Free Astronomy Magazine July-August 2020

JULY-AUGUST 2020 A schematic representa- tion of the bar and spiral arms of the Milky Way. The Sun is near the so-called Orion Spur. [NASA/ JPL-Cal- tech/R. Hurt (SSC/Caltech)] curs on a much larger scale than the dis- tinctly local one of a supernova. The more intense the gravitational interaction be- tween two galaxies is, the higher the star formation rate. The borderline case is the merging of galaxies, which represents the main triggering mechanism for star forma- tion within the galaxies themselves, while also yielding significant changes in their chemical and dynamic evolution. Like all galaxies, the Milky Way has experi- enced episodes of gravitational interactions and mergings, resulting in increased mass and the adoption of its current form. As far as we know, the most relevant of these episodes happened approximately 10- 11 billion years ago, when the young Milky Way collided with a galaxy four times less massive, called Gaia-Enceladus, completely engulfing it. Traces of this distant event are