2.3 Supernovae and cataclysmic variables

In the final segment of Jo’s video she describes the explosion of a supergiant star, such as Betelgeuse. These massive stars end their lives in a colossal explosion triggered by the collapse of the core when all nuclear fuel is exhausted. In the process most of the star is destroyed, with material rich in heavier elements thrown off into the interstellar medium. Only a small inner core remains, eventually collapsing to form a neutron star or even a black hole. The expanding cloud of gas thrown off in these supernova explosions forms a nebula known as a supernova remnant. The Crab Nebula (M1) that you first saw in Week 4 is a classic example of a supernova remnant.

While a supernova is a one-off and very catastrophic explosion, the term cataclysmic variable primarily refers to a type of binary system in which one component is a white dwarf, in orbit with a low-mass main sequence companion star.

Figure _unit8.3.3 Figure 8 An artist’s impression of accreting binary star Nova Aquilae, consisting of a white dwarf in orbit with a low-mass main sequence companion star. Material from the larger star is transferred and falls onto the surface of the white dwarf, causing repeated explosive events. (Credit: NASA/CXC/M.Weiss)

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Artist’s impression of accreting binary star Nova Aquilae. Against a black background, the larger and cooler star is shown on the left as a slightly elliptical red-orange shape, with a lobe extending to the right. On the right is a white ellipse surrounding the white dwarf star. The elliptical shape depicts a circular disk of swirling material, seen at an angle, surrounding the white dwarf. The central part of this accretion disk is shown in white with the colour changing first to blue and then to yellow-green moving outwards. A trail of material extends from the red-orange companion star, depicting material being transferred from the outer envelope of the larger star on the left to join the accretion disk on the right.

Figure 8 An artist’s impression of accreting binary star Nova Aquilae, consisting of a white dwarf in orbit with a low-mass main sequence...

In this type of system, material from the outer layers of the companion star falls onto the surface of the white dwarf, building up density and pressure. Since this material contains a lot of hydrogen the increase in density, temperature and pressure eventually triggers an outburst of nuclear fusion, producing an explosion that can cause the brightness of the binary system to briefly increase by hundreds of thousands of times (a classical nova). In some systems the process of accretion and explosion can occur repeatedly, producing a cataclysmic variable star with periodic, if perhaps irregular, outbursts. These are known as recurrent novae.