has been performed re- cently by a team of researchers lead by Tomás Ruiz-Lara (Instituto de Astrofísica de Canarias), who combined data from Gaia and modeling techniques based on color- magnitude diagrams. With this procedure, the team was able to characterize with suf- ficient precision the stellar content in a sphere of space about 13,000 light-years in diameter, centered on the Sun; a sample cer- tainly representative of the whole galactic disk. Ruiz-Lara and colleagues found that at the end of the Gaia-Enceladus event, the star formation in our galaxy had returned to the levels of other massive spiral galaxies, which are generally very low levels. Never- theless, within this constant behavior, three well-defined star formation bursts are rec- T omás Ruiz- Lara, Postdoc- toral Fellow, at the Instituto de Astrofísica de Ca- narias, coordi- nated the team of researchers who proposed the Sagittarius galaxy as the driving force for the formation of our Solar System. P hotomontage of the Gaia Space Observa- tory against the background of the Milky Way. [ESA/ATG media- lab; background: ESO/S. Brunier] ognizable whose central phases date back to 5.7, 1.9 and 1.0 billion years ago with es- timated durations of 0.8, 0.2 and 0.1 billion years, respectively. There are also traces of a possible fourth peak of star formation, which can be located in the last 100 million years. It is very interesting that the timings of these episodes coincide with the passages to the pericenter of Sagittarius deduced from simulations of the orbit, from the structure of the galactic disk and from the stellar contents of Sagittarius.