Problem with starry sky image

hello.

ive been creating a starry sky image.
ive done everything correct in creating it but when i save all the layers to a flattened single layer i lose the adjustment layers.

new document-fill with black-add noise-levels adjustment layer-adjust sliders-flatten image to a single layer.

for some reason the levels adjustment layer which creates the stars effect is lost.

any help would be appreciated.

phil.