H-alpha (also called Hydrogen-alpha) is a spectral line in the visible spectrum of light. It is caused by the emission of light from hydrogen atoms when they are excited by ultraviolet radiation. The H-alpha line is one of the most important lines in astronomical spectroscopy, because it can be used to observe regions of star formation and other phenomena in galaxies and nebulae.
The wavelength of the H-alpha line is 656.3 nanometers, which is red light. This makes it visible to the naked eye under dark skies. However, H-alpha emissions are often very faint, so they are best observed with telescopes and cameras that are sensitive to red light.
H-alpha emissions arise from several different astrophysical phenomena. One of the most important is star formation, which creates massive stars that emit large amounts of ultraviolet radiation. This UV radiation excites hydrogen atoms in nearby gas clouds, causing them to emit H-alpha photons. Another significant source of H-alpha emission is accretion onto black holes and neutron stars, where hot plasma emits copious amounts of UV radiation as it falls into these extremely dense objects. Finally, supernova explosions also produce significant amounts of H-alpha emission as their shock waves interact with surrounding gas clouds.
Astronomical objects that emit strong H-alpha emissions include planetary nebulae, active galactic nuclei (AGN), and young open clusters such as the Orion Nebula Cluster (ONC). These objects are all sites of intense energy release, whether from stellar winds or nuclear activity. As a result, they are some of the most interesting targets for professional astronomers and amateur observers alike!