On my MT I had years ago, you rotate that star several times, or less, which releases a ball bearing that locks into the grooves, to hold it in place. Mine also was locked up. It took more than several applications of penetrant to get it loosened, and rust flushed out. Then keep rocking the star back and forth until it loosens up.
If you're not too concerned with the paint, a 50/50 mix of Acetone & ATF would work great. The Acetone will take the paint off. Otherwise, some PB Blaster, or Kroil would be my second choices.
If memory serves me right, I may have bumped it back & forth with a medium sized rubber mallet to help things along. Once it's broken loose, a shot of oil occasionally will keep it free.
Just make sure that when you have it set to the position you want, make sure the detent is fully engaged, and/or the star is tightened, not riding on top of one of the notches. If not, it will pull out some, or push in with little effort.