A cataclysmic variable is a binary star system containing a
small, cool, red star in a tight binary orbit with a white dwarf star.
to see an artist's concept of a cataclysmic variable binary star.)
The stars are so close that they typically orbit one another in a few
hours! The gravitational attraction of the white dwarf is so strong
that material from the companion is captured and swirls onto the white
dwarf through a large disk of gas called an accretion disk.
As the gas falls in, it gets heated and releases optical and ultraviolet
The spectrum above is for a cataclysmic variable known as IX Velorum.
The downward "dips" in the spectrum are called absorption lines,
and arise from different chemical elements in the gas, such as carbon,
nitrogen, oxygen, silicon, etc. The lines are marked with element
abbreviations. (Roman numerals indicate the "ionization stage" of
the element.) Most of the light seen by HUT arises in the accretion disk
of IX Vel, while the absorption lines are due to the gaseous disk and
possibly a "wind" or "corona" about the disk.
[Data from K. S. Long et al. 1994, ApJ, 426, 704.]
[Note: earth symbols (a circle with a "+" sign) mark regions affected
by residual atmospheric emissions.]