A nova is caused by runaway thermonuclear reactions on the surface of a white dwarf star that has been accreting matter from a more normal companion star. The explosion sends a shell of hot, ionized plasma expanding into space at velocities of hundreds or even thousands of kilometers per second. Our spectra record the infrared radiation emitted by this shell. The radiation is in the form of both a continuum produced by electrons interacting with protons and other ions, and emission lines generated as electrons recombine with protons and other ions.
Nova Cygni was known as a ``neon nova'' because its ejecta evidently contained a much larger proportion of elements such as neon, oxygen, and nitrogen relative to hydrogen than is present in our Sun. One sign of a high neon abundance was a very bright emission line at 12.8 microns produced by neon ions with one electron removed (Ne+). This line is the brightest visible in our April 1992 low-resolution spectrum ( Plot 1 ), and it slowly fades on the following dates. The line is very broad ( Plot 2 ) due to the Doppler shift of the high-velocity expanding gas. As the shell expanded and thinned out, x-rays from the central white dwarf gradually ionized the ejecta to much higher levels, and lines from ions such as Ne+5 (at 7.65 microns) and Mg+6 (8.99 microns) appeared, while the once-bright 12.8 micron line faded from view (Plots 3 , 4 , and 5 ). Eventually, as the white dwarf cooled slightly, the Ne+5 ions recombined, the 7.65 micron line faded, and the Ne+ line reappeared ( Plot 6 ).
Novae provide one of the ways by which processed stellar material is injected back into the interstellar medium, where it may eventually become part of a cloud that collapses into a star or planetary system. Therefore, further study of these impressive objects will help us to understand the source of elements like oxygen and nitrogen in our own solar system.
