Cataclysmic Variables

The majority of stars are not solo stars like our Sun. Instead, they are binary or trinary systems, and in some cases, there can be many more. The stars don’t have to be equal, either. You might get a small dwarf star circulating a red giant, or perhaps a main-sequence star gradually being ripped apart by a black hole. Or they could be equal. But cataclysmic variable stars are essentially stars that go boom on a regular basis. What’s up with that?

The Discovery of Cataclysmic Variable Stars

Cataclysmic variable stars have long been observed, with the first known observation by Pére Dom Anthelme in 1670. Others carried on that work, particularly during the 19th century with bigger and better telescopes. More importantly, spectroscopy became useful to identify anomalies, such as those often found with novae. These stars were oddities that periodically brightened and dimmed, and spectroscopic measurements also varied.

During the 1940s, spectroscopic data showed that there were two distinct sets of emission spectra, which suggest that a known dwarf nova was actually two stars: a binary system. Further data backed this up.

In the 1960s, a series of missile launches were used to scan the heavens. Essentially, it was an X-ray detector strapped to an ICBM. The launches were programmed to scan a particular sector for 3 to 5 minutes, and then they would fall back to Earth with the data. One launch detected an X-ray transient, which is an X-ray burst that momentarily appeared and then disappeared when it was next scanned.

This provided the first inkling that the universe was even more violent than we had thought.

This system was replaced by the Vela series of satellites, and they detected even more X-ray bursts, as well as more than a few from their official job: nuclear weapon test detection. Other satellites supplanted them.

All of this data came in handy when eventually declassified. Scientists in the ’70s started to work out what these X-ray bursts were.

Stars That Go Boom

The answer is surprisingly simple. Take one main-sequence (usually dwarf) star, and then pair it with a white dwarf. Because the stars are orbiting very closely, they are very hard to tell apart. The main sequence star must also pass near or through the dwarf star’s accretion disc. This strips out a load of the main sequence star’s fusible material, which immediately reacts violently on the surface of the white dwarf. The result is a series of huge and controlled thermonuclear explosions that are many orders of magnitude bigger than the largest thermonuclear weapon humanity has ever created.

It’s this that creates a huge burst of X-rays and gamma rays. This has the effect of sterilizing all life in the system and perhaps further.

There are several types of CV system:

  • Novae are extremely bright. This creates a new, incredibly bright light in the sky that can last for several months. These include M31N 2008-12a.
  • Dwarf nova is not as bright, and their output can perhaps last a few hours or as long as a couple of weeks. These are the most common type of CV. These include U Geminorum and SS Cygni.
  • Polars are magnetic cataclysmic variables. They have strong magnetic fields that force the material to go to the north and south poles, where they continuously burn. They glow, but they do not produce outbursts. Polars include AM Herculis.

Binary systems are some of the most common in the Milky Way, and it’s almost inevitable that a good portion of them are some sort of cataclysmic variable star or will be. Despite the lack of mainstream news interest, understanding them is important if we are to understand our place in the galaxy.