A couple more posts on the AAS are coming, but for now I want to take a moment to answer a question asked by a reader yesterday. Paul asked “I have a question about how stars form. I’m confused about the answers I’ve read or heard about. Some people say that stars “condense” from molecular space gas. As they do, they heat up and ignite. Others say they “collapse” from gas. Still others say that the gas is “compressed” by a nearby supernova. … Can you help explain how stars are formed?”

As I understand it, there are a couple different models. In one, a molecular cloud will very slowly, over lots and lots of time collapse due to gravity (some clouds formed with our galaxy still haven’t collapsed all the way into stars!). Higher density regions will collapse faster, and lower density regions will either get sucked into higher density regions, or just collapse very very slowly. Pretty much everything in the galaxy has some angular momentum due to inherent rotation. As the densities within a giant molecular cloud collapse, they begin to spin and flatten. There is a period of time during which gravity is pulling material into the center of the density while the radiation pressure from the warm gas is ejecting the material in jets. Luckily, the system is able to not blow itself apart in the process, and gravity wins. When a star turns on – when nuclear reactions start up in the center, the light from the star creates so much pressure on surrounding material that the inflow of mass stops and the star clears out the area around it.

Now, if all the stars in the universe where formed simply through the very very slow gravitational collapse and fragmentation of molecular clouds, we would live in a very boring universe. Shocks (such as those from supernovae, spiral density waves, and collisions) can speed up the collapse of gas by pushing stuff together (condensing it). In this scenario, only the highest density regions survive to form stars, and the lower density gas dispersed. Here is a way to picture it: Imagine you have a rake with very flexible light wieght tines. Thanks to the help of a squirrel, you have one small patch of lawn with an over density of leaves. When you rack that one section of leaves, leaves that aren’t part of the original clump get pushed into it, and the force from the rake condences the pile. If the clump gets big enough, with a lot of large friction with the ground, the tines may bend and leave that clump behind. In a similar way, the shock wave can push together a large density of material, and if the material is dense enough gravity will hold it together and it will grow into a star.

In colliding galaxies, massive amounts of star formation will be triggered by shockwaves from the collapse, but the material that doesn’t get turned into stars may get strewn through space or pushed into the central black holes. No matter its fate, after the collision, the two galaxies will be dead, and star formation will have ceased.

image credit goes to HST.