Star Formation Begins When Clouds of Gas Crash Into Each Other

To trigger star formation, you need to compress a lot of gas into not a lot of volume. To make a lot of stars at once, you need to really pack it in. Until now, astronomers haven’t been sure how to pull this off. But a collection of 20 papers outlines how to do it: make giant clouds of gas crash into each other.

Stars come from nebulae, and nebulae come from stars. It’s a vast, slow circle of stellar life that constantly circulates material throughout each galaxy. When a nebula, which is just a cloud of gas and dust, collapses, it can fragment. These fragmented parts then individually collapse, eventually reaching the densities needed to hatch a newborn star.

A typical star-forming nebulae will produce 10 to 100 sibling stars in one go. But astronomers have seen massive star clusters, containing over 10,000 members, within the Milky Way and our nearest neighbor galaxies, Andromeda and Triangulum. The traditional collapse-of-a-gas-cloud approach doesn’t usually produce so many stars, so the question of their formation has been an open one.

Recently, a team of researchers led by Associate Professor Kengo Tachihara and Emeritus Professor Yasuo Fukui of Nagoya University examined a hypothetical scenario for making giant star clusters. In this scenario, it’s not just one gas cloud collapsing one its own, but two or more clouds colliding with each other, setting off a chain reaction that converts almost all the available mass into stars.

To test the hypothesis, the team looked in our galaxy and our nearest neighbors for signs of colliding gas clouds, and if those collision regions were sites of recent of star formation. Their work, encompassing 20 papers, appeared in Publications of the Astronomical Society of Japan in January 2021 as a special issue titled “Star Formation Triggered by Cloud-Cloud Collision ?.

From their observations, it does appear that colliding gas clouds supply enough energy and enough raw material to form giant star clusters. When gas clouds collide, it may be messy, but it sure is beautiful.