How do you get the mystery star in battle belt galaxy?

To get the mystery star in Battle Belt Galaxy, you need to first complete the main mission and unlock the second mission, titled "Battle Belt Galaxy: The Battle of the Bowsers." During this mission, look for a hidden star near the end of the level. After defeating the Bowser enemy, you may need to explore the area for a hidden launch star or specific actions that trigger the appearance of the mystery star. Collect it to complete the challenge!