Extreme ultraviolet and optical light curves of the 1994 August outburst of the dwarf nova SS Cygni. The EUV brightness is shown by the dots. The brightness at visible wavelengths as measured by the American Association of Variable Star Observers is shown by the solid line. Note the very fast and dramatic rise (by a factor of approximately 400) of the extreme ultraviolet brightness over a period of a few days.
Cataclysmic variables form a diverse class of close binary star systems composed of a white dwarf star and a cooler star (with spectral types G, K, or M). The stars in these binaries are so close that the time it takes for them to orbit about each other is measured in hours rather than days or years. Material is lost from the cooler star and is accreted by the white dwarf through a disk. As material spirals through this disk to the surface of the white dwarf it heats and becomes luminous; so luminous in fact that it outshines both the white dwarf and the cooler star. SS Cygni belongs to a subclass of cataclysmic variables called dwarf novae. In dwarf novae, the transport of material through the disk and onto the white dwarf is unstable: material alternately builds up in the disk and dumps out. While material builds up in the disk, it is faint, and as material dumps out of the disk, it becomes bright. These periodic brightenings of the disk are perceived at Earth as outbursts which last tens of days and which recur with periods of tens to hundreds of days.
It is important to observe dwarf novae in outburst with instruments that can detect various wavelengths, such as visible, ultraviolet, extreme ultraviolet, and X-ray. These different spectral regions are important to the study of the disks of dwarf novae because each probes a different region of the disk: the visible spectral region probes the outer disk, the ultraviolet region the middle disk, and the extreme ultraviolet and X-ray the inner disk and the very hot, very bright region between the disk and the surface of the white dwarf.