What is a cataclysmic variable star?

Bruno Cooke August 20, 2020

NASA’s Kepler spacecraft has detected a new light source. Further analysis revealed it to be a cataclysmic variable star — CV star for short. What exactly is a cataclysmic variable star, and why is it worth knowing about?

What is a CV star?

A cataclysmic variable star is actually a star system composed of a normal star and a white dwarf. The two orbit each other, like dancers. Compared with other star systems, CVs are quite small and spin around each other really quickly. CV orbital periods tend to sit in a range of 1-10 hours. 

Scientists have located CV binary systems in a variety of stellar environs: the centre of the Milky Way, in globular clusters, and even in our own solar neighbourhood. In terms of astral chronology, they pop in and out of vision. CV stars increase in brightness with sudden and erratic irregularity, and drop back down to a quiescent state just as quickly.

Photo by NASA’s Marshall Space Flight Center on Flickr

What make a cataclysmic variable star variable?

This oscillation in luminescence (the variable part of the name) is a result of two principal energy sources vying for dominance — accretion and nuclear fusion. 

The white dwarf in any given pairing is incredibly dense, and therefore exerts an extraordinary amount of gravitational potential energy on its companion. During the accretion process, some of the material that makes up the companion star is sucked in by the white dwarf. This part of the process is called mass transfer, and results in an accretion disc around the white dwarf. 

Image by Phillip D Hall via Wiki Commons

Explaining novae

In some cases, thermal instability in the disc triggers an unstable thermonuclear fusion reaction. Depending on whether the CV star pairing is in its fusion or accretion phase, this reaction can result in either a classical nova or a dwarf nova. 

These stellar outbursts can increase the brightness of the variable star system by ten thousand or even a million times. Classical novae are thought to recur over a period of 10,000 years.

Why you should care

Depending on which way you choose to look at them, such curiosities may seem distant and irrelevant. On the other hand, I think having a sense of perspective, wonder and awe can be life-enhancing. 

The beauty and magic of the stars have inspired the human imagination for millennia. It is a noble mission to attempt to discover more about them and a captivating past-time for anyone wanting to expand their comfort zone. 

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Bruno is a postgraduate student studying global journalism, with research interests in the intersection of the media, storytelling, culture and politics. His articles have appeared in Groundviews, Packs Light and Forge Press, and most are readable on Medium or onurbicycle.com. He is a Student Ambassador for Tortoise Media, a big fan of Freddie Mercury and a novelist – his debut novel, Reveries, is available on Amazon.