Hydrogen-alpha emissions (top graph)
Hydrogen-alpha emission was a component of a star's light originating from hydrogen.
When Doctor Timicin attempted to revitalize Beta Praxillus in 2367, two early increases of hydrogen-alpha emission levels were indications of neutron migration within the stellar core. This unexpected development ultimately led to Praxillus going nova. (TNG: "Half a Life")
As an in-joke, the console that La Forge was working on during the experiment on the star is labeled as sensor analysis 4077. This was the M*A*S*H unit that David Ogden Stiers was a member of while he played the character of Charles Winchester on the television series M*A*S*H. (Star Trek Encyclopedia)