My texture is slightly transparent for no apparent reason

I have no clue why, but the one of the textures I’m using is partially transparent, even though on the source image it’s perfectly white. (If it makes any difference, I’m using additive blend mode, though I have had no trouble with this in the past)

Also, whenever I try to increase the color values to make it more emmisive, it just colors the partially transparent parts of the source image.

This is the point of the additive blend mode, it adds the output value of your material to the pixels behind your mesh so it will always be slightly transparent.

If you need to have opaque I would rather use the masked blend mode with your star texture as the opacity mask.