Cataclysmic Variable Example Spectrum

A cataclysmic variable is a binary star system containing a small, cool, red star in a tight binary orbit with a white dwarf star. (Click here 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 emission.

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.]