You can get %chance not to use ammo by wearing necro armor, or any of the ranger-tailored hardmode sets.
And I assume that it doesn't disappear, because of how the stars fall from the sky. I mean, I think the ammo used for fallen stars that fall from the sky is just the same as the starcannon's, except beefed up to 1000k.
There are space cannons up there, with 1000 dmg each. The aliens shooting them practise their aim against birds and bunnies.
I still haven't made myself a starcannon, despite just having killed the destroyer for the first time. Would have been easier on a large map, since I had to fend off two wyverns during the battle as well, but I succeeded.
For those having trouble who do not wish to use the meteorite trick, build a longish platform about 100-150 blocks up. You may need to adjust, but ideally you want the destroyer doing passes underneath it. Adamantite repeater with Holy Arrows, Ranged armor, and buff up. Star cloak is a good choice too. Just remember to shoot probes first, and the destroyer second. You get hearts and prevent laser fire, while the falling stars from the arrows and the cloak will still do damage to the destroyer.
Now I've got myself an unreal megashark and almost dropped the twins. Stupid Retina deciding he wanted to block some shots for his brother.