The union of the Milky Way and the so-called dwarf galaxy Gaia-Enceladus increased our galaxy's mass by about a quarter and triggered a period of accelerated star formation lasting about 2 to 4 billion years, the scientists said. "Yes, indeed it was a pivotal moment," said astronomer Carme Gallart of Instituto de Astrofisica de Canarias in Spain, lead author of the research published in the journal Nature Astronomy. Galaxies of all types including the Milky Way began to form relatively soon after the Big Bang explosion that marked the beginning of the universe some 13.8 billion years ago, but were generally smaller than those seen today and were forming stars at a rapid rate. Subsequent galactic mergers were instrumental in configuring galaxies existing now. The merger of the Milky Way and the dwarf galaxy Gaia-Enceladus roughly 10 billion years ago, left, and the current appearance of the Milky Way galaxy, right, are shown in this artist's conception, July 22, 2019. Certain stars with higher content of elements other than hydrogen or helium arose in the Milky Way, they found, and others with lower such content originated in Gaia-Enceladus, owing to its smaller mass. While the merger was dramatic and helped shape what the Milky Way has become, it was not a star-destroying calamity. "This crash was big in cosmic terms, but if it was happening now, we could probably not even notice at a human or solar system level," Gallart said. "The distances between stars in a galaxy are so huge — a galaxy is basically empty space — that the two galaxies intermix, change their global shape, more star formation may happen in one, and maybe the small one stops forming stars. "But the individual stars in each galaxy don't collide, don't really notice the force of the event in a way that affects their individual evolution or the evolution of the planetary systems that may be attached to them," Gallart said. The Milky Way, spiral shaped with a central bar-like structure composed of stars, includes 100 to 400 billion stars, including the sun, which formed roughly 4.5 billion years ago, far after the merger.