The secret of fire: triggered star formation

Abstract

Let us turn back to fundamentals now, to keep our focus, avoiding the distractions along the many byways of the world of galaxies that we have wandered through in the last few chapters. The first of the two most important effects of galaxy collisions is that they build bigger galaxies from the small ones that formed first. They do this because mergers are inevitable, unless the collision happens to occur at very high relative velocity, e.g., in a dense galaxy cluster. The second important effect is that collisions and mergers evidently drive high rates of star formation, and increased nuclear activity in at least some cases. A major part of the meaning of the term “galaxy evolution” is the process of turning interstellar gas into stars, and increasing the heavy element abundance of gas and stars as a result of nuclear processing in successive stellar generations. Thus, collisions and mergers affect galaxy evolution, sometimes by a large factor. Induced star formation is also responsible for the beautiful imagery of colliding galaxies, with young star clusters scattered like jewels in some cases.