Cosmo Run

Every time you fail during a run, you'll be given the option to spend your stars on the Save Me feature to let you continue your run. The first time you use Save Me during a run it costs 100 stars, and each subsequent time you use it during the same run, the cost increases by an additional 100 stars.

Initially you won't have many stars and it'll take a bunch of runs to earn another 100, so it'll feel like it takes a while to collect enough stars to use Save Me. However, once you get better at the game, you'll be able to collect plenty of stars in each run to maintain your total.

I recommend using Save Me only once per run at most so you only spend 100 stars per save. If you do it this way, you'll need a total of 5000 stars to use Save Me 50 times. That'll likely seem like a huge amount at first, but as a point of reference, after I had unlocked every achievement, I still had 10,000 stars left. So you will have more than enough stars to use Save Me 50 times while working towards 200,000. The "Battlecruiser" achievement solution has some additional information about collecting stars quickly.