H-alpha is bright in the sky but does not stand out visually because it is so red as to be outside the most sensitive part of the eye's light sense.
An H-Alpha Filter (or Hydrogen-Alpha Filter) is centered on the H-alpha Wavelength.
HII Region (HII)
Palomar Four-Filter Survey (PC)
Star Formation Rate (SFR)
VLT Survey Telescope (VST)