Free Astronomy Magazine July-August 2020

28 JULY-AUGUST 2020 GALAXIES T his illustration and the video below summa- rize the last pas- sages to the pericenter of the Sagittarius dwarf galaxy, the re- sults of which were an observed increase in star formation in the Milky Way during each. [ESA, IAC] light-years around the Sun, it is not at all possible to obtain sufficient temporal reso- lution to isolate individual star formation events and place them precisely in the evo- lutionary history of the Milky Way. The ability to investigate these merger events has increased significantly in recent years, thanks to the release in 2016 and 2018 of the extremely accurate photomet- ric, astrometric and spectroscopic data collected for over one billion stars by the Gaia Space Ob- servatory. Gaia’s data allowed for the test of an intriguing hy- pothesis, according to which some star formation bursts that occurred in the Milky Way might have been related to the pericentric passages of one of its satel- lites, the Sagittarius dwarf galaxy. This small and evanescent stel- lar agglomerate, vaguely spheroidal, was discovered in 1994 and turned out to be the then-closest galaxy to the Milky Way, a record that lasted until 2003. Sagittarius is currently about 70,000 light-years from Earth and about 50,000 light-years from the galactic center. It follows a spiraling polar orbit which, due to gravitational interac-