You'll find this hidden star in Dreadnought's third star mission. From the start, run to the underside of the saucer to find a warp pipe which will teleport you to a garbage dump.
The droid here needs your help; it seems the planet's been covered in trash, and you'll have to blow it away within 30 seconds. This can be very, very difficult, since the Bob-ombs take forever to explode. Thankfully, there's a trick.
Do you see the gold emblems on the ground That's where you should toss the Bob-ombs. Toss one on each of the six gold emblems - start with the bottom left, then work your way along the lower row to the bottom-right. Now quickly toss two more bombs, one in both the top-left and top-right corners. if your're quick with your aim, the bombs will destroy all of the nearby trash, thereby completing this level well within the time limit.