I'm going for the ultimate in world building here: Creating a whole new star.
I have a race that's about a Type II civilization (as per the Kardashev scale). The important thing here is that they're capable of building megastructures, such as a Death Star. They could build a Dyson Sphere or something on that scale, but it would be extremely hard.
For some reason, this civilization wants to make a star in the middle of a giant molecular cloud, which is a large interstellar cloud composed of gas (molecular $H_2$) and dust where young stars are born. For a star to be born, a section of this cloud has to undergo gravitational collapse.$^1$
Scientists (in our world) don't know for sure what triggers this collapse, but there are some ideas. One is that a supernova shockwave hits the cloud. Another is that two clouds collide and collapse.
How can this civilization trigger any of these events and form a star? Is it even possible?
Note: A food page to look at is the Wikipedia page on Jeans instability. That might help you come up with an idea, if you can figure out what quantities need to change.
$^1$ You could just add matter, but I'd like to avoid that.