The term Ha (or Hα) stands for hydrogen alpha. It refers to a state of the hydrogen atom, when its only electron loses an energy level and emits a photon. Other possible states are called H-beta and H-gamma. Most nebulae are made of ionised hydrogen, which is where the Ha state occur. In the visible light spectrum, Ha appears on the red side (with a wavelength of about 656nm), and in astrophotography, it is possible to use a Ha filter that isolates this wavelength.