Question Soft fade shader

Could anyone hlep me with creating a shader where I want to have it be partially faded so one part is fully visible and then it fades near the edges. Everything I try just leave are harsh edge no soft fade.

Something like this, where it just fades away. Cause no matter what texture I make there is just a harsh cut off. I am using this for a particle system, would that be why the shader I make can't have a nice fade