sorry for late reply again.
Alpha channel is not needed by addtive blending.
The black is transparent by addtive blend, and the background is never darkened.
Each RGB of all pixels of the image is added to RGB of the background respectively.
And it will be seen a similar shine in the reality.
There are becoming white if adding bright lighter parts to bright background,
and white interrupts your view.
It means glare of the light pollution is more realistically simulated.